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Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2017	Mar. 09, 2018	Jun. 30, 2017
Document and Entity Information
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	Q4
Entity Registrant Name	SELECTA BIOSCIENCES INC
Entity Central Index Key	0001453687
Entity Voluntary Filers	No
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Well-known Seasoned Issuer	No
Entity Common Stock, Shares Outstanding		22,349,840
Entity Public Float			$ 287,783,534
Entity Current Reporting Status	Yes

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 70,622	$ 58,656
Short-term deposits and investments	25,940	25,485
Restricted cash	76	78
Accounts receivable	63	215
Prepaid expenses and other current assets	1,979	2,382
Total current assets	98,680	86,816
Property and equipment, net	2,091	2,047
Restricted cash and other deposits	329	316
Other assets	0	122
Total assets	101,100	89,301
Current liabilities:
Accounts payable	1,606	3,882
Accrued expenses	8,580	3,921
Loans payable, current portion	0	4,067
Deferred revenue, current portion	787	1,836
Total current liabilities	10,973	13,706
Non‑current liabilities:
Deferred rent and lease incentive	151	222
Loans payable, net of current portion	21,042	7,977
Deferred revenue, net of current portion	15,919	12,439
Other long‑term liabilities	1,201	0
Total liabilities	49,286	34,344
Commitments and contingencies (Notes 8 and 13)
Stockholders’ equity:
Preferred stock, $0.0001 par value; 10,000,000 shares authorized; no shares issued and outstanding at December 31, 2017 and December 31, 2016, respectively	0	0
Common stock, $0.0001 par value; 200,000,000 shares authorized; 22,343,254 and 18,438,742 shares issued and outstanding as of December 31, 2017 and December 31, 2016, respectively	3	1
Additional paid-in capital	273,128	211,125
Receivable from stock option exercises	0	(75)
Accumulated deficit	(216,897)	(151,576)
Accumulated other comprehensive loss	(4,420)	(4,518)
Total stockholders’ equity	51,814	54,957
Total liabilities and stockholders’ equity	$ 101,100	$ 89,301

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	22,343,254	18,438,742
Common stock, shares outstanding	22,343,254	18,438,742

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Grant and collaboration revenue	$ 207	$ 8,083	$ 6,011
Operating expenses:
Research and development	45,165	29,702	22,980
General and administrative	18,826	13,051	8,335
Total operating expenses	63,991	42,753	31,315
Loss from operations	(63,784)	(34,670)	(25,304)
Investment income	617	234	171
Loss on extinguishment of debt	(673)	0	0
Foreign currency transaction gain (loss), net	(123)	(525)	933
Interest expense	(1,206)	(1,253)	(948)
Other income (expense), net	(152)	4	(26)
Net loss	(65,321)	(36,210)	(25,174)
Other comprehensive loss:
Foreign currency translation adjustment	78	504	(1,110)
Unrealized gain (loss) on securities	20	(36)	0
Total comprehensive loss	(65,223)	(35,742)	(26,284)
Net loss	(65,321)	(36,210)	(25,174)
Accretion of redeemable convertible preferred stock	0	(4,566)	(7,335)
Net loss attributable to common stockholders	$ (65,321)	$ (40,776)	$ (32,509)
Net loss per share attributable to common stockholders:
Net loss per share attributable to common stockholders—basic and diluted (in dollars per share)	$ (3.20)	$ (3.89)	$ (15.13)
Weighted average common shares outstanding:
Basic and diluted (in shares)	20,425,050	10,493,939	2,150,422

Consolidated Statements of Stockholders' Equity - USD ($)	Total	Common stock	Additional paid-In Capital	Stock option receivable	Accumulated deficit	Accumulated other comprehensive loss	Series A Redeemable Convertible Preferred Stock Preferred Stock	Series B Redeemable Convertible Preferred Stock Preferred Stock	Series C Redeemable Convertible Preferred Stock Preferred Stock	Series D Redeemable Convertible Preferred Stock	Series D Redeemable Convertible Preferred Stock Preferred Stock	Series D Redeemable Convertible Preferred Stock Additional paid-In Capital	Series SRN Redeemable Convertible Preferred Stock Preferred Stock	Series E Redeemable Convertible Preferred Stock	Series E Redeemable Convertible Preferred Stock Preferred Stock	Series E Redeemable Convertible Preferred Stock Additional paid-In Capital
Temporary equity, shares outstanding at the beginning of the period (in shares) at Dec. 31, 2014							2,589,868	7,437,325	5,000,002		8,099,994		2,111,109
Temporary equity, outstanding at the beginning of the period at Dec. 31, 2014							$ 3,493,000	$ 20,533,000	$ 19,270,000		$ 40,570,000		$ 10,167,000
Beginning balance (in shares) at Dec. 31, 2014		2,123,997
Beginning balance at Dec. 31, 2014	$ (87,755,000)	$ 0	$ 1,000	$ 0	$ (83,880,000)	$ (3,876,000)
Increase (Decrease) in Stockholders' Equity
Vesting of restricted common stock (in shares)		7,688
Vesting of restricted common stock	23,000		23,000
Issuance of common stock upon exercise of options (in shares)		41,714
Issuance of common stock upon exercise of options	86,000		86,000
Stock‑based compensation expense	1,125,000		1,125,000
Accretion of preferred stock to redemption value	7,335,000						$ 151,000	$ 915,000	$ 908,000		$ 2,332,000		$ 1,915,000		$ 1,114,000
Accretion of preferred stock to redemption value	(7,335,000)		(4,881,000)		(2,454,000)
Issuance of Series E redeemable convertible preferred stock, net of issuance costs of $213,469 (in shares)															8,888,888
Issuance of Series E redeemable convertible preferred stock, net of issuance costs of $213,469	0														$ 36,114,000
Issuance of common stock warrants	3,647,000		3,647,000
Currency translation adjustment	(1,110,000)					(1,110,000)
Net loss	(25,174,000)				(25,174,000)
Unrealized gains (losses) on securities	0
Temporary equity, shares outstanding at the end of the period (in shares) at Dec. 31, 2015							2,589,868	7,437,325	5,000,002		8,099,994		2,111,109		8,888,888
Temporary equity, outstanding at the end of the period at Dec. 31, 2015							$ 3,644,000	$ 21,448,000	$ 20,178,000		$ 42,902,000		$ 12,082,000		$ 37,228,000
Ending balance (in shares) at Dec. 31, 2015		2,173,399
Ending balance at Dec. 31, 2015	(116,493,000)	$ 0	1,000	0	(111,508,000)	(4,986,000)
Increase (Decrease) in Stockholders' Equity
Vesting of restricted common stock (in shares)		8,535
Vesting of restricted common stock	21,000		21,000
Issuance of common stock upon exercise of options (in shares)		71,816
Issuance of common stock upon exercise of options	149,000		224,000	(75,000)
Stock‑based compensation expense	2,030,000		2,030,000
Accretion of preferred stock to redemption value	4,566,000						$ 75,000	$ 449,000	$ 446,000		$ 1,131,000		$ 913,000		$ 1,552,000
Accretion of preferred stock to redemption value	(4,566,000)		(708,000)		(3,858,000)
Currency translation adjustment	504,000					504,000
Net loss	(36,210,000)				(36,210,000)
Exercise of common warrants (in shares)		572,003
Exercise of common warrants	0
Temporary equity, conversion of convertible stock upon listing (in shares)							(2,589,868)	(7,437,325)	(20,624,000)		(8,099,994)		(12,995,000)		(8,888,888)
Conversion of convertible stock upon listing							$ (3,719,000)	$ (21,897,000)	$ (5,000,002)		$ (44,033,000)		$ (2,111,109)		$ (38,780,000)
Common stock, conversion of convertible stock upon listing (in shares)		10,126,118
Conversion of convertible stock upon listing	142,048,000	$ 1,000	142,047,000
Issuance of common stock, initial public offering net of issuance costs (in shares)		5,289,633
Issuance of common stock, Initial public offering net of issuance costs	64,465,000		64,465,000
Issuance of common stock, license agreement (in shares)		197,238
Issuance of common stock, license agreement	2,743,000		2,743,000
Issuance of common stock, Private placement net of issuance costs										$ 189,000		$ 189,000		$ 113,000		$ 113,000
Unrealized gains (losses) on securities	$ (36,000)					(36,000)
Temporary equity, shares outstanding at the end of the period (in shares) at Dec. 31, 2016							0	0	0		0		0		0
Temporary equity, outstanding at the end of the period at Dec. 31, 2016							$ 0	$ 0	$ 0		$ 0		$ 0		$ 0
Ending balance (in shares) at Dec. 31, 2016	18,438,742	18,438,742
Ending balance at Dec. 31, 2016	$ 54,957,000	$ 1,000	211,125,000	(75,000)	(151,576,000)	(4,518,000)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock upon exercise of options (in shares)		269,842
Issuance of common stock upon exercise of options	705,000		630,000	75,000
Stock‑based compensation expense	4,080,000		4,080,000
Accretion of preferred stock to redemption value	0
Currency translation adjustment	78,000					78,000
Net loss	(65,321,000)				(65,321,000)
Issuance of common stock, license agreement (in shares)		529,616
Issuance of common stock, license agreement	10,000,000		10,000,000
Issuance of common stock, Private placement net of issuance costs (in shares)		3,088,791
Issuance of common stock, Private placement net of issuance costs	47,115,000	$ 2,000										$ 47,113,000
Issuance of common stock under Employee Stock Purchase Plan (in shares)		16,263
Issuance of common stock under Employee Stock Purchase Plan	180,000		180,000
Unrealized gains (losses) on securities	$ 20,000					20,000
Temporary equity, shares outstanding at the end of the period (in shares) at Dec. 31, 2017							0	0	0		0		0		0
Temporary equity, outstanding at the end of the period at Dec. 31, 2017							$ 0	$ 0	$ 0		$ 0		$ 0		$ 0
Ending balance (in shares) at Dec. 31, 2017	22,343,254	22,343,254
Ending balance at Dec. 31, 2017	$ 51,814,000	$ 3,000	$ 273,128,000	$ 0	$ (216,897,000)	$ (4,420,000)

Consolidated Statements of Changes in Stockholders’ Equity (Deficit) - (Parenthetical) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($)
Issuance of redeemable convertible preferred stock, issuance costs	$ 302
Preferred Stock \| Series E Redeemable Convertible Preferred Stock
Issuance of redeemable convertible preferred stock, issuance costs	$ 213,469

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities
Net loss	$ (65,321)	$ (36,210)	$ (25,174)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	750	802	1,044
Amortization of premiums and accretion of discounts on investments	233	193	0
Loss on disposal of fixed assets	36	3	0
Stock‑based compensation expense	4,081	2,051	1,125
Non‑cash interest expense	390	263	198
Loss on extinguishment of debt	673	0	0
Change in fair value of redeemable convertible preferred stock warrant	0	12	(83)
Changes in operating assets and liabilities:
Accounts receivable	152	610	(153)
Prepaid expenses and other assets	525	(860)	(1,011)
Restricted cash and other deposits	689	103	977
Accounts payable	(2,327)	2,686	667
Deferred revenue	2,432	10,409	(507)
Contingently repayable grant funding	0	(461)	(805)
Accrued expenses and other liabilities	5,650	716	1,259
Net cash used in operating activities	(52,037)	(19,683)	(22,463)
Cash flows from investing activities
Maturities of short-term government obligations	60,158	6,900	0
Purchase of short-term investments	(61,527)	(28,416)	(3,516)
Purchases of property and equipment	(733)	(586)	(1,163)
Proceeds from the sale of property and equipment	4	2	0
Net cash used in investing activities	(2,098)	(22,100)	(4,679)
Cash flows from financing activities
Proceeds from issuance of long-term debt, net of expenses	20,957	0	6,674
Repayments of long-term debt	(12,934)	0	(2,336)
Proceeds from issuance of convertible note	0	0	7,092
Proceeds from Initial Public Offering, net of underwriters' discounts and commissions	0	68,870	0
Deferred IPO costs paid	0	(4,103)	(302)
Proceeds from Private Placement, net of issuance costs ($2.9 million)	47,114	0	0
Proceeds from issuance of common stock	10,000	2,743	0
Proceeds from exercise of stock options	706	149	109
Proceeds from issuance of common stock under Employee Stock Purchase Plan	180	0	0
Net proceeds from issuance of preferred stock and warrants	0	0	32,669
Net cash provided by financing activities	66,023	67,659	43,906
Effect of exchange rate changes on cash	78	443	(1,019)
Net increase in cash and cash equivalents	11,966	26,319	15,745
Cash and cash equivalents at beginning of period	58,656	32,337	16,592
Cash and cash equivalents at end of period	70,622	58,656	32,337
Supplement cash flow information
Cash paid for interest	905	972	531
Noncash investing and financing activities
Purchase of property and equipment not yet paid	103	147	0
Unrealized gain on marketable securities	20	(36)	0
Venture debt termination fee liability	0	0	270
Issuance of preferred warrants in connection with venture loans	0	0	137
Reclassification of deferred IPO costs from non-current assets to additional paid-in capital	0	4,404	0
Initial public offering costs in accounts payable and accrued liabilities	0	122	1,246
Accrued dividends and accretion of preferred stock to redemption value	0	4,566	7,335
Conversion of bridge loans into Series E preferred	$ 0	$ 0	$ 7,288

Nature of the Business and Basis of Presentation	12 Months Ended
Nature of the Business and Basis of Presentation	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of the Business and Basis of Presentation	Nature of the Business and Basis of Presentation Selecta Biosciences, Inc. (the “Company”) was incorporated in Delaware on December 10, 2007, and is based in Watertown, Massachusetts. The Company is a biopharmaceutical company dedicated to developing the first generation of nanoparticle immunomodulatory drugs for the treatment and prevention of human diseases. Since inception, the Company has devoted its efforts principally to research and development of its technology and product candidates, recruiting management and technical staff, acquiring operating assets, and raising capital. The Company is subject to risks common to companies in the biotechnology industry including, but not limited to, new technological innovations, protection of proprietary technology, dependence on key personnel, compliance with government regulations and the need to obtain additional financing. Product candidates currently under development will require significant additional research and development efforts, including extensive pre-clinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure and extensive compliance-reporting capabilities. The Company’s product candidates are in development. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be obtained, that any products developed will obtain necessary government regulatory approval or that any approved products will be commercially viable. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will generate significant revenue from product sales. The Company operates in an environment of rapid change in technology and substantial competition from pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees and consultants. Liquidity The Company has incurred losses since inception and negative cash flows from operating activities. As of December 31, 2017 and December 31, 2016, the Company had an accumulated deficit of $216.9 million and $151.6 million, respectively. The Company’s cash and cash equivalents as of December 31, 2017 and December 31, 2016, includes $1.2 million and $2.4 million of unrestricted cash held by its Russian subsidiary. The future success of the Company is dependent upon its ability to obtain additional capital through issuances of equity and debt securities and from collaboration and grant agreements in order to further the development of its technology and product candidates, and ultimately upon its ability to attain profitable operations. There can be no assurance that the Company will be able to obtain the necessary financing to successfully develop and market its product candidates or attain profitability. Based on the current operating plan, the Company expects that its cash, cash equivalents, short-term investments and short-term restricted cash as of December 31, 2017, will fund its operating expenses and capital expenditure requirements into mid-2019. The Company has based this estimate on assumptions that may prove to be wrong, and the Company could use its capital resources sooner than it currently expects. Additionally, the process of testing product candidates in clinical trials is costly, and the timing of progress in these trials is uncertain. Because the Company’s product candidates are in various stages of clinical and preclinical development and the outcome of these efforts is uncertain, the Company cannot estimate the actual amounts necessary to successfully complete the development and commercialization of its product candidates or whether, or when, it may achieve profitability. Guarantees and Indemnifications As permitted under Delaware law, the Company indemnifies its officers, directors, consultants and employees for certain events or occurrences that happen by reason of the relationship with, or position held at, the Company. Through December 31, 2017, the Company had not experienced any losses related to these indemnification obligations, and no claims were outstanding. The Company does not expect significant claims related to these indemnification obligations and, consequently, concluded that the fair value of these obligations is negligible, and no related reserves were established.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2017

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

Principles of Consolidation

The consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries, Selecta RUS, LLC (“Selecta (RUS)”), a Russian limited liability corporation, and Selecta Biosciences Security Corporation, a Massachusetts Security Corporation. All significant intercompany accounts and transactions have been eliminated.

Foreign Currency

The functional currency of Selecta (RUS) is the Russian ruble. Assets and liabilities of Selecta (RUS) are translated at period-end exchange rates, while revenues and expenses are translated at average exchange rates for the period. Translation gains and losses are reflected in accumulated other comprehensive loss within stockholders’ deficit. Foreign currency transaction gains or losses are reflected in the consolidated statements of operations and comprehensive loss.

Use of Estimates

The preparation of consolidated financial statements in conformity with U.S. GAAP requires the Company’s management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. The Company’s management considers many factors in selecting appropriate financial accounting policies and controls, and bases its estimates on historical experience and other market-specific or other relevant assumptions that it believes to be reasonable under the circumstances. In preparing these consolidated financial statements, management used significant estimates in the following areas, among others: revenue recognition, the fair value of common stock and other equity instruments, accounting for stock-based compensation and estimating accrued research and development expenses. The Company assesses the above estimates on an ongoing basis; however, actual results could materially differ from those estimates.

The Company’s management made significant estimates and assumptions in determining the fair value of its common stock for those periods reported prior to the completion of the initial public offering of the Company’s common stock (the “IPO”). The Company utilized various valuation methodologies in accordance with the framework of the 2004 American Institute of Certified Public Accountants’ Technical Practice Aid, Valuation of Privately-Held Company Equity Securities Issued as Compensation, to estimate the fair value of its common stock. Each valuation methodology included estimates and assumptions that require the Company’s judgment. These estimates and assumptions included a number of objective and subjective factors, including external market conditions affecting the biotechnology industry sector, the prices at which the Company sold shares of its preferred stock, the superior rights and preferences of securities senior to the Company’s common stock at the time and the likelihood of achieving a liquidity event, such as an initial public offering or sale. Significant changes to the key assumptions used in the valuations could result in different fair values of common stock at each valuation date.

Segment Information

Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision maker, the Company’s Chief Executive Officer, in making decisions regarding resource allocation and assessing performance. The Company views its operations and manages its business in one operating segment, the research and development of nanoparticle immunomodulatory drugs for the treatment and prevention of human diseases.

Reverse Stock Split

In connection with the initial public offering, the Company’s Board of Directors and stockholders approved a one-for-3.9 reverse stock split of the Company’s common stock. The reverse stock split became effective June 7, 2016. All share and per share amounts presented have been retroactively adjusted for all periods presented to give effect to this reverse stock split, including reclassifying an amount equal to the reduction in par value of common stock to additional paid-in capital.

Cash Equivalents and Short-term Investments

Cash equivalents include all highly liquid investments maturing within 90 days from the date of purchase. Investments consist of securities with remaining maturities greater than 90 days when purchased. The Company classifies these investments as available‑for‑sale and records them at fair value in the accompanying consolidated balance sheets. Unrealized gains or losses are included in accumulated other comprehensive income (loss). Premiums or discounts from par value are amortized to investment income over the life of the underlying investment.

The Company, as part of its cash management strategy, may invest in reverse repurchase agreements. All reverse repurchase agreements are tri-party and have maturities of three months or less at the time of investment. These agreements are collateralized by U.S. treasury securities for an amount no less than 102% of their value.

Although available to be sold to meet operating needs or otherwise, securities are generally held through maturity. The cost of securities sold is determined based on the specific identification method for purposes of recording realized gains and losses. During the reporting periods, there were no realized gains or losses on sales of investments, and no investments were adjusted for other than temporary declines in fair value.

Concentrations of Credit Risk and Off‑Balance Sheet Risk

Financial instruments that potentially subject the Company to concentration of credit risk consist primarily of cash, cash equivalents, and accounts receivable. Cash and cash equivalents are deposited with federally insured financial institutions in the United States and may, at times, exceed federally insured limits. Management believes that the financial institutions that hold the Company’s deposits are financially credit worthy and, accordingly, minimal risk exists with respect to those balances. Generally, these deposits may be redeemed upon demand and therefore bear minimal interest rate risk. As an integral part of operating its Russian subsidiary, the Company also maintains cash in Russian bank accounts in denominations of both Russian rubles and U.S. dollars. As of December 31, 2017, the Company maintained approximately $1.3 million in Russian bank accounts, of which $1.2 million was held in U.S. dollars.

The Company has minimal credit risk as the majority of accounts receivable relates to amounts due under a government sponsored grant, collaboration with large pharmaceutical companies or grants from well‑known and supported non‑profit organizations. The Company did not have any off balance sheet arrangements as of December 31, 2017 and December 31, 2016.

Fair Value of Financial Instruments

The Company’s financial instruments consist mainly of cash equivalents, short‑term investments, restricted cash, accounts receivable, accounts payable, loans payable, and common stock warrants. The carrying amounts of cash equivalents, short-term investments, restricted cash, accounts receivable, and accounts payable approximate their estimated fair value due to their short-term maturities. At December 31, 2017, the carrying amount of loans payable approximates its estimated fair value due to the consistency between the prevailing market rates in effect and the effective interest rate of 6.53% for the debt arrangement.

Accounting standards define fair value as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. A three‑level hierarchy is used to prioritize the inputs to valuation techniques used to measure fair value. The hierarchy gives the highest priority to unadjusted quoted prices in active markets for identical assets or liabilities (Level 1 measurements), and the lowest priority to unobservable inputs (Level 3 measurements). The three levels of the fair value hierarchy are described below:

Level 1—Level 1 inputs are quoted prices in active markets for identical assets or liabilities that the reporting entity has the ability to access at the measurement date.

Level 2—Level 2 inputs are inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly. If the asset or liability has a specified (contractual) term, a Level 2 input must be observable for substantially the full term of the asset or liability.

Level 3—Level 3 inputs are unobservable inputs for the asset or liability in which there is little, if any, market activity for the asset or liability at the measurement date.

To the extent that a valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. The fair value of the Company's loan payable was determined using Level 3 inputs.

Fair value is a market‑based measure considered from the perspective of a market participant rather than an entity‑specific measure. Therefore, even when market assumptions are not readily available, the Company’s own assumptions are set to reflect those that market participants would use in pricing the asset or liability at the measurement date. The Company uses prices and inputs that are current as of the measurement date, including during periods of market dislocation. In periods of market dislocation, the observability of prices and inputs may change for many instruments. This condition could cause an instrument to be reclassified within levels in the fair value hierarchy. There were no transfers within the fair value hierarchy during the years ended December 31, 2017 and December 31, 2016.

Property and Equipment

Property and equipment are recorded at cost and depreciated using the straight‑line method over the estimated useful lives of the respective assets, generally seven years for furniture and fixtures, five years for laboratory equipment and office equipment and three years for computer equipment. Leasehold improvements are amortized over their useful life or the life of the lease, whichever is shorter. Major additions and betterments are capitalized. Maintenance and repairs, which do not improve or extend the life of the respective assets, are charged to operations as incurred. Costs incurred for construction in progress are recorded as assets and are not amortized until the construction is substantially complete and the assets are ready for their intended use.

Impairment of Long‑Lived Assets

The Company periodically evaluates its long‑lived assets for potential impairment. Impairment is assessed when there is evidence that events or changes in circumstances indicate that the carrying amount of an asset may not be recovered. Recoverability of these assets is assessed based on undiscounted expected future cash flows from the assets, considering a number of factors, including past operating results, budgets and economic projections, market trends, and product development cycles. Impairment in the carrying value of each asset is assessed when the undiscounted expected future cash flows derived from the asset are less than their carrying value. The Company did not recognize any impairment charges as of December 31, 2017 and December 31, 2016.

Debt Issuance Costs

Debt issuance costs and fees paid to lenders are classified as a debt discount, and are recorded as a direct deduction from the face amount of the related debt. Issuance costs paid to third parties that are the direct result of the debt issuance are capitalized as a direct deduction from the face amount of the related debt. Debt issuance costs are amortized over the term of the related debt using the interest method and recorded as interest expense. Costs and fees paid to third parties are expensed as incurred.

Accumulated Other Comprehensive Income (Loss)

The components of accumulated other comprehensive income (loss), net of tax, were as follows (in thousands):



	Foreign currency translation adjustment			Unrealized gains (losses) on available-for-sale securities			Accumulated other comprehensive income (loss)
Balance at December 31, 2014	$	(3,876	)	$	—		$	(3,876	)
Other comprehensive loss during the year	(1,110		)	—			(1,110		)
Balance at December 31, 2015	$	(4,986	)	$	—		$	(4,986	)
Other comprehensive income (loss) during the year	504			(36		)	468
Balance at December 31, 2016	$	(4,482	)	$	(36	)	$	(4,518	)
Other comprehensive income during the year	78			20			98
Balance at December 31, 2017	$	(4,404	)	$	(16	)	$	(4,420	)

Comprehensive income (loss) is the total net earnings and all other non-owner changes in equity. Except for net income and unrealized gains and losses on available-for-sale securities and foreign exchange translation adjustments, the Company does not have any transactions or other economic events that qualify as comprehensive income (loss). There were no reclassifications out of accumulated other comprehensive income or loss as well as no tax effect for the period presented.

Revenue Recognition

The Company’s revenue is primarily generated from research grants in both the United States and Russia, and, prior to its termination, a license and research collaboration agreement with Sanofi. The Company recognizes revenue in accordance with ASC Topic 605, Revenue Recognition. Accordingly, revenue is recognized when all of the following criteria are met:

- Persuasive evidence of an arrangement exists;

- Delivery has occurred or services have been rendered;

- The seller’s price to the buyer is fixed or determinable; and

- Collectability is reasonably assured.

Amounts received prior to satisfying the revenue recognition criteria are recognized as deferred revenue in the Company’s consolidated balance sheets. Amounts expected to be recognized as revenue within the 12 months following the balance sheet date are classified as deferred revenue, current portion. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date are classified as deferred revenue, net of current portion.

Collaboration Revenue

The Company enters into collaborative arrangements for the development and commercialization of product candidates utilizing the Company’s Synthetic Vaccine Particles (SVP) technology. The terms of these agreements contain multiple deliverables which may include (i) licenses, or options to obtain licenses, to its technology platforms, (ii) rights to future technological improvements, (iii) research and development activities to be performed on behalf of the collaborative partner or as part of the collaboration, (iv) the manufacture of pre-clinical or clinical materials for the collaborative partner, and (v) options to acquire licenses for additional therapeutic areas. Payments to the Company under these agreements may include nonrefundable license fees, option fees, exercise fees, payments for research and development activities, payments for the manufacture of preclinical or clinical materials, payments based upon the achievement of certain milestones and royalties on product sales.

When evaluating multiple element arrangements such as the agreements discussed in Note 13, the Company considers whether the deliverables under the arrangement represent separate units of accounting. This evaluation requires subjective determinations and requires management to make judgments about the individual deliverables and whether such deliverables are separable from the other aspects of the contractual relationship. In determining the units of accounting, management evaluates certain criteria, including whether the deliverables have standalone value, based on the consideration of the relevant facts and circumstances for each arrangement. The consideration received is allocated among the separate units of accounting using the relative selling price method, and the applicable revenue recognition criteria are applied to each of the separate units. If deliverables have no standalone value, they are grouped together and revenue is recognized over the estimated performance period.

The Company determines the estimated selling price for deliverables within each agreement using vendor‑specific objective evidence (“VSOE”) of selling price, if available, third‑party evidence (“TPE”) of selling price if VSOE is not available, or best estimate of selling price if neither VSOE nor TPE is available. Determining the best estimate of selling price for a deliverable requires significant judgment. Since the Company is a clinical stage company without a marketable product, and currently there is no technologically comparative product on the market, to determine either VSOE or TPE, the Company has used its best estimate of selling price to estimate the selling price for licenses and deliverables related to the Company’s proprietary technology. Under the circumstances, the Company considers market conditions as well as entity‑specific factors, including those factors contemplated in negotiating the agreements, estimated development costs, probability of success and the time needed to commercialize a product candidate pursuant to the license. In validating the Company’s best estimate of selling price, the Company evaluates whether changes in the key assumptions used to determine the best estimate of selling price will have a significant effect on the allocation of arrangement consideration between multiple deliverables.

Options for future deliverables are considered substantive if, at the inception of the arrangement, the Company is at risk as to whether the collaboration partner will choose to exercise the option. Factors that the Company considers in evaluating whether an option is substantive include the overall objective of the arrangement, the benefit the collaborator might obtain from the arrangement without exercising the option, the cost to exercise the option and the likelihood that the option will be exercised. When an option is considered substantive and there is no significant incremental discount, the option is not considered a deliverable in the arrangement and no consideration is allocated to it. Conversely, when an option is not considered substantive or it is considered substantive but is priced at an incremental discount, it would be considered a deliverable at the inception of the arrangement and a corresponding amount would be included in the initial consideration.

The Company may receive upfront payments when licensing its intellectual property in conjunction with a manufacturing or a research and development agreement. When management believes the license to its intellectual property does not have stand‑alone value from the other deliverables to be provided in the arrangement, the Company generally recognizes revenue attributed to the license over the Company’s contractual or estimated performance period. When management believes the license to its intellectual property has stand‑alone value, the Company generally recognizes revenue attributed to the license upon delivery. The periods over which revenue should be recognized are subject to estimates by management and may change over the course of the research and development agreement. Such a change could have a material impact on the amount of revenue the Company records in future periods. Payments or reimbursements resulting from the Company’s deliveries of manufactured products and research and development efforts are recognized as the services are performed.

At the inception of each agreement that includes milestone payments, the Company evaluates whether each milestone is substantive and at risk to both parties based on the contingent nature of the milestone, specifically reviewing factors such as the scientific and other risks that must be overcome to achieve the milestone, whether the milestone consideration is reasonable relative to all deliverables and payment terms, as well as the level of effort and investment required. Revenues from milestones, if they are nonrefundable and deemed substantive, are recognized upon successful accomplishment of the milestones. Milestones that are not considered substantive are accounted for as license payments and recognized over the remaining period of performance.

Grant Agreements

Grant revenue is generally recognized as the related research and development work is performed. Grant arrangements frequently include payment milestones which the Company has judged to be non‑substantive milestones as they are typically entitled to receive payment regardless of the outcome of the research work. Revenue under such arrangements is recognized using a proportional performance method, but not more than cash received.

Amounts received prior to satisfying the above revenue recognition criteria are recorded as deferred revenue in the accompanying consolidated balance sheets.

Research and Development Costs

Costs incurred in the research and development of the Company’s products are expensed as incurred. Research and development expenses include costs incurred in performing research and development activities, including salaries and benefits, facilities cost, overhead costs, contract services, supplies and other outside costs. Nonrefundable advance payments for goods and services that will be used in future research and development activities are expensed when the activity has been performed or when the goods have been received rather than when the payment is made.

Clinical Trial Costs

Clinical trial costs are a component of research and development expenses. The Company accrues and expenses clinical trial activities performed by third parties based on an evaluation of the progress to completion of specific tasks using data such as patient enrollment, clinical site activation, and other information provided to the Company by its vendors.

Income Taxes

The Company provides deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the Company’s financial statement carrying amounts and the tax basis of assets and liabilities using enacted tax rates expected to be in effect in the years in which the differences are expected to reverse. A valuation allowance is provided to reduce the deferred tax assets to the amount that will more-likely-than-not be realized.

The Company determines whether it is more likely than not that a tax position will be sustained upon examination. If it is not more-likely-than-not that a position will be sustained, none of the benefit attributable to the position is recognized. The tax benefit to be recognized for any tax position that meets the more‑likely‑than‑not recognition threshold is calculated as the largest amount that is more than 50% likely of being realized upon resolution of the contingency. The Company accounts for interest and penalties related to uncertain tax positions as part of its provision for income taxes. To date, the Company has not incurred interest and penalties related to uncertain tax positions.

Preferred Stock

Prior to the completion of the IPO, the Company classified preferred stock as temporary equity and initially recorded it at the original issuance price, net of issuance costs and discounts. The carrying value was accreted up to the redemption value over the earliest redemption period. The carrying value was also adjusted for dividends expected to be paid upon redemption or liquidation according to the preferred stock terms on each balance sheet date.

Warrants

The Company determines the accounting classification of warrants that are issued, as either liability or equity, by first assessing whether the warrants meet liability classification in accordance with ASC 480-10, Accounting for Certain Financial Instruments with Characteristics of both Liabilities and Equity, and then in accordance with ASC 815-40, Accounting for Derivative Financial Instruments Indexed to, and Potentially Settled in, a Company’s Own Stock. Under ASC 480, warrants are considered liability classified if the warrants are mandatorily redeemable, obligate the issuer to settle the warrants or the underlying shares by paying cash or other assets, or warrants that must or may require settlement by issuing variable number of shares.

If warrants do not meet liability classification under ASC 480-10, the Company assesses the requirements under ASC 815-40, which states that contracts that require or may require the issuer to settle the contract for cash are liabilities recorded at fair value, irrespective of the likelihood of the transaction occurring that triggers the net cash settlement feature. If the warrants do not require liability classification under ASC 815-40, in order to conclude equity classification, the Company assesses whether the warrants are indexed to our common stock and whether the warrants are classified as equity under ASC 815-40 or other applicable GAAP. After all relevant assessments are made, the Company concludes whether the warrants are classified as liability or equity. Liability classified warrants are required to be accounted for at fair value both on the date of issuance and on subsequent accounting period ending dates, with all changes in fair value after the issuance date recorded in the statements of operations as a gain or loss. Equity classified warrants are accounted for at fair value on the issuance date with no changes in fair value recognized after the issuance date.

Stock‑Based Compensation

The Company accounts for all stock‑based compensation granted to employees and non‑employees using a fair value method. Stock‑based compensation awarded to employees is measured at the grant date fair value and is recognized over the requisite service period of the awards, usually the vesting period, on a straight‑line basis, net of estimated forfeitures. Stock‑based compensation awarded to non‑employees are subject to revaluation over their vesting terms. The Company reduces recorded stock‑based compensation for estimated forfeitures. To the extent that actual forfeitures differ from the Company’s estimates, the differences are recorded as a cumulative adjustment in the period the estimates were adjusted. Stock‑based compensation expense recognized in the consolidated financial statements is based on awards that are ultimately expected to vest.

Comprehensive Loss

Comprehensive loss is defined as the change in the equity of a business entity during a period from transactions and other events and circumstances from non‑owner sources. It includes all changes in equity during a period except those resulting from investments by owners and distributions to owners. Comprehensive loss consists of both: (i) all components of net loss and (ii) all components of comprehensive loss other than net loss, referred to as other comprehensive loss. For all periods presented, other comprehensive loss is comprised of foreign currency translation adjustments and the unrealized gains and losses on investments.

Net Loss Per Share

The Company has reported losses since inception and has computed basic net loss per share attributable to common stockholders by dividing net loss attributable to common stockholders by the weighted average number of common shares outstanding for the period. The Company has computed diluted net loss per common share after considering all potentially dilutive common shares, including stock options, convertible preferred stock, and warrants outstanding during the period except where the effect of including such securities would be antidilutive. Because the Company has reported net losses since inception, these potential common shares have been anti‑dilutive and basic and diluted loss per share have been the same.

Deferred Rent

Rent expense and lease incentives from operating leases are recognized on a straight‑line basis over the lease term. The difference between rent expense recognized and rental payments is recorded as deferred rent in the accompanying consolidated balance sheets.

Contingent Liabilities

The Company accounts for its contingent liabilities in accordance with ASC No. 450, Contingencies. A provision is recorded when it is both probable that a liability has been incurred and the amount of the loss can be reasonably estimated. With respect to legal matters, provisions are reviewed and adjusted to reflect the impact of negotiations, estimated settlements, legal rulings, advice of legal counsel and other information and events pertaining to a particular matter. As of December 31, 2017 and December 31, 2016, the Company was not a party to any litigation that could have a material adverse effect on the Company’s business, financial position, results of operations or cash flows.

Recent Accounting Pronouncements

In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers ("ASU 2014-09"). ASU 2014-09 and its related amendments (collectively referred to as ASC 606) requires that an entity recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. The new guidance requires the application of a five-step model to determine the amount and timing of revenue to be recognized. The underlying principle is that revenue is to be recognized for the transfer of goods or services to customers that reflects the amount of consideration that the Company expects to be entitled to in exchange for those goods or services. The Company adopted the standard as of the required effective date of January 1, 2018 using the modified retrospective method. Under this method, the new guidance was applied to contracts that were not yet completed as of January 1, 2018 with the cumulative effect recognized at the date of initial application.

The Company finalized its review of its existing revenue contracts. As a result of its assessment, the Company determined that there may be a significant impact related to the recognition of grant revenue and capitalization of contract acquisition costs. Specifically, the Company deferred certain amounts related to grants that were not fixed and determinable under ASC 605 and will finalize the application of ASC 606, including the amount of variable consideration to be recognized subject to a constraint, resulting in a transition adjustment upon adoption that the Company does not expect will exceed $3.0 million. The Company also anticipates that contract acquisition costs, such as sublicense fees due to MIT related to the Spark agreement, will be capitalized and amortized over the period of performance, resulting in a transition adjustment upon adoption that the Company does not expect will exceed $2.6 million. The adoption of ASC 606 is expected to have an immaterial impact on the Company’s Collaboration Revenues at the date of adoption. Consistent with ASC 605, the Company concluded that the license and delivery of the underlying particles are not distinct, therefore, are considered a combined performance obligation. In addition, consistent with ASC 605, the license options are material rights because they were issued at a discount. The Company allocated the consideration to each material right and performance obligations using a relative selling price allocation. The Company’s collaboration agreements contain significant amounts that will become due upon the occurrence of certain events. These amounts include development, and regulatory milestones, which are considered part of the variable consideration. Upon adoption of ASC 606, the Company expects to conclude that the development and regulatory milestones are fully constrained. The Company will continue to evaluate the likelihood of the development and regulatory milestone events occurring and will include such amounts in the transaction price when it is probable that a significant revenue reversal will not occur in future periods. The arrangement also includes sales milestones and royalties, which will be recognized as revenue when the related sales occur, or when the milestone thresholds have been met using the royalty recognition constraint. The impact on the Company's collaboration revenue in 2018 and forward will depend on the timing and delivery of performance obligations, as well as the estimates of variable consideration, subject to the constraint. The deferred tax assets and liabilities resulting from these adjustments will be substantially offset by an associated adjustment to the Company valuation allowance. Therefore, as the Company currently maintains a full valuation allowance against its net deferred tax assets, the Company does not expect the adoption of ASC 606 to have a significant impact on its deferred tax balances or income tax expense in 2018.

The potential impact listed above are estimates of the expected effects of the Company’s adoption of ASC 606 as of the time of preparation of this Annual Report on Form 10-K and will be finalized in connection with the first quarter. The anticipated accounting impacts described above will have no impact on cash flows.

In November 2015, the FASB issued ASU No. 2015-17, Income Taxes (Topic 740), Balance Sheet Classification of Deferred Taxes. The new standard requires that deferred tax assets and liabilities be classified as non-current in a classified statement of financial position. The Company adopted this standard on January 1, 2017, and the adoption did not have an impact on its consolidated financial statements due to the full valuation allowance position.

In January 2016, the FASB issued ASU No. 2016-01, Financial Instruments-Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities (“ASU 2016-01”). ASU 2016-01 supersedes the guidance to classify equity securities with readily determinable fair values into different categories (that is, trading or available-for-sale) and requires equity securities (including other ownership interests, such as partnerships, unincorporated joint ventures, and limited liability companies) to be measured at fair value with changes in fair value recognized through net income. The amendments allow equity investments that do not have readily determinable fair values to be remeasured at fair value either upon the occurrence of an observable price change or upon identification of an impairment. The amendments also require enhanced disclosures about those investments. ASU 2016-01 is effective for annual reporting beginning after December 15, 2017, including interim periods within the year of adoption, and calls for prospective application, with early application permitted. Accordingly, the standard is effective for the Company on January 1, 2018. Upon adoption of ASU 2016-01 on January 1, 2018, the Company expects the impact to be less than $0.1 million on its consolidated statement of operations.

In February 2016, FASB issued ASU No. 2016-02, Leases (Topic 842) (“ASU 2016-02”). ASU 2016-02 requires a lessee to separate the lease components from the non-lease components in a contract and recognize in the statement of financial position a liability to make lease payments (the lease liability) and a right-of-use asset representing its right to use the underlying asset for the lease term. It also aligns lease accounting for lessors with the revenue recognition guidance in ASU 2014-09. ASU 2016-02 is effective for fiscal years beginning after December 15, 2018, including interim periods within those fiscal years. The Company is currently evaluating the effect of the adoption of this guidance on its consolidated financial statements.

In March 2016, the FASB issued ASU No. 2016-9, Compensation-Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting (“ASU 2016-9”). ASU 2016-09 simplifies several aspects of the accounting for share-based payment transactions, including the accounting for income taxes, classification of awards as either equity or liabilities, the estimation of forfeitures, statutory tax withholding requirements, and classification in the statement of cash flows. This guidance is effective for the Company on January 1, 2017 and early adoption was permitted. The Company adopted ASU 2016-09 during the first fiscal quarter ended on March 31, 2017. As part of the adoption of this guidance, the Company elected to continue estimating forfeitures in its calculation of stock-based compensation expense.

The excess tax benefits and tax deficiencies from stock-based compensation awards accounted for under ASC 718 are recognized as income tax benefit or expense, respectively, in the statement of operations and comprehensive loss. The income tax related items had no effect on the current period presentation as the Company maintains a full valuation allowance against its deferred tax assets. In addition, excess tax benefits for share-based payments are presented as an operating activity in the statement of cash flows rather than financing activity. The changes have been applied prospectively in accordance with the ASU and prior periods have not been adjusted. As a result of the adoption on January 1, 2017, the net operating losses deferred tax assets were increased by $0.3 million, which was offset by a corresponding increase in the valuation allowance.

In August 2016, the FASB issued ASU No. 2016-15, Statements of Cash Flows (Topic 230) - Classifications of Certain Cash Receipts and Cash Payments ("ASU 2016-15"), to clarify how companies present and classify certain cash receipts and cash payments in the statement of cash flows. This guidance is effective for annual reporting periods beginning after December 15, 2017, including interim periods within that reporting period, with early adoption permitted. The Company does not expect the adoption of this guidance will have a material impact on its consolidated financial statements.

In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows, Restricted Cash ("ASU 2016-18"). This guidance requires that a statement of cash flows explain the total change during the period of cash, cash equivalents, and amounts generally described as restricted cash or restricted cash equivalents. Amounts described as restricted cash and restricted cash equivalents should be included with cash and cash equivalents when reconciling the beginning of period and end of period to total amounts shown on the statement of cash flows. This guidance is effective for annual reporting periods beginning after December 15, 2017, including interim periods within that reporting period, with early adoption permitted. We will adopt ASU 2016-18 during the first quarter ending March 31, 2018. Upon adoption, the Company will present $0.4 million of restricted cash in the "Cash and cash equivalents at beginning of period" line in the consolidated cash flow statement.

In January 2017, the FASB issued ASU No. 2017-01, “Business Combinations (Topic 805): Clarifying the Definition of a Business” (ASU 2017-01), which clarifies the definition of a business to assist entities with evaluating whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses. This ASU is effective for public entities for fiscal years beginning after December 15, 2017, with early adoption permitted. The Company does not expect the adoption of ASU 2017-01 will have a material impact on the its consolidated financial statements.

Available-for-Sale Marketable Securities

12 Months Ended

Dec. 31, 2017

Investments, Debt and Equity Securities [Abstract]

Available-for-Sale Marketable Securities

As of December 31, 2017, the Company’s available-for-sale marketable securities consisted of corporate debt securities and U.S. agency bonds. As of December 31, 2016, the Company's available-for-sale marketable securities consisted of corporate debt securities.

The following tables summarize the Company’s available-for-sale marketable securities by major type of security as of December 31, 2017 and December 31, 2016 (in thousands):



	December 31, 2017
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
U.S. government agency bonds	$	12,798	$	—	$	(10	)	$	12,788
Corporate bonds	13,158		—		(6		)	13,152
Total available-for-sale marketable securities	$	25,956	$	—	$	(16	)	$	25,940



	December 31, 2016
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Corporate bonds	$	24,821	$	17	$	(53	)	$	24,785
Total available-for-sale marketable securities	$	24,821	$	17	$	(53	)	$	24,785

All available-for-sale marketable securities are classified in the Company’s consolidated balance sheets as short-term deposits and investments.

The Company classifies its marketable debt securities based on their contractual maturity dates. As of December 31, 2017, the Company’s marketable debt securities mature at various dates through May 2018. The fair values and amortized cost of marketable debt securities by contractual maturity were as follows (in thousands):



	December 31, 2017				December 31, 2016
	Fair Value		Amortized Cost		Fair Value		Amortized Cost
Less than one year	$	25,940	$	25,956	$	24,785	$	24,821

As of December 31, 2017, the Company held a total of 12 corporate bond positions, all of which were in an unrealized loss position, none of which had been in an unrealized loss position for 12 months or greater. The aggregate fair value of corporate bond investments in an unrealized loss position is $13.2 million.

As of December 31, 2017, the Company held a total of 8 U.S. agency bond positions, all of which were in an unrealized loss position, none of which had been in an unrealized loss position for 12 months or greater. The aggregate fair value of U.S. agency bond investments in an unrealized loss position is $12.8 million.

Based on the Company’s review of these securities, the Company believes that the cost basis of its available-for-sale marketable securities is recoverable and it had no other-than-temporary impairments on these securities as of December 31, 2017. The Company does not intend to sell these debt securities and the Company believes it is not more-likely-than-not that it will be required to sell these securities before the recovery of their amortized cost basis, which may be at maturity.

Net Loss Per Share

12 Months Ended

Dec. 31, 2017

Earnings Per Share [Abstract]

Net Loss Per Share

The Company has reported a net loss attributable to common stockholders for the years ended December 31, 2017, 2016 and 2015, and for this reason basic and diluted net loss per share attributable to common stockholders are the same for all periods presented. The following table sets forth the computation of basic and diluted net loss per share (in thousands, except share and per‑share data):



	Year Ended December 31,
	2017			2016			2015
Numerator:
Net loss	$	(65,321	)	$	(36,210	)	$	(25,174	)
Less: accretion on preferred stock	—			(4,566		)	(7,335		)
Net loss attributable to common stockholders	$	(65,321	)	$	(40,776	)	$	(32,509	)
Denominator:
Weighted‑average common shares outstanding—basic and diluted	20,425,050			10,493,939			2,150,422
Net loss per share attributable to common stockholders—basic and diluted	$	(3.20	)	$	(3.89	)	$	(15.13	)

All potential dilutive common shares have been excluded from the computation of the diluted net loss per share for all periods presented, as the effect would have been anti-dilutive. Potential dilutive common share equivalents consist of the following:



	Year Ended December 31,
	2017	2016	2015
Stock options to purchase common stock	2,657,187	2,128,346	1,569,379
Stock warrants to purchase common stock	176,432	97,302	650,618
Redeemable convertible preferred stock	—	—	9,890,580
Redeemable convertible preferred stock warrants	—	—	26,832
Total	2,833,619	2,225,648	12,137,409

Fair Value Measurements

12 Months Ended

Dec. 31, 2017

Fair Value Disclosures [Abstract]

Fair Value Measurements

The tables below present information about the Company’s financial assets and liabilities that are measured and carried at fair value as of December 31, 2017 and December 31, 2016, and indicate the level within the fair value hierarchy where each measurement is classified. Below is a summary of assets measured at fair value on a recurring basis (in thousands):



	December 31, 2017
	(Level 1)		(Level 2)		(Level 3)		Total
Cash equivalents:
Money market funds	$	39,478	$	—	$	—	$	39,478
U.S. government agency bonds	—		1,000		—		1,000
Total cash equivalents	$	39,478	$	1,000	$	—	$	40,478
Short-term investments:
Corporate bonds	$	13,152	$	—	$	—	$	13,152
U.S. government agency bonds	—		12,788		—		12,788
Total short-term investments	$	13,152	$	12,788	$	—	$	25,940



	December 31, 2016
	(Level 1)		(Level 2)		(Level 3)		Total
Cash equivalents:
Money market funds	$	183	$	—	$	—	$	183
Tri-party repurchase agreements	—		27,000		—		27,000
Total cash equivalents	$	183	$	27,000	$	—	$	27,183
Short-term investments:
Corporate bonds	$	24,785	$	—	$	—	$	24,785
Total short-term investments	$	24,785	$	—	$	—	$	24,785

At December 31, 2017, cash and cash equivalent investments were held in money market funds and U.S. government agency bonds maturing within 90 days from the date of purchase. The tri-party repurchase agreements held at December 31, 2016 were collateralized by government securities for an amount not less than 102% of their value. All tri-party repurchase agreements had maturities of three months or less at the time of investment.

The average maturity date for Corporate Bonds, included in investments at December 31, 2017 and December 31, 2016 was 179 days and 297 days, respectively. Fair value of Corporate Bonds approximates amortized value.

The average maturity date for U.S. Government Agency Bonds, included in investments at December 31, 2017 was 168 days. Fair value of U.S. Government Agency Bonds approximates amortized value.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Property and Equipment

12 Months Ended

Dec. 31, 2017

Property, Plant and Equipment [Abstract]

Property and Equipment

Property and equipment consists of the following (in thousands):



	December 31,			December 31,
	2017			2016
Laboratory equipment	$	5,138		$	4,713
Computer equipment and software	480			562
Leasehold improvements	278			222
Furniture and fixtures	235			226
Office equipment	118			64
Total property and equipment	6,249			5,787
Less accumulated depreciation	(4,158		)	(3,740		)
Property and equipment, net	$	2,091		$	2,047

Depreciation expense was $0.8 million, $0.8 million and $1.0 million for the years ended December 31, 2017, 2016 and 2015, respectively.

Accrued Expenses

12 Months Ended

Dec. 31, 2017

Payables and Accruals [Abstract]

Accrued Expenses

Accrued expenses consist of the following (in thousands):



	December 31,		December 31,
	2017		2016
Payroll and employee related expenses	$	1,902	$	1,551
Current portion of deferred rent and lease incentive	72		29
Collaboration and licensing	1,020		—
Accrued patent fees	268		415
Accrued external research and development costs	3,578		794
Accrued professional and consulting services	859		459
Accrued grant refund	175		152
Accrued interest	89		81
Other	617		440
Accrued expenses	$	8,580	$	3,921

Commitments and Contingencies

12 Months Ended

Dec. 31, 2017

Commitments and Contingencies Disclosure [Abstract]

Commitments and Contingencies

Operating Leases

The Company has a non‑cancellable operating lease for its laboratory and office space located at 480 Arsenal Way, Watertown, Massachusetts ("Headquarters Lease"). As part of the Headquarters Lease agreement, the landlord provided the Company a tenant improvement allowance of up to $0.7 million, which the Company fully utilized during 2012. The tenant improvement allowance is accounted for as a lease incentive obligation and is being amortized as a reduction to rent expense over the lease term. The leasehold improvements are capitalized as a component of property and equipment.

In connection with the Headquarters Lease, the Company secured a letter of credit for $0.3 million which renews automatically each year and is classified in restricted cash and other deposits in the accompanying consolidated balance sheets.

In August 2016, the Company signed an amendment to the Headquarters Lease, which extends the term through March 31, 2020. The Headquarters Lease includes a rent escalation clause, and accordingly, rent expense is being recognized on a straight-line basis over the lease term.

In October 2017, the Company entered into a lease for approximately 5,100 square feet of additional office space located in Watertown, Massachusetts. The lease expires on March 31, 2020. The lease includes a rent escalation clause, and accordingly, rent expense is being recognized on a straight-line basis over the lease term.

Deferred rent and lease incentive liability totaled $0.2 million and $0.3 million as of December 31, 2017 and December 31, 2016, respectively. Included in that amount, the current portion of deferred rent and lease incentive liability is classified as accrued expenses and was less than $0.1 million at December 31, 2017 and December 31, 2016, respectively.

The Company has a month‑to‑month facility agreement for its Moscow, Russia facility. Rent expense is recognized as incurred.

Rent expense for the years ended December 31, 2017, 2016 and 2015 was $1.9 million, $1.6 million and $1.1 million, respectively.

As of December 31, 2017, the aggregate future minimum lease payments related to these leases are as follows (in thousands):



Year ended December 31,
2018	$	1,437
2019	1,482
2020	375
Total minimum lease payments	$	3,294

Other

As permitted under Delaware law, the Company indemnifies its directors for certain events or occurrences while the director is, or was, serving at the Company’s request in such capacity. The term of the indemnification is for the director’s lifetime. The maximum potential amount of future payments the Company could be required to make is unlimited; however, the Company has directors’ insurance coverage that limits its exposure and enables it to recover a portion of any future amounts paid. The Company also has indemnification arrangements under certain of its facility leases that require it to indemnify the landlord against certain costs, expenses, fines, suits, claims, demands, liabilities, and actions directly resulting from certain breaches, violations, or non‑performance of any covenant or condition of the Company’s lease. The term of the indemnification is for the term of the related lease agreement. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is unlimited. To date, the Company had not experienced any material losses related to any of its indemnification obligations, and no material claims with respect thereto were outstanding.

The Company is a party in various other contractual disputes and potential claims arising in the ordinary course of business. The Company does not believe that the resolution of these matters will have a material adverse effect the Company's business, financial position, results of operations or cash flows.

Debt

12 Months Ended

Dec. 31, 2017

Debt Disclosure [Abstract]

Debt

2015 Term Loan

On August 9, 2013, the Company entered into a loan and security agreement with Oxford Finance LLC (“Oxford”) and Square 1 Bank (“Square 1”) to borrow up to $7.5 million. The Company initially borrowed $3.0 million in August 2013 and subsequently borrowed an additional $4.5 million in July 2014. In December 2015, the Company refinanced this debt facility to increase the amount of the borrowing to $12.0 million and to extend the repayment term. The amounts borrowed are collectively referred to as the “2015 Term Loan”. The lenders for the refinanced debt facility are Oxford and Pacific Western Bank (“Pacific Western”). Pacific Western had acquired Square 1 since the time of the original loan. Such a change in lender does not constitute third party financing on its own, and does not require extinguishment accounting. As a result of the refinancing, the stated interest rate was also adjusted to reflect the current market borrowing rate.

Prior to the extinguishment of the 2015 Term Loan debt in September 2017 that is discussed in greater detail below, the 2015 Term Loan was collateralized by substantially all of the assets of the Company and bore interest at 8.1% per annum. The monthly payments for the 2015 Term Loan were initially interest only through January 2017. Principal repayments for the 2015 Term Loan were due over 30 monthly installments beginning on February 1, 2017.

The 2015 Term Loan could be prepaid at the Company’s option at any time prior to maturity subject to a prepayment fee of 2% if prepaid after the first anniversary but before the second anniversaries, and 1% if prepaid after the second anniversary. The 2015 Term Loan did not include any financial covenants. The 2015 Term Loan required a final payment fee of 6.0% on the aggregate principal amounts borrowed upon repayment at maturity, on a prepayment date, or upon default. The final payment fee totaling $0.7 million was recorded as a loan discount.

In connection with the 2015 Term Loan, the Company granted the lenders warrants in August 2013 to purchase up to 26,668 shares of the Company’s Series D Preferred and additional warrants in July 2014 to purchase up to 40,000 shares of the Company’s Series D Preferred. As of the IPO, the warrants to purchase up to 66,668 shares of the Company’s Series D Preferred were converted to warrants to purchase up to 17,888 shares of the Company’s common stock at an exercise price of $16.77 per share. These warrants are classified as permanent equity in the accompanying consolidated balance sheets and will expire ten years from the date of issuance.

Additionally, with the refinancing of the 2015 Term Loan at December 31, 2015, the Company granted the lenders 37,978 shares of the Company’s Series E Preferred which also was converted at the IPO to warrants to purchase up to 15,094 shares of Company’s common stock at an exercise price of $11.32 per share. These warrants are classified as permanent equity in the accompanying consolidated balance sheets and will expire ten years from the date of issuance.

The initial grant date fair value of the Series D Preferred and Series E Preferred warrants was $0.1 million and $0.1 million for each issuance, respectively and was recorded as a loan discount.

In December 2016, a total of 16,493 warrants to purchase common stock were exercised under a cashless exercise, resulting in a net issuance of 4,697 shares of common stock. The warrant exercise price had been established at the time the warrants were converted.

2017 Term Loan

On September 12, 2017, the Company entered into a term loan facility of up to $21.0 million (the “2017 Term Loan”) with Silicon Valley Bank, a California corporation (“SVB”), the proceeds of which were used to repay the 2015 Term Loan and for general corporate and working capital purposes. The Company refers to the 2015 Term Loan and the 2017 Term Loan, collectively, as the “Term Loans.” The 2017 Term Loan is governed by a loan and security agreement, dated September 12, 2017, between the Company and SVB (the “Loan Agreement”). The 2017 Term Loan was funded in full on September 13, 2017 (the “Funding Date”).

On the Funding Date, the Company entered into a payoff letter with SVB, pursuant to which SVB utilized $10.0 million of the 2017 Term Loan to pay off all outstanding obligations under the 2015 Term Loan. The Company recognized a loss on extinguishment of debt in the amount of $0.7 million during the year ended December 31, 2017, which is reflected in the consolidated statements of operations and comprehensive loss under the header “Loss on extinguishment of debt”.

The Company incurred less than $0.1 million in debt issuance costs in connection with the closing of the 2017 Term Loan. Debt issuance costs are presented in the consolidated balance sheet as a direct deduction from the associated liability and amortized to interest expense over the term of the related debt.

The 2017 Term Loan will mature on February 1, 2022. Each advance under the 2017 Term Loan accrues interest at a floating per annum rate equal to one-half of one percent above the prime rate (as published in the money rates section of The Wall Street Journal). The 2017 Term Loan provides for interest-only payments monthly until August 31, 2019. Thereafter, amortization payments will be payable monthly in equal installments of principal and variable interest to fully amortize the outstanding principal over the remaining term of the loan. The monthly interest is subject to recalculation upon a change in the prime rate. The Company may prepay the 2017 Term Loan in full but not in part provided that the Company (i) provides five business days’ prior written notice to SVB, (ii) pays on the date of such prepayment (A) all outstanding principal plus accrued and unpaid interest, and (B) a prepayment premium of 3% if prepaid before the first anniversary, 2% if prepaid after the first anniversary but before the second anniversaries, and 1% if prepaid after the second anniversary.

Amounts outstanding during an event of default are payable upon SVB’s demand and shall accrue interest at an additional rate of 4.0% per annum of the past due amount outstanding. The events of default under the Loan Agreement include, but are not limited to, the Company’s failure to make any payments of principal or interest under the Loan Agreement or other transaction documents, the Company’s breach or default in the performance of any covenant under the Loan Agreement or other transaction documents, the occurrence of a material adverse effect, the Company making a false or misleading representation or warranty in any material respect under the Loan Agreement, the Company’s insolvency or bankruptcy, any attachment or judgment on the Company’s assets in excess of approximately $0.3 million, or the occurrence of any default under any agreement or obligation of the Company involving indebtedness in excess of approximately $0.3 million. If an event of default occurs, SVB is entitled to take enforcement action, including acceleration of amounts due under the Loan Agreement.

The 2017 Term Loan is secured by a lien on substantially all of the assets of the Company, other than intellectual property, provided that such lien on substantially all assets includes any rights to payments and proceeds from the sale, licensing or disposition of intellectual property. The Company has also granted SVB a negative pledge with respect to its intellectual property.

The 2017 Term Loan does not include any financial covenants. The 2017 Term Loan requires a final payment fee of 5% on the aggregate principal amounts borrowed upon repayment at maturity, on a prepayment date, or upon default. The final payment fee totaling $1.1 million is recorded as a loan discount. Under the 2017 Term Loan, the Company is not required to maintain a minimum cash balance. In addition, the 2017 Term Loan contains a subjective acceleration clause whereby in an event of default, an immediate acceleration of repayment occurs if there is a material impairment of the lenders’ lien or the value of the collateral, a material adverse change in the business condition or operations, or a material uncertainty exists that any portion of the loan may not be repaid. To date, there have been no such events and the lender has not exercised its right under this clause.

The Company assessed all terms and features of the 2017 Term Loan in order to identify any potential embedded features that would require bifurcation. As part of this analysis, the Company assessed the economic characteristics and risks of the 2017 Term Loan, including any put and call features. The Company determined that all features of the 2017 Term Loan were clearly and closely associated with the debt host and did not require bifurcation as a derivative liability, or the fair value of the embedded feature was immaterial to the Company's consolidated financial statements. The Company reassesses the identified features on a quarterly basis to determine if they require bifurcation.

As of December 31, 2017 and December 31, 2016, the outstanding principal balance under the Term Loans was $21.0 million and $12.0 million, respectively.

Future minimum payments on the 2017 Term Loan as of December 31, 2017 are as follows (in thousands):



2018	$	1,065
2019	4,535
2020	9,093
2021	8,665
2022	1,753
Total minimum debt payments	$	25,111
Less: Amount representing interest	(3,061		)
Less: Debt discount and deferred charges	(1,008		)
Less: Current portion of loans payable	—
Loans payable, net of current portion	$	21,042

During the years ended December 31, 2017, 2016 and 2015, the Company recognized $1.2 million, $1.3 million and $0.7 million, respectively, of interest expense related to the Term Loans.

Convertible Notes

In April 2015, the Company issued convertible notes as a bridge loan to be automatically converted into the Company’s capital stock upon the consummation of a private placement of the Company’s Preferred Stock. The convertible notes bore interest at 8% per annum, compounding monthly. In the event the Company was unable to consummate the private placement by July 15, 2015, the Company would be required to issue warrants to purchase shares of the Company’s common stock equal to 20% of the convertible note principal divided by $17.55. On July 24, 2015, the Company issued warrants to the convertible note holders to purchase up to 80,813 shares of the Company’s common stock at an exercise price of $17.55 per share for a term of three years. The carrying value and accrued interest of the outstanding convertible notes were automatically converted into 1,619,550 shares of Series E Preferred. As part of the Series E Preferred issuance, the convertible note holders also received warrants to purchase up to 103,817 shares of the Company’s common stock (Note 10). The difference between the carrying value and accrued interest of the convertible notes that were converted and the combined fair value of the Series E Preferred shares and common stock warrants issued were negligible. There was no interest expense related to the convertible notes for the years ended December 31, 2017 and December 31, 2016, and $0.3 million for the year ended December 31, 2015.

Preferred Stock	12 Months Ended
Preferred Stock	Dec. 31, 2017
Temporary Equity Disclosure [Abstract]
Preferred Stock	Preferred Stock Prior to the completion of its IPO, the Company had 37,835,623 authorized shares of preferred stock, par value $0.0001 per share (the "Preferred Stock") as of December 31, 2015. The Company issued Preferred Stock to investors for cash or as settlement for outstanding debt under convertible notes. As of December 31, 2015, the Company had issued and outstanding Preferred Stock of (i) 2,589,868 shares of Series A redeemable convertible preferred stock (“Series A Preferred”), (ii) 7,437,325 shares of Series B redeemable convertible preferred stock (“Series B Preferred”), (iii) 5,000,002 shares of Series C redeemable convertible preferred stock (“Series C Preferred”), (iv) 8,099,994 shares of Series D Preferred, (v) 2,111,109 shares of Series SRN Redeemable Convertible Preferred Stock (“Series SRN Preferred”) and (vi) 8,888,888 shares of Series E Preferred. All outstanding shares of the Company’s convertible preferred stock automatically converted into 10,126,118 shares of the Company’s common stock upon the closing of the IPO on June 27, 2016.

Common Stock

12 Months Ended

Dec. 31, 2017

Equity [Abstract]

Common Stock

2017 Shelf Registration Statement

On August 10, 2017, the Company filed a universal shelf registration statement on Form S-3 with the SEC to sell up to $200 million of equity and/or debt securities and entered into a sales agreement with Jefferies LLC, as sales agent, pursuant to which the Company may, from time to time, issue and sell common stock with an aggregate value of up to $50 million in an at-the-market, or ATM, offering. The shelf registration statement was declared effective by the SEC on August 28, 2017. As of December 31, 2017, no securities have been issued from this shelf registration statement.

PIPE Financing

On June 26, 2017, the Company entered into a securities purchase agreement (the "Institutional Purchase Agreement") with a select group of institutional investors (the “Institutional Investors”) and a securities purchase agreement with Timothy Springer, Ph.D., a member of the board of directors (the "Springer Purchase Agreement") for a private placement of the Company's securities (the "2017 PIPE"). The closing of the 2017 PIPE occurred on June 27, 2017.

Pursuant to the Institutional Purchase Agreement, the Company agreed to sell an aggregate of 2,750,000 shares of its common stock, par value $0.0001 per share, at a purchase price equal to $16.00 per share. Pursuant to the Springer Purchase Agreement, the Company agreed to sell to Dr. Springer an aggregate of 338,791 shares of common stock at a purchase price equal to $17.71 per share, which was equal to the most recent consolidated closing bid price on the Nasdaq Global Market on June 23, 2017, and warrants to purchase up to 79,130 shares of common stock (“Warrant Shares”), exercisable at $17.71 per Warrant Share, and with a term of five years. The purchase price for each warrant was equal to $0.125 for each Warrant Share, consistent with Nasdaq Global Market requirements for an “at the market” offering. Under the terms of the Common Stock Purchase Warrant, the warrants can be settled in unregistered shares. The Warrant Shares qualify for equity classification. The fair value of the allocated proceeds was determined on the relative fair value basis.

After deducting for placement agent fees and offering expenses, the aggregate net proceeds from the 2017 PIPE was approximately $47.1 million. The Company expects to use the proceeds from the 2017 PIPE towards working capital requirements and general corporate purposes.

On June 27, 2017, in connection with the 2017 PIPE, the Company entered into a Registration Rights Agreement (the “Registration Rights Agreement”) with the Institutional Investors and Dr. Springer (together, the “Investors”). Pursuant to the Registration Rights Agreement, the Company agreed to prepare and file a registration statement with the Securities and Exchange Commission (the “SEC”) within 20 days after the closing of the 2017 PIPE for purposes of registering the resale of the shares of common stock issued and sold in the 2017 PIPE (the “Shares”), the Warrant Shares, and any shares of common stock issued as a dividend or other distribution with respect to the Shares or Warrant Shares. The registration statement was declared effective by the SEC on July 21, 2017.

The Company agreed to indemnify the Investors, their officers, directors, members, employees and agents, successors and assigns under the registration statement from certain liabilities and to pay all fees and expenses (excluding any legal fees of the selling holder(s), and any underwriting discounts and selling commissions) incident to the Company’s obligations under the Registration Rights Agreement.

Initial Public Offering

On June 21, 2016, the Company completed its IPO and issued and sold 5,000,000 shares of common stock at a price to the public of $14.00 per share for net proceeds of $60.8 million after deducting underwriting discounts and commissions and offering expenses. On July 25, 2016, 289,633 additional shares of the Company’s common stock were sold to the underwriters pursuant to the exercise of their option to purchase additional shares of common stock at a price to the public of $14.00 per share resulting in additional net proceeds of approximately $3.7 million after deducting underwriting discounts, commissions and offering expenses, bringing the total IPO net proceeds to $64.5 million. Upon the closing of the IPO on June 27, 2016, all outstanding shares of the Company’s convertible preferred stock automatically converted into 10,126,118 shares of the Company’s common stock. In addition, at this time, the warrants to purchase shares of the Company’s Series D and Series E convertible preferred stock were converted into warrants to purchase shares of the Company’s common stock.

As of December 31, 2017, the Company has 200,000,000 shares of common stock authorized for issuance, $0.0001 par value per share, with 22,343,254 shares issued and outstanding. The voting, dividend and liquidation rights of the common stockholders are subject to and qualified by the rights, powers and preferences of the preferred stock. The common stock has the following characteristics:

Voting

The common stockholders are entitled to one vote for each share of common stock held with respect to all matters voted on by the stockholders of the Company.

Dividends

The common stockholders are entitled to receive dividends, if and when declared by the Board of Directors. Through December 31, 2017, no dividends have been declared or paid on common stock.

Liquidation

Upon liquidation of the Company, the common stockholders are entitled to receive all assets of the Company available for distribution to such stockholders.

Reserved Shares

The Company has authorized shares of common stock for future issuance as follows:



	Period ending
	December 31, 2017	December 31, 2016
Exercise of common warrants	176,432	97,302
Shares available for future stock incentive awards	923,440	939,317
Outstanding common stock options	2,657,187	2,128,346
Total	3,757,059	3,164,965

Stock Incentive Plans

12 Months Ended

Dec. 31, 2017

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Stock Incentive Plans

Stock Options

The Company maintains the 2008 Stock Incentive Plan (the “2008 Plan”) for employees, consultants, advisors, and directors. The 2008 Plan provided for the granting of incentive and non‑qualified stock option and restricted stock awards as determined by the Board. At inception of the 2008 Plan, a total of 2,213,412 shares of common stock were authorized for grants under the 2008 Plan. As of December 31, 2017, 18,947 shares remain available for future issuance under the 2008 Plan; however, the Company ceased granting awards under the 2008 Plan upon the effectiveness of the 2016 Plan (as defined below). Shares subject to awards that were granted under the 2008 Plan and that expire, lapse or terminate following the effectiveness of the 2016 Plan become available under the 2016 Plan as shares available for future grants. All unvested stock options granted under the 2008 Plan may be exercised into restricted stock subject to forfeiture upon termination prior to vesting.

The 2008 Plan provided that the exercise price of incentive stock options cannot be less than 100% of the fair market value of the Company's common stock on the grant date for participants who own 10% or less of the total combined voting power of the Company, and not less than 110% for participants who own more than 10% of the Company’s voting power. Options and restricted stock awards granted under the 2008 Plan vest over periods as determined by the Board, which are generally four years and, for options, with terms that generally expire ten years from the grant date.

As of the effective date of the Company's Registration Statement on Form S-1 relating to the initial public offering of its common stock on June 21, 2016, the Company ceased granting awards under the 2008 Plan; however, awards issued under the 2008 Plan remain subject to the terms of the 2008 Plan and the applicable 2008 Plan agreement.

On June 7, 2016, the Company’s stockholders approved the 2016 Incentive Award Plan (the “2016 Plan”), which became effective June 20, 2016. The 2016 Plan provides for the granting of incentive and non‑qualified stock option, restricted stock and other stock and cash based awards as determined by the Board. At inception of the 2016 Plan, a total of 1,947,779 shares of common stock were authorized for future grant under the 2016 Plan. The number of shares of common stock that may be issued under the 2016 Plan automatically increases on the first day of each calendar year, beginning in 2017 and ending in and including 2026, by an amount equal to the lesser of: (i) 4% of the number of shares of the Company’s common stock outstanding on the last day of the applicable preceding calendar year and (ii) such smaller number of shares as is determined by the Board. As of December 31, 2017, 563,300 shares remain available for future issuance under the 2016 Plan.

The 2016 Plan provides that the exercise price of incentive stock options cannot be less than 100% of the fair market value of the Company's common stock on the grant date for participants who own 10% or less of the total combined voting power of the Company, and not less than 110% for participants who own more than 10% of the Company’s voting power. Options and restricted stock awards granted under the 2016 Plan vest over periods as determined by the Board, which are generally four years and, for options, with terms that generally expire ten years from the grant date.

The fair value of each option award was estimated on the grant date using Black‑Scholes. Expected volatilities were based on historical volatilities from guideline companies, since there was no active market for the Company’s common stock. The Company used the “simplified” method to estimate the expected life of options granted and are expected to be outstanding. The risk‑free interest rate used is the rate for a U.S. Treasury zero coupon issue with a remaining life consistent with the options expected life on the grant date. The Company has not paid, and does not expect to pay in the foreseeable future, any cash dividends. Forfeitures were estimated at the time of grant and were adjusted, if necessary, in subsequent periods if actual forfeitures differed from those estimates. The Company had estimated a forfeiture rate of 10% based on historical attrition trends. The Company records stock‑based compensation expense only on awards that are expected to vest.

The estimated grant date fair values of employee stock option awards granted under the 2008 Plan and 2016 Plan were calculated using the Black-Scholes option pricing model, based on the following weighted-average assumptions:



	Year Ended December 31,
	2017			2016			2015
Risk-free interest rate	2.03		%	1.42		%	1.79		%
Dividend yield	—			—			—
Expected term	5.90			6.04			6.02
Expected volatility	84.52		%	94.97		%	79.80		%
Weighted-average fair value of common stock	$	15.32		$	13.25		$	7.35

The resulting weighted average grant date fair value of stock options granted to employees during the years ended December 31, 2017, 2016 and 2015 was $10.97, $10.23 and $5.03, respectively. The aggregate intrinsic value of stock options exercised during the years ended December 31, 2017, 2016 and 2015 was $2.6 million, $0.8 million and $0.3 million, respectively.

As of December 31, 2017 and 2016, total unrecognized compensation expense related to unvested employee stock options was $10.2 million and $5.6 million, respectively, which is expected to be recognized over a weighted average period of 2.9 years and 3.0 years, respectively.

During the year ended December 31, 2017, the status of an individual changed from an employee to a consultant (non-employee). Stock-based compensation expense relating to the individual's stock option awards was recognized through the employee's termination date. In accordance with ASC 505-50, the consultant's 55,073 unvested stock option awards were subsequently remeasured using the then-current fair value of the Company's common stock and updated assumption inputs in the Black-Scholes option pricing model. The Company will remeasure the consultant's stock option awards quarterly based on the fair value of the award on the date at which the related service is complete. Stock-based compensation expense will be recognized over the consulting period until services are completed. The 55,073 unvested stock option awards are included in the employee awards stock option rollforward above.

During the year ended December 31, 2017, there were no stock option awards granted to non-employees. The unvested options held by non‑employees are revalued using the Company’s estimate of fair value on each vesting and reporting date through the remaining vesting period. The weighted average assumptions used in the Black-Scholes option pricing model for unvested non‑employee stock option awards were as follows:



	Year Ended December 31,
	2017	2016		2015
Risk‑free interest rate	—	2.06	%	1.57	%
Dividend yield	—	—		—
Expected term	0.0	8.80		6.46
Expected volatility	—	87.33	%	98.18	%

As of December 31, 2017 and 2016, total unrecognized compensation expense related to unvested non‑employee stock options was $0.6 million and $1.1 million, respectively.

The following table summarizes the activity under the 2008 Plan and the 2016 Plan:



					Weighted‑average
					remaining	Aggregate
	Number of		Weighted-average		contractual term	intrinsic value
	options		exercise price ($)		(in years)	(in thousands)
Employee Awards
Outstanding at December 31, 2016	1,809,945		$	7.57	7.26	$	17,459
Granted	882,900		$	15.32
Exercised	(203,378	)	$	2.54
Forfeited	(78,230	)	$	14.72
Outstanding at December 31, 2017	2,411,237		$	10.58	7.50	$	4,729
Vested at December 31, 2017	1,094,117		$	6.80	5.76	$	4,215
Vested and expected to vest at December 31, 2017	2,232,527		$	10.31	7.37	$	4,729

Non‑Employee Awards
Outstanding at December 31, 2016	318,342		$	4.55	5.96	$	4,044
Granted	—		$	—
Exercised	(67,392	)	$	3.97
Forfeited	(5,000	)	$	23.95
Outstanding at December 31, 2017	245,950		$	4.32	5.10	$	1,351
Vested at December 31, 2017	207,502		$	3.82	4.53	$	1,243
Vested and expected to vest at December 31, 2017	245,950		$	4.32	5.10	$	1,351

Employee Stock Purchase Plan

On June 7, 2016, the Company’s stockholders approved the 2016 Employee Stock Purchase Plan (the “ESPP”), which became effective June 20, 2016. The ESPP is intended to qualify as an "employee stock purchase plan" under Section 423 of the Internal Revenue Code of 1986 with the purpose of providing employees with an opportunity to purchase the Company's common stock through accumulated payroll deductions.

Under the ESPP, the Company has set two six-month offering periods during each calendar year, one beginning March 1^st and the other beginning September 1^st of each calendar year, during which employees may elect to have up to 25% of their eligible compensation deducted on each payday on an after-tax basis for use in purchasing the Company's common stock on the last trading day of each offering period, subject to limits imposed by the Internal Revenue Code. The purchase price of the shares may not be less than 85% of the fair market value on the first or last trading day of the offering period, whichever is lower.

At inception of the ESPP, a total of 357,456 shares of common stock were authorized and reserved for future issuance under the ESPP. The number of shares of common stock that may be issued under the ESPP will automatically increase on the first day of each calendar year, beginning in 2017 and ending in and including 2026, by an amount equal to the lesser of: (i) 1% of the number of shares of the Company’s common stock outstanding on the last day of the applicable preceding calendar year and (ii) such smaller number of shares as is determined by the Company’s Board of Directors. During the year ended December 31, 2017, the Company issued 16,263 shares of common stock under the ESPP. As of December 31, 2017, 341,193 shares remain available for future issuance under the ESPP.

The fair value of the purchase rights granted under the ESPP for each of the six month offering periods beginning March 1, 2017 and September 1, 2017 were estimated by applying the Black-Scholes option-pricing model using the following range of assumptions:



Fair value of common stock	$13.08	-	$18.02
Risk-free interest rate	0.79%	-	1.19%
Expected volatility	46.73%	-	75.78%
Expected term	0.1	-	0.5
Dividend yield	—	-	—

For the year ended December 31, 2017, the Company recognized $0.1 million of stock-based compensation expense under the ESPP. There was no stock-based compensation expense related to the ESPP for the years ended December 31, 2016 and 2015.

Restricted Stock

During the year ended December 31, 2013, the Company issued 30,317 shares of restricted common stock to employees upon the early exercise of stock options. During the year ended December 31, 2014, the Company issued 2,564 shares of restricted common stock to employees. Under the terms of each agreement, the Company had the right to repurchase any unvested shares when/if the shareholders terminate their employment relationship with the Company, at a price equal to the original exercise price. Accordingly, the Company recorded the cumulative payments received of $0.1 million for the purchase of the restricted shares as a liability, which was amortized over the vesting period. The liability became fully amortized as of December 31, 2016. As of December 31, 2016, there was no unrecognized compensation cost related to unvested restricted stock.

The Company recorded stock-based compensation expense related to stock option awards, the ESPP and restricted stock in the following expense categories of its consolidated statements of operations and comprehensive loss (in thousands):



	Year Ended December 31,
	2017		2016		2015
Research and development	$	1,779	$	1,183	$	495
General and administrative	2,302		847		630
Total	$	4,081	$	2,030	$	1,125

Revenue Arrangements

12 Months Ended

Dec. 31, 2017

Revenue Recognition [Abstract]

Revenue Arrangements

Spark License Agreement

In December 2016, the Company entered into a License and Option Agreement (“Spark License Agreement”) with Spark Therapeutics, Inc. (“Spark”) pursuant to which the Company and Spark agreed to collaborate on the development of gene therapies for certain targets utilizing the SVP™ technology. The Spark License Agreement provides Spark with certain exclusive, worldwide, royalty bearing licenses to the Company’s intellectual property, allowing Spark to develop and commercialize gene therapies for an initial identified target.

In addition to an upfront cash payment of $10.0 million under the Spark License Agreement, additional payments of an aggregate of $5.0 million in two payments of $2.5 million each were paid within twelve months of December 2, 2016 (“Contract Date”). The first of the two additional payments was scheduled to be made on or before May 31, 2017 (the “May 2017 License Payment”) (see “Spark Letter Agreement” below) and the second was made on October 31, 2017. Spark may also exercise options to research, develop and commercialize gene therapies utilizing the SVP technology for up to four additional targets. The Company is eligible to receive a variable fee up to $2.0 million for each additional target option elected, dependent on the incidence of the applicable indication. As per the agreement, the election period in which Spark can exercise additional targets is a term of three years from the Contract Date, or December 1, 2019.

Assuming successful development and commercialization, the Company could receive up to an additional $65.0 million in development and regulatory milestone payments and $365.0 million in commercialization milestone payments for each indication. If commercialized, the Company would be eligible to receive tiered royalties on global net sales at percentages ranging from mid-single to low-double digits, all of which apply on a target-by-target basis. Under the terms of the agreement, the Company will be eligible to receive these royalties commencing on the first commercial sale of the licensed product and terminating upon the later of (i) ten years after the first commercial sale, (ii) expiration of the last to expire valid claim on patents covering the jointly invented field specific improvements, or (iii) the expiration of regulatory exclusivity in the applicable country for the licensed product.

The License Agreement may be terminated by Spark for convenience upon ninety days’ notice. Either party may terminate the License Agreement on a target-by-target basis for material breach with respect to such target.

In December 2016, the Company also entered into a Share Purchase Agreement (the “Spark Purchase Agreement”) with Spark. Pursuant to the Spark Purchase Agreement, the Company sold 197,238 shares of the Company’s common stock to Spark for gross proceeds of $5.0 million, or $25.35 per share of common stock, at an initial closing (the “Initial Closing”). The purchase price per share represents an amount equal to 115% of the average daily volume weighted average price (“VWAP”) of the common stock during the thirty consecutive calendar days leading up to and ending on the day prior to the Contract Date. Under the Spark Purchase Agreement, Spark has agreed not to dispose of any of the Initial Closing Shares or any Acquisition Right Shares that it may acquire until January 1, 2018 and, thereafter, transfers will be contractually subject to volume limitations applicable to an “affiliate” under Rule 144 of the Securities Act. Closings under the Spark Purchase Agreement are subject to customary conditions.

Beyond the Initial Closing, the Spark Purchase Agreement contemplated potential future sales of shares by the Company to Spark as follows:


•	First Acquisition Right. During the period beginning on May 1, 2017 and ending on June 1, 2017, Spark had the right (the “First Acquisition Right”) to purchase a number of shares of common stock equal to an aggregate price of $5.0 million. See "Spark Letter Agreement" below.

•

Second Acquisition Right. During the period beginning on October 1, 2017 and ending on November 1, 2017, Spark had the right (the “Second Acquisition Right”) to purchase a number of shares of common stock equal to an aggregate price of $5.0 million. On October 31, 2017 Spark exercised this right and purchased 205,254 shares of common stock from the Company for $5.0 million, or $24.36 per share of common stock. The purchase price per share represents an amount equal to 115.0% of the average daily VWAP of the common stock during the thirty consecutive calendar days leading up to and ending on the day prior to the Second Acquisition Right notification date.

The First Acquisition Rights and Second Acquisition Rights are collectively referred to herein as the “Acquisition Rights”. The aggregate number of shares that the Company may issue pursuant to the Stock Purchase Agreement may not exceed the lesser of (i) 2,758,112 shares and (ii) such number of shares that would require the Company to obtain prior shareholder approval under the Nasdaq Marketplace Rules.

In connection with the Spark License Agreement and Spark Purchase Agreement, the Company has made contractual payments defined in the MIT license agreement (Note 15) totaling $2.2 million for the MIT sub-license provided to Spark, and $0.4 million relative to the calculated premium paid by Spark for the equity investments made under the Spark Purchase Agreement.

Accounting Analysis

The Spark License Agreement contains the following deliverables: (1) certain exclusive, worldwide, royalty bearing licenses to the Company’s intellectual property and a license to conduct certain research activities under the collaboration, or the License Deliverable, (2) options to research, develop and commercialize gene therapies utilizing the SVP technology for up to four additional target therapy options, or the Option Deliverable, and (3) manufactured supply of pre-clinical and clinical SVP, or the Supply Deliverable. The Company considered the provisions of the multiple-element arrangement guidance in determining how to recognize the revenue associated with the identified deliverables. The Company has accounted for access to certain of the Company's technology through the various licenses and rights to use under the license deliverables listed above and the delivery of the manufactured supply as a single unit of accounting. The deliverables are not considered to have standalone value from one another and amounts allocated to these obligations will be recognized throughout the estimated period of delivery of the supply.

The Option Deliverable is determined to be substantive at the outset of the agreement, and therefore will not be considered an element of the arrangement when allocating the consideration as there is considerable uncertainty that the options would be exercised. Although the options are not considered a deliverable at inception, it was determined that there is a significant and incremental discount that requires inclusion as an element of the initial allocation of contract consideration.

In addition, the Company evaluated the Spark Purchase Agreement and the Spark License agreement as one arrangement and determined that the initial purchase of common stock combined with the embedded future stock Acquisition Rights had a fair value of $2.7 million and this amount was recorded in equity.

The Company evaluated and determined that the Acquisition Rights under the Spark Purchase Agreement are not freestanding instruments as these rights are not legally detachable from the Company’s common stock. In addition, the Company has further assessed that the Acquisition Rights were clearly and closely related to the economic characteristic and risk of the common stock. The Company determined that the Acquisition Rights are embedded and inseparable from the initial stock purchase and no subsequent remeasurement is necessary.

Allocable arrangement consideration at inception consisted of the total non-contingent payments aggregating to $15 million. The Company allocated $2.7 million to equity (representing the fair value of the initial purchase of common stock combined with the embedded future stock Acquisition Rights), $7.1 million to the License and Supply Deliverable combined unit of accounting and $5.2 million to the discount on the Option Deliverable. The total arrangement consideration for the License and Supply deliverable and the Option deliverable was allocated using the relative best estimate of selling price method to each deliverable. The best estimate of selling price for the License and Supply deliverable was determined using a discounted cash flow model that includes Level 3 fair value measurements. The best estimate of selling price for the Option Deliverable was determined based on the fair value of the license minus the strike price of the option (the probability of exercise was included in the valuation). The consideration allocated to the combined unit of accounting for the License and Supply Deliverable, totaling $7.1 million, will be recognized based on the proportion of actual deliveries to the total expected deliveries over the initial term which is estimated to be approximately four years. The discount associated with the Option Deliverable, totaling $5.2 million, will be recognized, if the options are exercised, over the related expected deliveries of supply. If the options expire without exercise, the related deferred revenue will be recognized upon expiration (December 1, 2019).

The Company recognized less than $0.1 million in revenues related to the Spark License Agreement during the years ended December 31, 2017 and 2016. There were no revenues recognized during the year ended December 31, 2015.

The Company determined that each potential future clinical and regulatory milestone was non-substantive, therefore, any consideration received will be allocated to the License and Supply Deliverable and Option Deliverable using the relative selling price.

Sales-based milestones and royalty payments are expected to be recognized when earned.

As of December 31, 2017, there was $14.7 million of deferred revenue related to this agreement. A total of $0.8 million was recorded as a current liability and $13.9 million was classified as a long term liability in the accompanying consolidated balance sheet.

Spark Letter Agreement

On June 6, 2017, the Company and Spark entered into a letter agreement (the “Letter Agreement”), pursuant to which the parties agreed that Spark would make the May 2017 License Payment by June 6, 2017. The May 2017 License Payment was received, and recorded as a liability as of June 30, 2017, of which some or all may potentially constitute the reimbursement described below. The parties also agreed that Spark would be deemed to have delivered notice on May 31, 2017 exercising its right to purchase the shares pursuant to the First Acquisition Right. The Letter Agreement further outlines a cost reimbursement arrangement, pursuant to which the Company agreed to reimburse Spark for all costs and expenses, including the cost of materials provided by the Company, associated with the preclinical research and toxicology studies being performed by Spark for any licensed products for a specified amount of time (the “Reimbursement Period”), up to an agreed upon cap.

Consistent with the First Acquisition Right, Spark purchased 324,362 shares of common stock pursuant to the Spark Purchase Agreement, as amended by the Letter Agreement, for an aggregate purchase price of $5.0 million, or $15.41 per share of common stock. The purchase price per share represents an amount equal to 115.0% of the average daily volume weighted average price (“VWAP”) of the common stock during the thirty consecutive calendar days leading up to and ending on the day prior to the First Acquisition Right notification date. At the initial contract assessment, the Company allocated $2.7 million to equity (representing the fair value of the initial purchase of common stock combined with the embedded future stock Acquisition Rights). Upon exercise of the First Acquisition Right, the Company recorded the purchase amount to stockholders’ equity.

On October 31, 2017, Spark paid the Company a $2.5 million milestone payment pursuant to the license agreement with Spark. In addition, Spark exercised the Second Acquisition Right set forth in Section 2.4 of the Spark Purchase Agreement and purchased 205,254 shares of common stock from the Company for $5.0 million, or $24.36 per share of common stock. The purchase price per share represents an amount equal to 115.0% of the average daily VWAP of the common stock during the thirty consecutive calendar days leading up to and ending on the day prior to the Second Acquisition Right notification date.

Sanofi Collaboration Agreement

On November 27, 2012, the Company and Sanofi entered into a license and research collaboration agreement focused on the identification and development of vaccines against food allergies (the “Sanofi Agreement”). Under the arrangement, the Company agreed to perform research to identify an initial vaccine candidate for development and commercialization by Sanofi under an exclusive license.

Pursuant to the Sanofi Agreement, the Company received an upfront payment of $2.0 million for the initial indication in November 2012 and an additional payment of $3.0 million in August 2013. In November 2014, Sanofi exercised the option to include celiac disease as an additional indication, and in May 2015, the Sanofi Agreement was amended to add terms specific to the celiac disease indication and to terminate Sanofi's right to exercise its option for any additional indications. Sanofi paid the Company an additional $2.0 million upon the exercise of the option in May 2015 and an additional $1.0 million in July 2016 upon attaining the first milestone for the celiac disease indication. To date, Sanofi has paid the Company $8.0 million in the aggregate under the Sanofi Agreement.

Except as authorized by Sanofi or permitted under the Sanofi Agreement, during the term of the Sanofi Agreement, exclusivity obligations prevent the Company from researching, developing, or commercializing products in these indications or granting third party licenses under the intellectual property rights and technologies licensed to Sanofi for use in these indications.

As per the agreement, the research term expired for the first indication on the third anniversary (November 27, 2015) of the agreement. The Company completed its research obligations within the initial three year period and is not obligated to perform any further research on the specific indication under the agreement. A vaccine candidate for development and commercialization was not selected by Sanofi by the end of the research plan, and therefore no further milestone payments have been received.

The Company identified the deliverables under the arrangement as the license, the research necessary to identify the development candidate, and participation of the Joint Research Committee (“JRC”). The Company determined that the exclusive license granted to Sanofi did not have standalone value from the research to be performed to identify the vaccine development candidate. As a result, each upfront and milestone consideration was allocated to the combined unit of account comprising the license and research services, and is being recognized over the estimated development period using a proportional performance method. The consideration allocated to participation on the JRC was not material. The Company recognized no revenue for the year ended December 31, 2017, and $2.8 million and $2.1 million for the years ended December 31, 2016 and 2015, respectively.

Termination of the Sanofi Collaboration Agreement

On November 9, 2016, the Company received written notice from Sanofi that Sanofi had elected to terminate in its entirety the Sanofi Agreement. The termination of the Sanofi Agreement was effective on May 8, 2017, or the Termination Date, which was six months from the date of the notice.

As discussed above, Sanofi has paid the Company $8.0 million in the aggregate under the Sanofi Agreement to date. The Company would have been eligible to receive additional development-based, regulatory-based and sales-based milestone payments and tiered royalties on net sales of any approved product generated by the collaboration had the Sanofi Agreement not been terminated. As of December 31, 2016, the Company recognized the remaining $2.2 million in revenue associated with the Sanofi Agreement, as the Company had no further performance obligation under the contact.

All rights granted to Sanofi terminated and reverted to the Company effective on the Termination Date, and Sanofi is required to grant to the Company a royalty bearing, exclusive license, with the right to grant sublicenses, under certain Sanofi intellectual property solely to the extent necessary to research, develop, make, have made, use, offer for sale, import, export and otherwise commercialize the vaccine candidates developed under the Sanofi Agreement. The exclusivity obligations discussed above also expired on the Termination Date.

The Company has exercised its right to acquire the development programs under the Sanofi Agreement. The Company is solely responsible for performing and funding any development and clinical trial activities relating to further development of vaccine candidates that it chooses to undertake after the Termination Date.

Other Research and Collaboration Agreements

During 2017, the Company did not participate in other research and collaboration agreements, and thus no revenue was recognized during the year ended December 31, 2017. During the years ended December 31, 2016 and 2015, the Company participated in and recognized revenues from other research or collaboration agreements in the amount of $0.1 million and $0.3 million, respectively.

Grant Agreements

The Company receives funding in the form of grants from the National Institutes of Health (“NIH”), the Juvenile Diabetes Research Foundation (“JDRF”), the Bill and Melinda Gates Foundation, the Russian Ministry of Industry and Trade (“Minpromtorg”), and the Russia-based Development Fund of New Technologies Development and Commercialization Center (“Skolkovo”).

NIH

The Company has two grants through the NIH. The first grant, for an aggregate amount of $8.1 million, was awarded in May 2014 to support research in the development of a next generation vaccine for smoking cessation and relapse prevention. The Company recognized no revenue for the year ended December 31, 2017. For the years ended December 31, 2016 and 2015, the Company recognized revenue of $4.1 million and $2.4 million, respectively.

The second grant is for an aggregate amount of $0.2 million, which was awarded in September 2015 for the development of nanoparticles for immune tolerance to factor VIII. The Company recognized less than $0.1 million of revenue for the year ended December 31, 2017. For the years ended December 31, 2016 and 2015, the Company recognized less than $0.1 million and $0.1 million, respectively, related to this grant.

JDRF

The Company, together with Sanofi, entered into a joint grant with JDRF in September 2014 for $0.4 million to conduct Type 1 Diabetes research. The Company recognized less than $0.1 million of revenue for the year ended December 31, 2017. For the years ended December 31, 2016 and 2015 the Company recognized $0.1 million and $0.2 million related to this grant, respectively.

Bill and Melinda Gates Foundation

The Company received a grant in 2013 from the Bill and Melinda Gates Foundation for $1.2 million to fund the Company’s immunology research on malaria antigens. During 2014, the grant amount was increased to a total of $1.6 million and the term was extended to a three-year research term. Revenue is recognized on a proportional performance basis as it relates to employee time expended on the research, along with reimbursement for external costs directly related to, and approved, by the grant terms. Total revenue recognized for the years ended December 31, 2017, 2016 and 2015 was $0.1 million, $0.5 million and $0.6 million, respectively.

Minpromtorg

The Company had a contract awarded from Minpromtorg for approximately $4.6 million to fund the Company’s nicotine cessation vaccine clinical trial to be conducted in Russia. The grant covered a term from July 9, 2013 through December 31, 2015, and provided for reimbursement of expenses incurred by the Company from the clinical trial. Under the agreement term, the Company was subject to a penalty in the event that the clinical trial was delayed or terminated prior to completion. As a result of the penalty provision, the Company concluded the amounts received under the agreement were not fixed or determinable. In 2014, the Company terminated its plan to conduct the clinical trial in Russia subjecting the Company to the penalty obligation.

In February 2015, the Company received an executed final settlement agreement from Minpromtorg that included the repayment of funds previously received by the Company totaling $0.2 million, and a penalty fee that equaled to 10% of the contract value, or $0.2 million. The Company paid the settlement payment in March 2015 and all mutual claims under the contract were terminated. According to the terms of the agreement, Minpromtorg has the right to audit the expenditure incurred under the agreement for a period up to three years from each research milestone date. All grant funding received in excess of the penalty settlement will remain as a liability on the balance sheet until such time the audit period has expired and at which time, the amount will be recognized as revenue. Through December 31, 2017, the Company received payments totaling approximately $1.4 million. No additional payments are expected to be received under the terms of the contract.

The first audit period expired on December 31, 2015, and as a result $0.4 million of revenue was recognized for the year ended December 31, 2015. The second and third audit periods expired during the twelve months ended December 31, 2016, and as a result $0.5 million of revenue was recognized for the year ended December 31, 2016. The third audit period is the final audit to be conducted under the contract. As of December 31, 2016, all remaining deferred revenue associated with the Minpromtorg contract was fully recognized.

Skolkovo

On November 28, 2014, the Company executed a grant awarded by Skolkovo for the development of a therapeutic vaccine using nanoparticles to treat chronic infection caused by human papillomavirus (HPV) and diseases associated with this infection. The grant covers the period from August 1, 2014 through July 21, 2017. The grant provides for up to $2.7 million that covers 48.5% of the estimated total cost of the research plan with the remaining 51.5% of estimated costs to be contributed by the Company. From grant inception through December 31, 2017, the Company has received $1.8 million from Skolkovo.

At any time during the term of the grant agreement, but not more than once per quarter, Skolkovo has the right to request information related to the project and to conduct an audit of the expenses incurred by the Company. In the event the project or the expenses do not meet predefined requirements, the Company may be required to reimburse the funds received up to three years after the completion of the project. As a result, the Company has determined that the grant funding is not fixed or determinable and all amounts received to date are recorded as deferred revenue in the consolidated balance sheet until the completion of the Skolkovo audit or the expiration of the audit term.

Related-Party Transactions	12 Months Ended
Related-Party Transactions	Dec. 31, 2017
Related Party Transactions [Abstract]
Related-Party Transactions	Related‑Party Transactions As part of the Series B Preferred and Series D Preferred financings (as described in Note 11), the Company’s landlord (the “Landlord”) purchased 49,254 shares of Series B Preferred at $2.03 per share for total proceeds of $0.1 million and 488,888 shares of Series D Preferred at $4.50 per share for total proceeds of $2.2 million. Additionally, in April 2015, the Landlord participated in the Company’s bridge loan in the amount of $0.2 million, which converted into Series E Preferred (see Note 11). The Landlord paid the same price as the price paid by other investors in each of these Preferred Stock purchases. At the IPO, all preferred stock was converted to common stock. In connection with the 2017 PIPE, the Company sold to Timothy Springer, Ph.D., a member of its Board of Directors, an aggregate of 338,791 shares of common stock at a purchase price equal to $17.71 per share, which was equal to the most recent consolidated closing bid price on the Nasdaq Global Market on June 23, 2017. In addition, the Company sold warrants to Dr. Springer to purchase up to 79,130 shares of common stock (“Warrant Shares”), exercisable at $17.71 per Warrant Share, and with a term of five years. The purchase price for each warrant was equal to $0.125 for each Warrant Share, consistent with Nasdaq Global Market requirements for an “at the market” offering. The Company incurred expenses for consulting services provided by its founders totaling $0.2 million, $0.3 million and $0.3 million during the years ended December 31, 2017, 2016 and 2015, respectively.

Technology License Agreements	12 Months Ended
Technology License Agreements	Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Technology License Agreements	Technology License Agreements Massachusetts Institute of Technology On November 25, 2008, the Company entered into an Exclusive Patent License agreement with the Massachusetts Institute of Technology (“MIT”), which is referred to as the Exclusive Patent License. The Company received an exclusive royalty‑bearing license to utilize patents held by MIT in exchange for upfront consideration and annual license maintenance fees. Such fees are expensed as incurred and have not been material to any period presented. In the event the Company sublicenses the MIT patents to a third party, it will be required to remit to MIT a percentage (ranging from 10% to 30%) of sublicense income. In addition, the Company is obligated to pay MIT a certain amount upon the achievement of defined clinical milestones, up to a total of $1.5 million. On December 18, 2008, the Company entered into a patent‑cross‑license agreement with BIND Therapeutics, Inc. whereby each party receives a license for the use of the other patents in their respective fields of use. In exchange for this license, the Company paid a one‑time expense in 2008. As of December 31, 2017, and in connection with the execution of the Spark License Agreement, the Company has made contractual payments pursuant to the Exclusive Patent License totaling $2.2 million for the sublicense granted to Spark, and $0.4 million relative to the calculated premium paid by Spark for the equity investments made under the Spark Purchase Agreement. Shenyang Sunshine Pharmaceutical Co., Ltd In May 2014, the Company entered into a license agreement with Shenyang Sunshine Pharmaceutical Co., Ltd. (“3SBio”), which is referred to as the 3SBio License. Pursuant to the 3SBio License, which was amended and restated in May 2017, the Company was granted an exclusive license to certain pegsiticase‑related patents and related “know‑how” owned or in‑licensed by 3SBio for the worldwide (except for Greater China and Japan) development and commercialization of products based thereupon for human therapeutic, diagnostic and prophylactic use. The Company was also granted a worldwide (except for Greater China) exclusive license to develop, commercialize and manufacture or have manufactured products combining the Company’s proprietary SVP technology with pegsiticase or related compounds supplied by 3SBio (or otherwise supplied if the Company’s rights to manufacture are in effect) for human therapeutic, diagnostic and prophylactic use. The Company was also granted a co‑exclusive license to manufacture and have manufactured pegsiticase and related compounds for preclinical and clinical use or, if the 3SBio License is terminated for 3SBio’s material breach, for any use under the 3SBio License. Otherwise, the Company is obligated to obtain all of its supply of such compounds for Phase 3 clinical trials and commercial use from 3SBio under the terms of supply agreements to be negotiated. Pursuant to the 3SBio License, the Company is required to use commercially reasonable efforts to develop and commercialize a product containing pegsiticase or a related compound. If the Company does not commercialize any such product in a particular country in Asia, Africa or South America within 48 months after approval of any such product in the United States or a major European country, then 3SBio will have the right to do so, but only until the Company commercializes a product combining the Company’s SVP technology with any such compound in such country. The Company has paid to 3SBio an aggregate of $1.0 million in upfront and milestone‑based payments under the 3SBio License. An additional liability totaling $2.0 million for milestone payments was expensed in 2016, and is included with Accounts Payable on the balance sheet as of December 31, 2016. The Company is required to make future payments to 3SBio contingent upon the occurrence of events related to the achievement of clinical and regulatory approval milestones of up to an aggregate of $21.0 million for products containing the Company’s SVP technology, and up to an aggregate of $41.5 million for products without the Company’s SVP technology. The Company is also required to pay 3SBio tiered royalties on annual worldwide net sales (on a country‑by‑country and product‑by‑product basis) related to the pegsiticase component of products at percentages ranging from the low‑to‑mid single digits for products containing the Company’s SVP technology, and a range of no more than ten percent points from the mid‑single digits to low double‑digits for products without the Company’s SVP technology. The Company will pay these royalties to 3SBio, subject to specified reductions, on a country‑by‑country and product‑by‑product basis until the later of (i) the date that all of the patent rights for that product have expired in that country, or (ii) a specified number of years from the first commercial sale of such product in such country. The 3SBio License expires on the date of expiration of all of the Company’s royalty payment obligations unless earlier terminated by either party for an uncured material default or for the other party’s bankruptcy. Any such termination by 3SBio for material default may be on a country‑by‑country or product‑by‑product basis in certain circumstances. The Company may also terminate the 3SBio License on a country‑by‑country or product‑by‑product basis for any reason effective upon 60 days’ prior written notice to 3SBio or, with respect to a given product, immediately upon written notice to 3SBio if the Company identifies a safety or efficacy concern related to such product. Massachusetts Eye and Ear Infirmary and The Schepens Eye Research Institute, Inc. In May 2016, the Company entered into a license agreement with the Massachusetts Eye and Ear Infirmary and The Schepens Eye Research Institute, Inc. (collectively, “MEE”), which is referred to as the MEE License. Under the MEE License, the Company was granted an exclusive commercial worldwide license, with the right to grant sublicenses through multiple tiers, to make, have made, use, offer to sell, sell and import certain products and to practice certain processes, the sale, use or practice of which are covered by patents and proprietary know‑how owned or controlled by MEE, for use of Anc80 gene therapy vectors for gene augmentation therapies expressing certain target sequences. MEE also granted the Company exclusive options to exclusively license certain of their intellectual property rights relating to several additional target sequences and variations thereof each linked to a specified disease. During a defined option period, the Company may exercise this right for up to a designated number of target sequences. If the Company exercises its options, under certain circumstances, the Company may substitute alternative target sequences for previously selected target sequences. The Company agreed to use commercially reasonable efforts to develop and commercialize licensed products pursuant to a development plan, and to market and sell at least one product for each target sequence for which the Company exercised its option as soon as reasonably practicable. Subject to certain exceptions, following commercial launch, the Company must use commercially reasonable efforts to market, sell, and maintain public availability of licensed products in a certain number of specified major markets. Pursuant to the MEE License, the Company agreed to pay MEE a license fee in the low six figures, annual license maintenance fees ranging from the mid‑twenty thousands to mid‑seventy thousands and an option maintenance fee in the low five figures for each exercisable option. The Company also agreed to reimburse MEE for a specified percentage of the past patent expenses for the patents licensed to the Company. The Company also agreed to pay development milestones on a licensed product‑by‑licensed product basis, totaling up to an aggregate of between $4.2 million to $37.0 million and sales milestones on a licensed product‑by‑licensed product basis, totaling up to an aggregate of between $50.0 million to $70.0 million; tiered royalties on a licensed product‑by‑licensed product and country‑by‑country basis equal to a percentage of net sales ranging from mid‑single digits to mid‑teens, subject to the prevalence of the targeted disease and certain reductions; and a percentage, in a range expected to be in the mid‑teens depending on timing, of any sublicense income the Company receives from sublicensing its rights granted thereunder, subject to certain reductions and exclusions. Upon exercise of each option, the Company agreed to pay MEE an option exercise fee ranging from low‑six figures to mid‑six figures, depending on the prevalence of the targeted disease. The MEE License will continue until the expiration of the last to expire of the patent rights licensed thereunder. The Company may terminate the MEE License in whole or in part upon prior written notice. MEE may terminate the MEE License on a target sequence‑by‑target sequence basis if the Company fails to make any scheduled payments in respect of such target sequence or if the Company materially breaches a diligence obligation in respect of such target sequence, in each case if the Company fails to cure within a specified time period. MEE may terminate the MEE License in its entirety if the Company materially breaches certain of its obligations related to diligence, representations and warranties, and maintenance of insurance; if the Company challenges the validity or enforceability of any patents licensed thereunder; if any of the Company’s executive officers are convicted of a felony relating to manufacture, use, sale or importation of licensed products; or upon the Company’s insolvency or bankruptcy. As of June 30, 2016, the Company had paid a total of $0.1 million in license fees due under the MEE License. No additional license fees are due under the MEE License as of December 31, 2017.

Income Taxes

12 Months Ended

Dec. 31, 2017

Income Tax Disclosure [Abstract]

Income Taxes

The Company provides for income taxes under ASC 740, Income Taxes ("ASC 740"). Under ASC 740, the Company provides deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the Company’s financial statement carrying amounts and the tax bases of assets and liabilities using enacted tax rates expected to be in effect in the years in which the differences are expected to reverse.

For the years ended December 31, 2017, 2016 and 2015, the Company did not record a current or deferred income tax expense or benefit. The following table reconciles the federal statutory income rate to the Company's effective income tax rate:



	Year Ended December 31,
	2017		2016		2015
Tax at U.S. statutory rate	34.0	%	34.0	%	34.0	%
Changes from statutory rate:
State taxes, net of federal benefit	5.8	%	5.8	%	5.7	%
Permanent items	(0.1	)%	(1.1	)%	(0.7	)%
Research tax credits/other	0.5	%	1.3	%	1.0	%
Change in enacted rates	(35.8	)%	—	%	—	%
Valuation allowance, net	(6.7	)%	(39.8	)%	(40.0	)%
Other	2.3	%	(0.2	)%	—	%
Effective income tax rate	—	%	—	%	—	%

Significant components of the Company's deferred tax assets (liabilities) consists of the following:



	December 31,
(In thousands)	2017			2016
Deferred tax assets
Net operating loss carryforwards	$	44,146		$	39,420
Research and development credits	4,092			3,052
Stock-based compensation expense	959			904
Deferred rent and other expenses	263			713
Deferred revenue	4,021			5,607
Patent costs/amortization	4,264			4,934
Total deferred tax assets	57,745			54,630

Deferred tax liabilities
Depreciation	$	(108	)	$	(131	)
Debt discount	—			(69		)
Unrealized foreign exchange gain	—			(1,525		)
Total deferred tax liabilities	(108		)	(1,725		)
Net deferred tax assets	$	57,637		$	52,905
Valuation allowance	(57,637		)	(52,905		)
Net deferred tax assets	$	—		$	—

The Company has provided a full valuation allowance against its net deferred tax assets because the Company believes it is more likely than not that its deferred tax assets will not be realized. Realization of future tax benefits is dependent on many factors, including the Company’s ability to generate taxable income within the net operating loss carryforward period. The Company has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets and concluded that it is more likely than not that the Company will not realize the benefit of its deferred tax assets. The valuation allowance increased by $4.7 million and $14.4 million for the years ended December 31, 2017 and 2016, respectively, primarily as a result of an increase in net operating loss. In 2014, the Company's Russian subsidiary was granted a 10 year tax holiday in Russia. The Company's foreign operations continue to benefit from the tax holiday, which is set to expire on December 31, 2023.

At December 31, 2017, the Company had federal and state net operating loss carryforwards of $162.9 million and $157.9 million, respectively, which will expire at various times through 2037. The Company also has federal and state research and development tax credit carryforwards of $2.5 million and $2.0 million, respectively, available to reduce future tax liabilities, which will expire at various dates through 2037.

Included in the net operating loss carryforwards above is approximately $0.8 million related to excess stock option deductions. On March 30, 2016, the FASB released ASU 2016-09, which the Company adopted during the first quarter of 2017. Upon adoption of ASU 2016-09, all excess tax benefits and tax deficiencies (including tax benefits of dividends on share-based awards) will be recognized as income tax expense or benefit in the income statement. Prior to adoption of ASU 2016-09, such excess tax benefits were recognized as an adjustment to paid-in capital. Further, each award needed to be evaluated to determine if there was a deficiency or an excess. Although ASU 2016-09 will certainly reduce the complexity surrounding the accounting for excess tax benefits and deficiencies, it will create significant volatility in a company’s tax rate. The standard provides that the tax effects of exercised or vested awards will be treated as a discrete item in the reporting period in which they occur. Thus, the tax effect of excess benefits and deficiencies on a company’s tax rate will not be part of the overall effective tax rate.

Prior to December 31, 2016, the Company had federal and state NOLs relating to stock-based compensation in the amount of $0.8 million and $0.8 million, respectively, that were not included in deferred tax assets. Under the Company's previous accounting policy, when the excess stock-based compensation relating to the NOL carryforward were ultimately realized, the benefit would be credited directly to stockholders’ equity. During the first quarter of 2017, the Company adopted ASU 2016-09, which resulted in an increase to NOL deferred tax assets of $0.3 million, offset by a corresponding increase in the valuation allowance. The adoption of ASU 2016-09 had no impact to the Company’s consolidated statements of operations and comprehensive loss, balance sheet, or retained earnings.

On December 22, 2017, the President of the United States signed into law the Tax Cuts and Jobs Act ("Tax Reform Act"). The legislation significantly changes U.S. tax law by, among other things, lowering corporate income tax rates, implementing a territorial tax system, expanding the tax base and imposing a tax on deemed repatriated earnings of foreign subsidiaries. The Tax Reform Act permanently reduces the U.S. corporate federal income tax rate from a maximum of 35% to a flat 21% rate, effective January 1, 2018. The Company has recognized the impact of the Tax Reform Act in these consolidated financial statements and related disclosures. Due to the complexities involved in accounting for the enactment of the Tax Reform Act, the SEC staff issued Staff Accounting Bulletin No. 118 ("SAB 118"), which allows a registrant to record provisional amounts during a measurement period not to extend beyond one year of the enactment date. In accordance with SAB 118, the Company recorded provisional amounts reflecting the impact of the Tax Reform Act in these consolidated financial statements and related disclosures. The impact of the remeasurement of the Company’s U.S. deferred tax assets and liabilities to 21% resulted in the reduction of deferred tax assets of approximately $23.4 million, which is offset by a full valuation allowance, thus there is no net effect. The Company recorded no tax expense related to the deemed repatriation tax because its foreign entity, Selecta (RUS) is a foreign disregarded entity, which is not subject to the repatriation tax.

The Company's preliminary estimate of the Tax Reform Act and the remeasurement of its deferred tax assets and liabilities is subject to the finalization of management’s analysis related to certain matters, such as developing interpretations of the provisions of the Tax Reform Act, changes to certain estimates and the filing of its tax returns. U.S. Treasury regulations, administrative interpretations or court decisions interpreting the Tax Reform Act may require further adjustments and changes in the Company's estimates. The final determination of the Tax Reform Act and the remeasurement of the Company's deferred assets and liabilities will be completed as additional information becomes available, but no later than one year from the enactment of the Tax Reform Act.

Utilization of the net operating loss and research and development credit carryforwards may be subject to a substantial annual limitation under Section 382 of the Internal Revenue Code of 1986 due to ownership change limitations that have occurred previously, or that could occur in the future. These ownership changes may limit the amount of net operating loss and research and development credit carryforwards that can be utilized annually to offset future taxable income and tax, respectively.

The Company applies ASC 740 to uncertain tax positions. As of the adoption date of January 1, 2010 and through December 31, 2017, the Company had no unrecognized tax benefits or related interest and penalties accrued.

The Company has not, as of yet, conducted a study of its research and development credit carryforwards. This study may result in an adjustment to the Company’s research and development credit carryforwards; however, until a study is completed and any adjustment is known, no amounts are being presented as an uncertain tax position. A full valuation allowance has been provided against the Company’s research and development credits and, if an adjustment is required, this adjustment would be offset by an adjustment to the valuation allowance. As a result, there would be no impact to the consolidated balance sheets, statements of operations and comprehensive loss, or cash flows if an adjustment was required.

Interest and penalty charges, if any, related to unrecognized tax benefits would be classified as income tax expense in the accompanying statement of operations and comprehensive loss. As of December 31, 2017, the Company had no accrued interest related to uncertain tax positions.

The statute of limitations for assessment by the Internal Revenue Service and Massachusetts tax authorities is open for tax years since inception. The Company files income tax returns in the United States and Massachusetts. There are currently no federal, state or foreign audits in progress.

Defined Contribution Plan	12 Months Ended
Defined Contribution Plan	Dec. 31, 2017
Retirement Benefits [Abstract]
401(k) Savings Plan	Defined Contribution Plan The Company maintains a defined contribution plan under Section 401(k) of the Internal Revenue Code (the “401(k) Plan”). The 401(k) Plan covers all employees who meet defined minimum age and service requirements, and allows participants to defer a portion of their annual compensation on a pretax basis. The 401(k) Plan provides for matching contributions on a portion of participant contributions pursuant to the 401(k) Plan’s matching formula. All matching contributions vest ratably over 4 years and participant contributions vest immediately. Contributions by the Company totaling $0.2 million for the year ended December 31, 2017, and less than $0.1 million for each of the years ended December 31, 2016 and 2015, have been recorded in the consolidated statements of operations and comprehensive.

Selected Quarterly Financial Data

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

Selected Quarterly Financial Data (Unaudited)

The following table summarizes unaudited quarterly financial data for the years ended December 31, 2017 and 2016 (in thousands, except per share data).



	Three Months Ended (unaudited)
	March 31,			June 30,			September 30,			December 31,
	2017			2017			2017			2017
Grant and collaboration revenue	$	137		$	26		$	27		$	17
Operating expenses	$	14,919		$	15,897		$	13,881		$	19,294
Net loss	$	(15,134	)	$	(15,967	)	$	(14,676	)	$	(19,544	)
Net loss attributable to common stockholders	$	(15,134	)	$	(15,967	)	$	(14,676	)	$	(19,544	)

Net loss per share attributable to common stockholders, basic and diluted	$	(0.82	)	$	(0.85	)	$	(0.66	)	$	(0.88	)

	Three Months Ended (unaudited)
	March 31,			June 30,			September 30,			December 31,
	2016			2016			2016			2016
Grant and collaboration revenue	$	2,088		$	2,017		$	1,048		$	2,930
Operating expenses	$	9,029		$	8,418		$	8,516		$	16,790
Net loss	$	(7,476	)	$	(6,923	)	$	(7,728	)	$	(14,083	)
Accretion on preferred stock	$	(2,356	)	$	(2,210	)	$	—		$	—
Net loss attributable to common stockholders	$	(9,832	)	$	(9,133	)	$	(7,728	)	$	(14,083	)

Net loss per share attributable to common stockholders, basic and diluted	$	(4.52	)	$	(2.75	)	$	(0.43	)	$	(0.77	)

Summary of Significant Accounting Policies (Policies)	12 Months Ended
Summary of Significant Accounting Policies (Policies)	Dec. 31, 2017
Accounting Policies [Abstract]
Principles of Consolidation	Principles of Consolidation The consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries, Selecta RUS, LLC (“Selecta (RUS)”), a Russian limited liability corporation, and Selecta Biosciences Security Corporation, a Massachusetts Security Corporation. All significant intercompany accounts and transactions have been eliminated.
Foreign Currency	Foreign Currency The functional currency of Selecta (RUS) is the Russian ruble. Assets and liabilities of Selecta (RUS) are translated at period-end exchange rates, while revenues and expenses are translated at average exchange rates for the period. Translation gains and losses are reflected in accumulated other comprehensive loss within stockholders’ deficit. Foreign currency transaction gains or losses are reflected in the consolidated statements of operations and comprehensive loss.
Use of Estimates	Use of Estimates The preparation of consolidated financial statements in conformity with U.S. GAAP requires the Company’s management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. The Company’s management considers many factors in selecting appropriate financial accounting policies and controls, and bases its estimates on historical experience and other market-specific or other relevant assumptions that it believes to be reasonable under the circumstances. In preparing these consolidated financial statements, management used significant estimates in the following areas, among others: revenue recognition, the fair value of common stock and other equity instruments, accounting for stock-based compensation and estimating accrued research and development expenses. The Company assesses the above estimates on an ongoing basis; however, actual results could materially differ from those estimates. The Company’s management made significant estimates and assumptions in determining the fair value of its common stock for those periods reported prior to the completion of the initial public offering of the Company’s common stock (the “IPO”). The Company utilized various valuation methodologies in accordance with the framework of the 2004 American Institute of Certified Public Accountants’ Technical Practice Aid, Valuation of Privately-Held Company Equity Securities Issued as Compensation, to estimate the fair value of its common stock. Each valuation methodology included estimates and assumptions that require the Company’s judgment. These estimates and assumptions included a number of objective and subjective factors, including external market conditions affecting the biotechnology industry sector, the prices at which the Company sold shares of its preferred stock, the superior rights and preferences of securities senior to the Company’s common stock at the time and the likelihood of achieving a liquidity event, such as an initial public offering or sale. Significant changes to the key assumptions used in the valuations could result in different fair values of common stock at each valuation date.
Segment Information	Segment Information Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision maker, the Company’s Chief Executive Officer, in making decisions regarding resource allocation and assessing performance. The Company views its operations and manages its business in one operating segment, the research and development of nanoparticle immunomodulatory drugs for the treatment and prevention of human diseases.
Reverse Stock Split	Preferred Stock Prior to the completion of the IPO, the Company classified preferred stock as temporary equity and initially recorded it at the original issuance price, net of issuance costs and discounts. The carrying value was accreted up to the redemption value over the earliest redemption period. The carrying value was also adjusted for dividends expected to be paid upon redemption or liquidation according to the preferred stock terms on each balance sheet date. Reverse Stock Split In connection with the initial public offering, the Company’s Board of Directors and stockholders approved a one-for-3.9 reverse stock split of the Company’s common stock. The reverse stock split became effective June 7, 2016. All share and per share amounts presented have been retroactively adjusted for all periods presented to give effect to this reverse stock split, including reclassifying an amount equal to the reduction in par value of common stock to additional paid-in capital.
Cash Equivalents and Short Term Investments	Cash Equivalents and Short-term Investments Cash equivalents include all highly liquid investments maturing within 90 days from the date of purchase. Investments consist of securities with remaining maturities greater than 90 days when purchased. The Company classifies these investments as available‑for‑sale and records them at fair value in the accompanying consolidated balance sheets. Unrealized gains or losses are included in accumulated other comprehensive income (loss). Premiums or discounts from par value are amortized to investment income over the life of the underlying investment. The Company, as part of its cash management strategy, may invest in reverse repurchase agreements. All reverse repurchase agreements are tri-party and have maturities of three months or less at the time of investment. These agreements are collateralized by U.S. treasury securities for an amount no less than 102% of their value. Although available to be sold to meet operating needs or otherwise, securities are generally held through maturity. The cost of securities sold is determined based on the specific identification method for purposes of recording realized gains and losses. During the reporting periods, there were no realized gains or losses on sales of investments, and no investments were adjusted for other than temporary declines in fair value.
Concentrations of Credit Risk and Off-Balance Sheet Risk	Concentrations of Credit Risk and Off‑Balance Sheet Risk Financial instruments that potentially subject the Company to concentration of credit risk consist primarily of cash, cash equivalents, and accounts receivable. Cash and cash equivalents are deposited with federally insured financial institutions in the United States and may, at times, exceed federally insured limits. Management believes that the financial institutions that hold the Company’s deposits are financially credit worthy and, accordingly, minimal risk exists with respect to those balances. Generally, these deposits may be redeemed upon demand and therefore bear minimal interest rate risk. As an integral part of operating its Russian subsidiary, the Company also maintains cash in Russian bank accounts in denominations of both Russian rubles and U.S. dollars. As of December 31, 2017, the Company maintained approximately $1.3 million in Russian bank accounts, of which $1.2 million was held in U.S. dollars. The Company has minimal credit risk as the majority of accounts receivable relates to amounts due under a government sponsored grant, collaboration with large pharmaceutical companies or grants from well‑known and supported non‑profit organizations. The Company did not have any off balance sheet arrangements as of December 31, 2017 and December 31, 2016
Fair Value of Financial Instruments	Fair Value of Financial Instruments The Company’s financial instruments consist mainly of cash equivalents, short‑term investments, restricted cash, accounts receivable, accounts payable, loans payable, and common stock warrants. The carrying amounts of cash equivalents, short-term investments, restricted cash, accounts receivable, and accounts payable approximate their estimated fair value due to their short-term maturities. At December 31, 2017, the carrying amount of loans payable approximates its estimated fair value due to the consistency between the prevailing market rates in effect and the effective interest rate of 6.53% for the debt arrangement. Accounting standards define fair value as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. A three‑level hierarchy is used to prioritize the inputs to valuation techniques used to measure fair value. The hierarchy gives the highest priority to unadjusted quoted prices in active markets for identical assets or liabilities (Level 1 measurements), and the lowest priority to unobservable inputs (Level 3 measurements). The three levels of the fair value hierarchy are described below: Level 1—Level 1 inputs are quoted prices in active markets for identical assets or liabilities that the reporting entity has the ability to access at the measurement date. Level 2—Level 2 inputs are inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly. If the asset or liability has a specified (contractual) term, a Level 2 input must be observable for substantially the full term of the asset or liability. Level 3—Level 3 inputs are unobservable inputs for the asset or liability in which there is little, if any, market activity for the asset or liability at the measurement date. To the extent that a valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. The fair value of the Company's loan payable was determined using Level 3 inputs. Fair value is a market‑based measure considered from the perspective of a market participant rather than an entity‑specific measure. Therefore, even when market assumptions are not readily available, the Company’s own assumptions are set to reflect those that market participants would use in pricing the asset or liability at the measurement date. The Company uses prices and inputs that are current as of the measurement date, including during periods of market dislocation. In periods of market dislocation, the observability of prices and inputs may change for many instruments. This condition could cause an instrument to be reclassified within levels in the fair value hierarchy. There were no transfers within the fair value hierarchy during the years ended December 31, 2017 and December 31, 2016.
Property and Equipment	Property and Equipment Property and equipment are recorded at cost and depreciated using the straight‑line method over the estimated useful lives of the respective assets, generally seven years for furniture and fixtures, five years for laboratory equipment and office equipment and three years for computer equipment. Leasehold improvements are amortized over their useful life or the life of the lease, whichever is shorter. Major additions and betterments are capitalized. Maintenance and repairs, which do not improve or extend the life of the respective assets, are charged to operations as incurred. Costs incurred for construction in progress are recorded as assets and are not amortized until the construction is substantially complete and the assets are ready for their intended use.
Impairment of Long-Lived Assets	Impairment of Long‑Lived Assets The Company periodically evaluates its long‑lived assets for potential impairment. Impairment is assessed when there is evidence that events or changes in circumstances indicate that the carrying amount of an asset may not be recovered. Recoverability of these assets is assessed based on undiscounted expected future cash flows from the assets, considering a number of factors, including past operating results, budgets and economic projections, market trends, and product development cycles. Impairment in the carrying value of each asset is assessed when the undiscounted expected future cash flows derived from the asset are less than their carrying value. The Company did not recognize any impairment charges as of December 31, 2017 and December 31, 2016.
Debt Issuance Costs	Debt Issuance Costs Debt issuance costs and fees paid to lenders are classified as a debt discount, and are recorded as a direct deduction from the face amount of the related debt. Issuance costs paid to third parties that are the direct result of the debt issuance are capitalized as a direct deduction from the face amount of the related debt. Debt issuance costs are amortized over the term of the related debt using the interest method and recorded as interest expense. Costs and fees paid to third parties are expensed as incurred.
Accumulated Other Comprehensive Income (Loss) and Comprehensive Loss	Comprehensive Loss Comprehensive loss is defined as the change in the equity of a business entity during a period from transactions and other events and circumstances from non‑owner sources. It includes all changes in equity during a period except those resulting from investments by owners and distributions to owners. Comprehensive loss consists of both: (i) all components of net loss and (ii) all components of comprehensive loss other than net loss, referred to as other comprehensive loss. For all periods presented, other comprehensive loss is comprised of foreign currency translation adjustments and the unrealized gains and losses on investments. Comprehensive income (loss) is the total net earnings and all other non-owner changes in equity. Except for net income and unrealized gains and losses on available-for-sale securities and foreign exchange translation adjustments, the Company does not have any transactions or other economic events that qualify as comprehensive income (loss). There were no reclassifications out of accumulated other comprehensive income or loss as well as no tax effect for the period presented.
Revenue Recognition	Revenue Recognition The Company’s revenue is primarily generated from research grants in both the United States and Russia, and, prior to its termination, a license and research collaboration agreement with Sanofi. The Company recognizes revenue in accordance with ASC Topic 605, Revenue Recognition. Accordingly, revenue is recognized when all of the following criteria are met: - Persuasive evidence of an arrangement exists; - Delivery has occurred or services have been rendered; - The seller’s price to the buyer is fixed or determinable; and - Collectability is reasonably assured. Amounts received prior to satisfying the revenue recognition criteria are recognized as deferred revenue in the Company’s consolidated balance sheets. Amounts expected to be recognized as revenue within the 12 months following the balance sheet date are classified as deferred revenue, current portion. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date are classified as deferred revenue, net of current portion.
Collaboration Revenue	Collaboration Revenue The Company enters into collaborative arrangements for the development and commercialization of product candidates utilizing the Company’s Synthetic Vaccine Particles (SVP) technology. The terms of these agreements contain multiple deliverables which may include (i) licenses, or options to obtain licenses, to its technology platforms, (ii) rights to future technological improvements, (iii) research and development activities to be performed on behalf of the collaborative partner or as part of the collaboration, (iv) the manufacture of pre-clinical or clinical materials for the collaborative partner, and (v) options to acquire licenses for additional therapeutic areas. Payments to the Company under these agreements may include nonrefundable license fees, option fees, exercise fees, payments for research and development activities, payments for the manufacture of preclinical or clinical materials, payments based upon the achievement of certain milestones and royalties on product sales. When evaluating multiple element arrangements such as the agreements discussed in Note 13, the Company considers whether the deliverables under the arrangement represent separate units of accounting. This evaluation requires subjective determinations and requires management to make judgments about the individual deliverables and whether such deliverables are separable from the other aspects of the contractual relationship. In determining the units of accounting, management evaluates certain criteria, including whether the deliverables have standalone value, based on the consideration of the relevant facts and circumstances for each arrangement. The consideration received is allocated among the separate units of accounting using the relative selling price method, and the applicable revenue recognition criteria are applied to each of the separate units. If deliverables have no standalone value, they are grouped together and revenue is recognized over the estimated performance period. The Company determines the estimated selling price for deliverables within each agreement using vendor‑specific objective evidence (“VSOE”) of selling price, if available, third‑party evidence (“TPE”) of selling price if VSOE is not available, or best estimate of selling price if neither VSOE nor TPE is available. Determining the best estimate of selling price for a deliverable requires significant judgment. Since the Company is a clinical stage company without a marketable product, and currently there is no technologically comparative product on the market, to determine either VSOE or TPE, the Company has used its best estimate of selling price to estimate the selling price for licenses and deliverables related to the Company’s proprietary technology. Under the circumstances, the Company considers market conditions as well as entity‑specific factors, including those factors contemplated in negotiating the agreements, estimated development costs, probability of success and the time needed to commercialize a product candidate pursuant to the license. In validating the Company’s best estimate of selling price, the Company evaluates whether changes in the key assumptions used to determine the best estimate of selling price will have a significant effect on the allocation of arrangement consideration between multiple deliverables. Options for future deliverables are considered substantive if, at the inception of the arrangement, the Company is at risk as to whether the collaboration partner will choose to exercise the option. Factors that the Company considers in evaluating whether an option is substantive include the overall objective of the arrangement, the benefit the collaborator might obtain from the arrangement without exercising the option, the cost to exercise the option and the likelihood that the option will be exercised. When an option is considered substantive and there is no significant incremental discount, the option is not considered a deliverable in the arrangement and no consideration is allocated to it. Conversely, when an option is not considered substantive or it is considered substantive but is priced at an incremental discount, it would be considered a deliverable at the inception of the arrangement and a corresponding amount would be included in the initial consideration. The Company may receive upfront payments when licensing its intellectual property in conjunction with a manufacturing or a research and development agreement. When management believes the license to its intellectual property does not have stand‑alone value from the other deliverables to be provided in the arrangement, the Company generally recognizes revenue attributed to the license over the Company’s contractual or estimated performance period. When management believes the license to its intellectual property has stand‑alone value, the Company generally recognizes revenue attributed to the license upon delivery. The periods over which revenue should be recognized are subject to estimates by management and may change over the course of the research and development agreement. Such a change could have a material impact on the amount of revenue the Company records in future periods. Payments or reimbursements resulting from the Company’s deliveries of manufactured products and research and development efforts are recognized as the services are performed. At the inception of each agreement that includes milestone payments, the Company evaluates whether each milestone is substantive and at risk to both parties based on the contingent nature of the milestone, specifically reviewing factors such as the scientific and other risks that must be overcome to achieve the milestone, whether the milestone consideration is reasonable relative to all deliverables and payment terms, as well as the level of effort and investment required. Revenues from milestones, if they are nonrefundable and deemed substantive, are recognized upon successful accomplishment of the milestones. Milestones that are not considered substantive are accounted for as license payments and recognized over the remaining period of performance.
Grant Agreements	Grant Agreements Grant revenue is generally recognized as the related research and development work is performed. Grant arrangements frequently include payment milestones which the Company has judged to be non‑substantive milestones as they are typically entitled to receive payment regardless of the outcome of the research work. Revenue under such arrangements is recognized using a proportional performance method, but not more than cash received. Amounts received prior to satisfying the above revenue recognition criteria are recorded as deferred revenue in the accompanying consolidated balance sheets.
Research and Development Costs	Research and Development Costs Costs incurred in the research and development of the Company’s products are expensed as incurred. Research and development expenses include costs incurred in performing research and development activities, including salaries and benefits, facilities cost, overhead costs, contract services, supplies and other outside costs. Nonrefundable advance payments for goods and services that will be used in future research and development activities are expensed when the activity has been performed or when the goods have been received rather than when the payment is made.
Clinical Trial Costs	Clinical Trial Costs Clinical trial costs are a component of research and development expenses. The Company accrues and expenses clinical trial activities performed by third parties based on an evaluation of the progress to completion of specific tasks using data such as patient enrollment, clinical site activation, and other information provided to the Company by its vendors.
Income Taxes	Income Taxes The Company provides deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the Company’s financial statement carrying amounts and the tax basis of assets and liabilities using enacted tax rates expected to be in effect in the years in which the differences are expected to reverse. A valuation allowance is provided to reduce the deferred tax assets to the amount that will more-likely-than-not be realized. The Company determines whether it is more likely than not that a tax position will be sustained upon examination. If it is not more-likely-than-not that a position will be sustained, none of the benefit attributable to the position is recognized. The tax benefit to be recognized for any tax position that meets the more‑likely‑than‑not recognition threshold is calculated as the largest amount that is more than 50% likely of being realized upon resolution of the contingency. The Company accounts for interest and penalties related to uncertain tax positions as part of its provision for income taxes. To date, the Company has not incurred interest and penalties related to uncertain tax positions.
Warrants	Warrants The Company determines the accounting classification of warrants that are issued, as either liability or equity, by first assessing whether the warrants meet liability classification in accordance with ASC 480-10, Accounting for Certain Financial Instruments with Characteristics of both Liabilities and Equity, and then in accordance with ASC 815-40, Accounting for Derivative Financial Instruments Indexed to, and Potentially Settled in, a Company’s Own Stock. Under ASC 480, warrants are considered liability classified if the warrants are mandatorily redeemable, obligate the issuer to settle the warrants or the underlying shares by paying cash or other assets, or warrants that must or may require settlement by issuing variable number of shares. If warrants do not meet liability classification under ASC 480-10, the Company assesses the requirements under ASC 815-40, which states that contracts that require or may require the issuer to settle the contract for cash are liabilities recorded at fair value, irrespective of the likelihood of the transaction occurring that triggers the net cash settlement feature. If the warrants do not require liability classification under ASC 815-40, in order to conclude equity classification, the Company assesses whether the warrants are indexed to our common stock and whether the warrants are classified as equity under ASC 815-40 or other applicable GAAP. After all relevant assessments are made, the Company concludes whether the warrants are classified as liability or equity. Liability classified warrants are required to be accounted for at fair value both on the date of issuance and on subsequent accounting period ending dates, with all changes in fair value after the issuance date recorded in the statements of operations as a gain or loss. Equity classified warrants are accounted for at fair value on the issuance date with no changes in fair value recognized after the issuance date.
Stock-Based Compensation	Stock‑Based Compensation The Company accounts for all stock‑based compensation granted to employees and non‑employees using a fair value method. Stock‑based compensation awarded to employees is measured at the grant date fair value and is recognized over the requisite service period of the awards, usually the vesting period, on a straight‑line basis, net of estimated forfeitures. Stock‑based compensation awarded to non‑employees are subject to revaluation over their vesting terms. The Company reduces recorded stock‑based compensation for estimated forfeitures. To the extent that actual forfeitures differ from the Company’s estimates, the differences are recorded as a cumulative adjustment in the period the estimates were adjusted. Stock‑based compensation expense recognized in the consolidated financial statements is based on awards that are ultimately expected to vest.
Net Loss Per Share	Net Loss Per Share The Company has reported losses since inception and has computed basic net loss per share attributable to common stockholders by dividing net loss attributable to common stockholders by the weighted average number of common shares outstanding for the period. The Company has computed diluted net loss per common share after considering all potentially dilutive common shares, including stock options, convertible preferred stock, and warrants outstanding during the period except where the effect of including such securities would be antidilutive. Because the Company has reported net losses since inception, these potential common shares have been anti‑dilutive and basic and diluted loss per share have been the same.
Deferred Rent	Deferred Rent Rent expense and lease incentives from operating leases are recognized on a straight‑line basis over the lease term. The difference between rent expense recognized and rental payments is recorded as deferred rent in the accompanying consolidated balance sheets.
Contingent Liabilities	Contingent Liabilities The Company accounts for its contingent liabilities in accordance with ASC No. 450, Contingencies. A provision is recorded when it is both probable that a liability has been incurred and the amount of the loss can be reasonably estimated. With respect to legal matters, provisions are reviewed and adjusted to reflect the impact of negotiations, estimated settlements, legal rulings, advice of legal counsel and other information and events pertaining to a particular matter. As of December 31, 2017 and December 31, 2016, the Company was not a party to any litigation that could have a material adverse effect on the Company’s business, financial position, results of operations or cash flows.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers ("ASU 2014-09"). ASU 2014-09 and its related amendments (collectively referred to as ASC 606) requires that an entity recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. The new guidance requires the application of a five-step model to determine the amount and timing of revenue to be recognized. The underlying principle is that revenue is to be recognized for the transfer of goods or services to customers that reflects the amount of consideration that the Company expects to be entitled to in exchange for those goods or services. The Company adopted the standard as of the required effective date of January 1, 2018 using the modified retrospective method. Under this method, the new guidance was applied to contracts that were not yet completed as of January 1, 2018 with the cumulative effect recognized at the date of initial application. The Company finalized its review of its existing revenue contracts. As a result of its assessment, the Company determined that there may be a significant impact related to the recognition of grant revenue and capitalization of contract acquisition costs. Specifically, the Company deferred certain amounts related to grants that were not fixed and determinable under ASC 605 and will finalize the application of ASC 606, including the amount of variable consideration to be recognized subject to a constraint, resulting in a transition adjustment upon adoption that the Company does not expect will exceed $3.0 million. The Company also anticipates that contract acquisition costs, such as sublicense fees due to MIT related to the Spark agreement, will be capitalized and amortized over the period of performance, resulting in a transition adjustment upon adoption that the Company does not expect will exceed $2.6 million. The adoption of ASC 606 is expected to have an immaterial impact on the Company’s Collaboration Revenues at the date of adoption. Consistent with ASC 605, the Company concluded that the license and delivery of the underlying particles are not distinct, therefore, are considered a combined performance obligation. In addition, consistent with ASC 605, the license options are material rights because they were issued at a discount. The Company allocated the consideration to each material right and performance obligations using a relative selling price allocation. The Company’s collaboration agreements contain significant amounts that will become due upon the occurrence of certain events. These amounts include development, and regulatory milestones, which are considered part of the variable consideration. Upon adoption of ASC 606, the Company expects to conclude that the development and regulatory milestones are fully constrained. The Company will continue to evaluate the likelihood of the development and regulatory milestone events occurring and will include such amounts in the transaction price when it is probable that a significant revenue reversal will not occur in future periods. The arrangement also includes sales milestones and royalties, which will be recognized as revenue when the related sales occur, or when the milestone thresholds have been met using the royalty recognition constraint. The impact on the Company's collaboration revenue in 2018 and forward will depend on the timing and delivery of performance obligations, as well as the estimates of variable consideration, subject to the constraint. The deferred tax assets and liabilities resulting from these adjustments will be substantially offset by an associated adjustment to the Company valuation allowance. Therefore, as the Company currently maintains a full valuation allowance against its net deferred tax assets, the Company does not expect the adoption of ASC 606 to have a significant impact on its deferred tax balances or income tax expense in 2018. The potential impact listed above are estimates of the expected effects of the Company’s adoption of ASC 606 as of the time of preparation of this Annual Report on Form 10-K and will be finalized in connection with the first quarter. The anticipated accounting impacts described above will have no impact on cash flows. In November 2015, the FASB issued ASU No. 2015-17, Income Taxes (Topic 740), Balance Sheet Classification of Deferred Taxes. The new standard requires that deferred tax assets and liabilities be classified as non-current in a classified statement of financial position. The Company adopted this standard on January 1, 2017, and the adoption did not have an impact on its consolidated financial statements due to the full valuation allowance position. In January 2016, the FASB issued ASU No. 2016-01, Financial Instruments-Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities (“ASU 2016-01”). ASU 2016-01 supersedes the guidance to classify equity securities with readily determinable fair values into different categories (that is, trading or available-for-sale) and requires equity securities (including other ownership interests, such as partnerships, unincorporated joint ventures, and limited liability companies) to be measured at fair value with changes in fair value recognized through net income. The amendments allow equity investments that do not have readily determinable fair values to be remeasured at fair value either upon the occurrence of an observable price change or upon identification of an impairment. The amendments also require enhanced disclosures about those investments. ASU 2016-01 is effective for annual reporting beginning after December 15, 2017, including interim periods within the year of adoption, and calls for prospective application, with early application permitted. Accordingly, the standard is effective for the Company on January 1, 2018. Upon adoption of ASU 2016-01 on January 1, 2018, the Company expects the impact to be less than $0.1 million on its consolidated statement of operations. In February 2016, FASB issued ASU No. 2016-02, Leases (Topic 842) (“ASU 2016-02”). ASU 2016-02 requires a lessee to separate the lease components from the non-lease components in a contract and recognize in the statement of financial position a liability to make lease payments (the lease liability) and a right-of-use asset representing its right to use the underlying asset for the lease term. It also aligns lease accounting for lessors with the revenue recognition guidance in ASU 2014-09. ASU 2016-02 is effective for fiscal years beginning after December 15, 2018, including interim periods within those fiscal years. The Company is currently evaluating the effect of the adoption of this guidance on its consolidated financial statements. In March 2016, the FASB issued ASU No. 2016-9, Compensation-Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting (“ASU 2016-9”). ASU 2016-09 simplifies several aspects of the accounting for share-based payment transactions, including the accounting for income taxes, classification of awards as either equity or liabilities, the estimation of forfeitures, statutory tax withholding requirements, and classification in the statement of cash flows. This guidance is effective for the Company on January 1, 2017 and early adoption was permitted. The Company adopted ASU 2016-09 during the first fiscal quarter ended on March 31, 2017. As part of the adoption of this guidance, the Company elected to continue estimating forfeitures in its calculation of stock-based compensation expense. The excess tax benefits and tax deficiencies from stock-based compensation awards accounted for under ASC 718 are recognized as income tax benefit or expense, respectively, in the statement of operations and comprehensive loss. The income tax related items had no effect on the current period presentation as the Company maintains a full valuation allowance against its deferred tax assets. In addition, excess tax benefits for share-based payments are presented as an operating activity in the statement of cash flows rather than financing activity. The changes have been applied prospectively in accordance with the ASU and prior periods have not been adjusted. As a result of the adoption on January 1, 2017, the net operating losses deferred tax assets were increased by $0.3 million, which was offset by a corresponding increase in the valuation allowance. In August 2016, the FASB issued ASU No. 2016-15, Statements of Cash Flows (Topic 230) - Classifications of Certain Cash Receipts and Cash Payments ("ASU 2016-15"), to clarify how companies present and classify certain cash receipts and cash payments in the statement of cash flows. This guidance is effective for annual reporting periods beginning after December 15, 2017, including interim periods within that reporting period, with early adoption permitted. The Company does not expect the adoption of this guidance will have a material impact on its consolidated financial statements. In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows, Restricted Cash ("ASU 2016-18"). This guidance requires that a statement of cash flows explain the total change during the period of cash, cash equivalents, and amounts generally described as restricted cash or restricted cash equivalents. Amounts described as restricted cash and restricted cash equivalents should be included with cash and cash equivalents when reconciling the beginning of period and end of period to total amounts shown on the statement of cash flows. This guidance is effective for annual reporting periods beginning after December 15, 2017, including interim periods within that reporting period, with early adoption permitted. We will adopt ASU 2016-18 during the first quarter ending March 31, 2018. Upon adoption, the Company will present $0.4 million of restricted cash in the "Cash and cash equivalents at beginning of period" line in the consolidated cash flow statement. In January 2017, the FASB issued ASU No. 2017-01, “Business Combinations (Topic 805): Clarifying the Definition of a Business” (ASU 2017-01), which clarifies the definition of a business to assist entities with evaluating whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses. This ASU is effective for public entities for fiscal years beginning after December 15, 2017, with early adoption permitted. The Company does not expect the adoption of ASU 2017-01 will have a material impact on the its consolidated financial statements.

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2017

Accounting Policies [Abstract]

Components of Accumulated Other Comprehensive Income (Loss), Net of Tax

The components of accumulated other comprehensive income (loss), net of tax, were as follows (in thousands):



	Foreign currency translation adjustment			Unrealized gains (losses) on available-for-sale securities			Accumulated other comprehensive income (loss)
Balance at December 31, 2014	$	(3,876	)	$	—		$	(3,876	)
Other comprehensive loss during the year	(1,110		)	—			(1,110		)
Balance at December 31, 2015	$	(4,986	)	$	—		$	(4,986	)
Other comprehensive income (loss) during the year	504			(36		)	468
Balance at December 31, 2016	$	(4,482	)	$	(36	)	$	(4,518	)
Other comprehensive income during the year	78			20			98
Balance at December 31, 2017	$	(4,404	)	$	(16	)	$	(4,420	)

Available-for-Sale Marketable Securities (Tables)

12 Months Ended

Dec. 31, 2017

Investments, Debt and Equity Securities [Abstract]

Summary of Available-for-Sale Marketable Securities

The following tables summarize the Company’s available-for-sale marketable securities by major type of security as of December 31, 2017 and December 31, 2016 (in thousands):



	December 31, 2017
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
U.S. government agency bonds	$	12,798	$	—	$	(10	)	$	12,788
Corporate bonds	13,158		—		(6		)	13,152
Total available-for-sale marketable securities	$	25,956	$	—	$	(16	)	$	25,940



	December 31, 2016
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Corporate bonds	$	24,821	$	17	$	(53	)	$	24,785
Total available-for-sale marketable securities	$	24,821	$	17	$	(53	)	$	24,785

Schedule of Fair Values and Amortized Cost of Marketable Debt Securities by Contractual Maturity

The fair values and amortized cost of marketable debt securities by contractual maturity were as follows (in thousands):



	December 31, 2017				December 31, 2016
	Fair Value		Amortized Cost		Fair Value		Amortized Cost
Less than one year	$	25,940	$	25,956	$	24,785	$	24,821

Net Loss Per Share (Tables)

12 Months Ended

Dec. 31, 2017

Earnings Per Share [Abstract]

Computation of Basic and Diluted Net Loss Per Share

The following table sets forth the computation of basic and diluted net loss per share (in thousands, except share and per‑share data):



	Year Ended December 31,
	2017			2016			2015
Numerator:
Net loss	$	(65,321	)	$	(36,210	)	$	(25,174	)
Less: accretion on preferred stock	—			(4,566		)	(7,335		)
Net loss attributable to common stockholders	$	(65,321	)	$	(40,776	)	$	(32,509	)
Denominator:
Weighted‑average common shares outstanding—basic and diluted	20,425,050			10,493,939			2,150,422
Net loss per share attributable to common stockholders—basic and diluted	$	(3.20	)	$	(3.89	)	$	(15.13	)

Schedule of Potential Common Shares Issuable Upon Conversion of Warrants

Potential dilutive common share equivalents consist of the following:



	Year Ended December 31,
	2017	2016	2015
Stock options to purchase common stock	2,657,187	2,128,346	1,569,379
Stock warrants to purchase common stock	176,432	97,302	650,618
Redeemable convertible preferred stock	—	—	9,890,580
Redeemable convertible preferred stock warrants	—	—	26,832
Total	2,833,619	2,225,648	12,137,409

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2017

Fair Value Disclosures [Abstract]

Summary of Assets Measured at Fair Value on a Recurring Basis

Below is a summary of assets measured at fair value on a recurring basis (in thousands):



	December 31, 2017
	(Level 1)		(Level 2)		(Level 3)		Total
Cash equivalents:
Money market funds	$	39,478	$	—	$	—	$	39,478
U.S. government agency bonds	—		1,000		—		1,000
Total cash equivalents	$	39,478	$	1,000	$	—	$	40,478
Short-term investments:
Corporate bonds	$	13,152	$	—	$	—	$	13,152
U.S. government agency bonds	—		12,788		—		12,788
Total short-term investments	$	13,152	$	12,788	$	—	$	25,940



	December 31, 2016
	(Level 1)		(Level 2)		(Level 3)		Total
Cash equivalents:
Money market funds	$	183	$	—	$	—	$	183
Tri-party repurchase agreements	—		27,000		—		27,000
Total cash equivalents	$	183	$	27,000	$	—	$	27,183
Short-term investments:
Corporate bonds	$	24,785	$	—	$	—	$	24,785
Total short-term investments	$	24,785	$	—	$	—	$	24,785

Property and Equipment (Tables)

12 Months Ended

Dec. 31, 2017

Property, Plant and Equipment [Abstract]

Schedule of Property and Equipment

Property and equipment consists of the following (in thousands):



	December 31,			December 31,
	2017			2016
Laboratory equipment	$	5,138		$	4,713
Computer equipment and software	480			562
Leasehold improvements	278			222
Furniture and fixtures	235			226
Office equipment	118			64
Total property and equipment	6,249			5,787
Less accumulated depreciation	(4,158		)	(3,740		)
Property and equipment, net	$	2,091		$	2,047

Accrued Expenses (Tables)

12 Months Ended

Dec. 31, 2017

Payables and Accruals [Abstract]

Schedule of Accrued Expenses

Accrued expenses consist of the following (in thousands):



	December 31,		December 31,
	2017		2016
Payroll and employee related expenses	$	1,902	$	1,551
Current portion of deferred rent and lease incentive	72		29
Collaboration and licensing	1,020		—
Accrued patent fees	268		415
Accrued external research and development costs	3,578		794
Accrued professional and consulting services	859		459
Accrued grant refund	175		152
Accrued interest	89		81
Other	617		440
Accrued expenses	$	8,580	$	3,921

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2017

Commitments and Contingencies Disclosure [Abstract]

Schedule of Future Minimum Lease Payments

As of December 31, 2017, the aggregate future minimum lease payments related to these leases are as follows (in thousands):



Year ended December 31,
2018	$	1,437
2019	1,482
2020	375
Total minimum lease payments	$	3,294

Debt (Tables)

12 Months Ended

Dec. 31, 2017

Debt Disclosure [Abstract]

Schedule of Future Minimum Payments on the Term Loans

Future minimum payments on the 2017 Term Loan as of December 31, 2017 are as follows (in thousands):



2018	$	1,065
2019	4,535
2020	9,093
2021	8,665
2022	1,753
Total minimum debt payments	$	25,111
Less: Amount representing interest	(3,061		)
Less: Debt discount and deferred charges	(1,008		)
Less: Current portion of loans payable	—
Loans payable, net of current portion	$	21,042

Common Stock (Tables)

12 Months Ended

Dec. 31, 2017

Equity [Abstract]

Schedule of Authorized Shares of Common Stock for Future Issuance

The Company has authorized shares of common stock for future issuance as follows:



	Period ending
	December 31, 2017	December 31, 2016
Exercise of common warrants	176,432	97,302
Shares available for future stock incentive awards	923,440	939,317
Outstanding common stock options	2,657,187	2,128,346
Total	3,757,059	3,164,965

- Definition

Tabular disclosure of an entity's stock, including par or stated value per share, number and dollar amount of share subscriptions, shares authorized, shares issued, shares outstanding, number and dollar amount of shares held in an employee trust, dividend per share, total dividends, share conversion features, par value plus additional paid in capital, the value of treasury stock and other information necessary to a fair presentation, and EPS information. Stock by class includes common, convertible, and preferred stocks which are not redeemable or redeemable solely at the option of the issuer. Includes preferred stock with redemption features that are solely within the control of the issuer and mandatorily redeemable stock if redemption is required to occur only upon liquidation or termination of the reporting entity. If more than one issue is outstanding, state the title of each issue and the corresponding dollar amount; dollar amount of any shares subscribed but unissued and the deduction of subscriptions receivable there from; number of shares authorized, issued, and outstanding.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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-Section 50
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-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21506-112644

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 480
-SubTopic 10
-Section S99
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=65877616&loc=d3e177068-122764

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(d))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 4: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21553-112644

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
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-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21475-112644

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
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-Subparagraph (SX 210.5-02(28))
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Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=65888546&loc=d3e21300-112643

Reference 10: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(27))
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Reference 11: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21521-112644

Reference 12: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 505
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-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21484-112644

Reference 13: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
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-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21538-112644

Reference 14: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21488-112644

+ Details

Stock Incentive Plans (Tables)

12 Months Ended

Dec. 31, 2017

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Schedule of Weighted Average Assumptions Used for Stock Option Grants



	Year Ended December 31,
	2017			2016			2015
Risk-free interest rate	2.03		%	1.42		%	1.79		%
Dividend yield	—			—			—
Expected term	5.90			6.04			6.02
Expected volatility	84.52		%	94.97		%	79.80		%
Weighted-average fair value of common stock	$	15.32		$	13.25		$	7.35

The weighted average assumptions used in the Black-Scholes option pricing model for unvested non‑employee stock option awards were as follows:



	Year Ended December 31,
	2017	2016		2015
Risk‑free interest rate	—	2.06	%	1.57	%
Dividend yield	—	—		—
Expected term	0.0	8.80		6.46
Expected volatility	—	87.33	%	98.18	%

Summary of Activity Under the 2008 Plan and the 2016 Plan

The following table summarizes the activity under the 2008 Plan and the 2016 Plan:



					Weighted‑average
					remaining	Aggregate
	Number of		Weighted-average		contractual term	intrinsic value
	options		exercise price ($)		(in years)	(in thousands)
Employee Awards
Outstanding at December 31, 2016	1,809,945		$	7.57	7.26	$	17,459
Granted	882,900		$	15.32
Exercised	(203,378	)	$	2.54
Forfeited	(78,230	)	$	14.72
Outstanding at December 31, 2017	2,411,237		$	10.58	7.50	$	4,729
Vested at December 31, 2017	1,094,117		$	6.80	5.76	$	4,215
Vested and expected to vest at December 31, 2017	2,232,527		$	10.31	7.37	$	4,729

Non‑Employee Awards
Outstanding at December 31, 2016	318,342		$	4.55	5.96	$	4,044
Granted	—		$	—
Exercised	(67,392	)	$	3.97
Forfeited	(5,000	)	$	23.95
Outstanding at December 31, 2017	245,950		$	4.32	5.10	$	1,351
Vested at December 31, 2017	207,502		$	3.82	4.53	$	1,243
Vested and expected to vest at December 31, 2017	245,950		$	4.32	5.10	$	1,351

Schedule of Assumptions Used to Estimate the Fair Value of Purchase Rights Granted Under the ESPP



Fair value of common stock	$13.08	-	$18.02
Risk-free interest rate	0.79%	-	1.19%
Expected volatility	46.73%	-	75.78%
Expected term	0.1	-	0.5
Dividend yield	—	-	—

Schedule of Stock-Based Compensation Expense Related to Stock Options and Restricted Common Stock



	Year Ended December 31,
	2017		2016		2015
Research and development	$	1,779	$	1,183	$	495
General and administrative	2,302		847		630
Total	$	4,081	$	2,030	$	1,125

Income Taxes - (Tables)

12 Months Ended

Dec. 31, 2017

Income Tax Disclosure [Abstract]

Schedule of Effective Income Tax Rate Reconciliation



	Year Ended December 31,
	2017		2016		2015
Tax at U.S. statutory rate	34.0	%	34.0	%	34.0	%
Changes from statutory rate:
State taxes, net of federal benefit	5.8	%	5.8	%	5.7	%
Permanent items	(0.1	)%	(1.1	)%	(0.7	)%
Research tax credits/other	0.5	%	1.3	%	1.0	%
Change in enacted rates	(35.8	)%	—	%	—	%
Valuation allowance, net	(6.7	)%	(39.8	)%	(40.0	)%
Other	2.3	%	(0.2	)%	—	%
Effective income tax rate	—	%	—	%	—	%

Schedule of Tax Effects of Temporary Differences

Significant components of the Company's deferred tax assets (liabilities) consists of the following:



	December 31,
(In thousands)	2017			2016
Deferred tax assets
Net operating loss carryforwards	$	44,146		$	39,420
Research and development credits	4,092			3,052
Stock-based compensation expense	959			904
Deferred rent and other expenses	263			713
Deferred revenue	4,021			5,607
Patent costs/amortization	4,264			4,934
Total deferred tax assets	57,745			54,630

Deferred tax liabilities
Depreciation	$	(108	)	$	(131	)
Debt discount	—			(69		)
Unrealized foreign exchange gain	—			(1,525		)
Total deferred tax liabilities	(108		)	(1,725		)
Net deferred tax assets	$	57,637		$	52,905
Valuation allowance	(57,637		)	(52,905		)
Net deferred tax assets	$	—		$	—

Selected Quarterly Financial Data (Tables)

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

Schedule of Quarterly Financial Information

The following table summarizes unaudited quarterly financial data for the years ended December 31, 2017 and 2016 (in thousands, except per share data).



	Three Months Ended (unaudited)
	March 31,			June 30,			September 30,			December 31,
	2017			2017			2017			2017
Grant and collaboration revenue	$	137		$	26		$	27		$	17
Operating expenses	$	14,919		$	15,897		$	13,881		$	19,294
Net loss	$	(15,134	)	$	(15,967	)	$	(14,676	)	$	(19,544	)
Net loss attributable to common stockholders	$	(15,134	)	$	(15,967	)	$	(14,676	)	$	(19,544	)

Net loss per share attributable to common stockholders, basic and diluted	$	(0.82	)	$	(0.85	)	$	(0.66	)	$	(0.88	)

	Three Months Ended (unaudited)
	March 31,			June 30,			September 30,			December 31,
	2016			2016			2016			2016
Grant and collaboration revenue	$	2,088		$	2,017		$	1,048		$	2,930
Operating expenses	$	9,029		$	8,418		$	8,516		$	16,790
Net loss	$	(7,476	)	$	(6,923	)	$	(7,728	)	$	(14,083	)
Accretion on preferred stock	$	(2,356	)	$	(2,210	)	$	—		$	—
Net loss attributable to common stockholders	$	(9,832	)	$	(9,133	)	$	(7,728	)	$	(14,083	)

Net loss per share attributable to common stockholders, basic and diluted	$	(4.52	)	$	(2.75	)	$	(0.43	)	$	(0.77	)

Nature of the Business and Basis of Presentation - Liquidity (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Accumulated deficit	$ 216,897	$ 151,576
Cash and Cash Equivalents [Line Items]
Unrestricted cash held by subsidiary	70,622	58,656	$ 32,337	$ 16,592
Russian subsidiary \| Unrestricted cash
Cash and Cash Equivalents [Line Items]
Unrestricted cash held by subsidiary	$ 1,200	$ 2,400

Summary of Significant Accounting Policies - Segment Information (Details)	12 Months Ended
	Dec. 31, 2017 segment
Accounting Policies [Abstract]
Number of operating segments	1

Summary of Significant Accounting Policies - Reverse Stock Split (Details)	Jun. 07, 2016
Accounting Policies [Abstract]
Reverse stock split	3.9

Summary of Significant Accounting Policies - Cash Equivalents and Short Term Investments (Details)	12 Months Ended
	Dec. 31, 2017 USD ($)
Cash and Cash Equivalents [Line Items]
Realized gains or losses on sale of investments	$ 0
Minimum
Cash and Cash Equivalents [Line Items]
Agreements collateralized by U.S. treasury securities (no less than) (as a percent)	102.00%

Summary of Significant Accounting Policies - Concentrations of Credit Risk and Off-Balance Sheet Risk (Details) - Russian subsidiary $ in Millions	Dec. 31, 2017 USD ($)
Concentration Risk [Line Items]
Cash maintained in Russian bank accounts	$ 1.3
U.S. dollars
Concentration Risk [Line Items]
Cash maintained in Russian bank accounts	$ 1.2

Summary of Significant Accounting Policies - Fair Value of Financial Instruments (Details)	Dec. 31, 2017
Accounting Policies [Abstract]
Effective interest rate for debt arrangement	6.53%

Summary of Significant Accounting Policies - Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2017
Furniture
Property, Plant and Equipment [Line Items]
Estimated useful life	7 years
Equipment
Property, Plant and Equipment [Line Items]
Estimated useful life	5 years
Computer and office equipment
Property, Plant and Equipment [Line Items]
Estimated useful life	3 years

Summary of Significant Accounting Policies - Components of Accumulated Other Comprehensive Income (Loss), Net of Tax (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
AOCI Attributable to Parent, Net of Tax
Beginning balance	$ 54,957	$ (116,493)	$ (87,755)
Ending balance	51,814	54,957	(116,493)
Foreign currency translation adjustment
AOCI Attributable to Parent, Net of Tax
Beginning balance	(4,482)	(4,986)	(3,876)
Other comprehensive loss during the year	78	504	(1,110)
Ending balance	(4,404)	(4,482)	(4,986)
Unrealized gains (losses) on available-for-sale securities
AOCI Attributable to Parent, Net of Tax
Beginning balance	(36)	0	0
Other comprehensive loss during the year	20	(36)	0
Ending balance	(16)	(36)	0
Accumulated other comprehensive income (loss)
AOCI Attributable to Parent, Net of Tax
Beginning balance	(4,518)	(4,986)	(3,876)
Other comprehensive loss during the year	98	468	(1,110)
Ending balance	$ (4,420)	$ (4,518)	$ (4,986)

Summary of Significant Accounting Policies - Recent Accounting Pronouncements (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
	Jan. 01, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Mar. 31, 2018	Jan. 01, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accumulated deficit		$ (216,897)				$ (151,576)				$ (216,897)	$ (151,576)
Adjustment to the consolidated statements of operations		(19,544)	$ (14,676)	$ (15,967)	$ (15,134)	(14,083)	$ (7,728)	$ (6,923)	$ (7,476)	(65,321)	(36,210)	$ (25,174)
Increase in net operating losses deferred tax assets		44,146				$ 39,420				44,146	$ 39,420
Accounting Standards Update 2015-01 \| Restatement Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustment to the consolidated statements of operations	$ 100
Accounting Standards Update 2016-09, Excess Tax Benefit Component
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase in net operating losses deferred tax assets														$ 300
Increase in valuation allowance		$ 300								$ 300
Accounting Standards Update 2016-18 \| Scenario, Forecast
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cash and cash equivalents at beginning of period, presented upon implementation of ASU 2016-18													$ 400
Difference between Revenue Guidance in Effect before and after Topic 606 \| Accounting Standards Update 2014-09 \| Scenario, Forecast
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accumulated deficit	3,000
MIT \| Difference between Revenue Guidance in Effect before and after Topic 606 \| Accounting Standards Update 2014-09 \| Scenario, Forecast
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accumulated deficit	$ 2,600

Available-for-Sale Marketable Securities - Summary of Available-for-Sale Marketable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 25,956	$ 24,821
Gross Unrealized Gains	0	17
Gross Unrealized Losses	(16)	(53)
Estimated Fair Value	25,940	24,785
U.S. government agency bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	12,798
Gross Unrealized Gains	0
Gross Unrealized Losses	(10)
Estimated Fair Value	12,788
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	13,158	24,821
Gross Unrealized Gains	0	17
Gross Unrealized Losses	(6)	(53)
Estimated Fair Value	$ 13,152	$ 24,785

Available-for-Sale Marketable Securities - Schedule of Fair Values and Amortized Cost of Marketable Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value
Less than one year	$ 25,940	$ 24,785
Amortized Cost
Less than one year	$ 25,956	$ 24,821

Available-for-Sale Marketable Securities - Narrative (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) position
Investments, Debt and Equity Securities [Abstract]
Number of investment positions \| position	12
Fair value of investments, unrealized loss position	$ 13,200,000
Number of investment position in an unrealized loss position, less than twelve months \| position	8
Fair value investments, unrealized loss position, less than twelve months	$ 12,800,000
Other-than-temporary impairments	$ 0

Net Loss Per Share - Computation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net loss	$ (19,544)	$ (14,676)	$ (15,967)	$ (15,134)	$ (14,083)	$ (7,728)	$ (6,923)	$ (7,476)	$ (65,321)	$ (36,210)	$ (25,174)
Less: accretion on preferred stock					0	0	(2,210)	(2,356)	0	(4,566)	(7,335)
Net loss attributable to common stockholders	$ (19,544)	$ (14,676)	$ (15,967)	$ (15,134)	$ (14,083)	$ (7,728)	$ (9,133)	$ (9,832)	$ (65,321)	$ (40,776)	$ (32,509)
Denominator:
Weighted‑average common shares outstanding—basic and diluted (in shares)									20,425,050	10,493,939	2,150,422
Net loss per share attributable to common stockholders—basic and diluted (in dollars per share)	$ (0.88)	$ (0.66)	$ (0.85)	$ (0.82)	$ (0.77)	$ (0.43)	$ (2.75)	$ (4.52)	$ (3.20)	$ (3.89)	$ (15.13)

Net Loss Per Share - Schedule of Potential Common Shares Issuable Upon Conversion of Warrants (Details) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Potential common shares
Total (in shares)	2,833,619	2,225,648	12,137,409
Stock options to purchase common stock
Potential common shares
Total (in shares)	2,657,187	2,128,346	1,569,379
Stock warrants to purchase common stock
Potential common shares
Total (in shares)	176,432	97,302	650,618
Redeemable convertible preferred stock
Potential common shares
Total (in shares)	0	0	9,890,580
Redeemable convertible preferred stock warrants
Potential common shares
Total (in shares)	0	0	26,832

Fair Value Measurements - Summary of Assets Measured at Fair Value on a Recurring Basis (Details) - Recurring - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in cash equivalents	$ 40,478	$ 27,183
Included in investments	25,940	24,785
(Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in cash equivalents	39,478	183
Included in investments	13,152	24,785
(Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in cash equivalents	1,000	27,000
Included in investments	12,788	0
(Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in cash equivalents	0	0
Included in investments	0	0
Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in investments	13,152	24,785
Corporate bonds \| (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in investments	13,152	24,785
Corporate bonds \| (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in investments	0	0
Corporate bonds \| (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in investments	0	0
U.S. government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in investments	12,788
U.S. government agency bonds \| (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in investments	0
U.S. government agency bonds \| (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in investments	12,788
U.S. government agency bonds \| (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in investments	0
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in cash equivalents	39,478	183
Money market funds \| (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in cash equivalents	39,478	183
Money market funds \| (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in cash equivalents	0	0
Money market funds \| (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in cash equivalents	0	0
U.S. government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in cash equivalents	1,000
U.S. government agency bonds \| (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in cash equivalents	0
U.S. government agency bonds \| (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in cash equivalents	1,000
U.S. government agency bonds \| (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in cash equivalents	$ 0
Tri-party repurchase agreements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in cash equivalents		27,000
Tri-party repurchase agreements \| (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in cash equivalents		0
Tri-party repurchase agreements \| (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in cash equivalents		27,000
Tri-party repurchase agreements \| (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in cash equivalents		$ 0

Fair Value Measurements - Narrative (Details)	12 Months Ended
Fair Value Measurements - Narrative (Details)	Dec. 31, 2017	Dec. 31, 2016
Corporate Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Average maturity, term	179 days	297 days
U.S. government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Average maturity, term	168 days
Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Tri-party repurchase agreements collateralized by government securities (not less than) (as a percent)	102.00%

Property and Equipment - Schedule of Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 6,249	$ 5,787
Less accumulated depreciation	(4,158)	(3,740)
Property and equipment, net	2,091	2,047
Laboratory equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	5,138	4,713
Computer equipment and software
Property, Plant and Equipment [Line Items]
Total property and equipment	480	562
Leasehold improvements
Property, Plant and Equipment [Line Items]
Total property and equipment	278	222
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Total property and equipment	235	226
Office equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 118	$ 64

Property and Equipment - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Abstract]
Depreciation expense	$ 750	$ 802	$ 1,044

Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Payables and Accruals [Abstract]
Payroll and employee related expenses	$ 1,902	$ 1,551
Current portion of deferred rent and lease incentive	72	29
Collaboration and licensing	1,020	0
Accrued patent fees	268	415
Accrued external research and development costs	3,578	794
Accrued professional and consulting services	859	459
Accrued grant refund	175	152
Accrued interest	89	81
Other	617	440
Accrued expenses	$ 8,580	$ 3,921

Commitments and Contingencies - Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2012 USD ($)	Oct. 31, 2017 ft²
Operating Leased Assets [Line Items]
Area of additional office space leased \| ft²					5,100
Deferred rent and lease incentive liability	$ 200	$ 300
Current portion of deferred rent and lease incentive liability (less than)	72	29
Rent expense, net of sublease payments	1,900	1,600	$ 1,100
Restricted cash and other deposits \| Letter of credit
Operating Leased Assets [Line Items]
Letter of credit	300
Accrued expenses
Operating Leased Assets [Line Items]
Current portion of deferred rent and lease incentive liability (less than)	$ 100	$ 100
Maximum \| Laboratory and office space
Operating Leased Assets [Line Items]
Tenant improvement allowance				$ 700

Commitments and Contingencies - Schedule of Future Minimum Lease Payments (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Year ended December 31,
2018	$ 1,437
2019	1,482
2020	375
Total minimum lease payments	$ 3,294

Debt - Term Loans (Narrative) (Details) - USD ($)		1 Months Ended			12 Months Ended
Debt - Term Loans (Narrative) (Details) - USD ($)	Jun. 21, 2016	Dec. 31, 2016	Jul. 31, 2014	Aug. 31, 2013	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Sep. 12, 2017	Aug. 09, 2013
Line of Credit Facility [Line Items]
Borrowings on loan and security agreement					$ 20,957,000	$ 0	$ 6,674,000
Loss on extinguishment of debt					673,000	0	0
Minimum assets trigger					300,000
Minimum amount in excess of indebtedness trigger					300,000
Interest expense related to the Term Loan					$ 1,206,000	1,253,000	$ 948,000
Conversion of Series D Preferred
Line of Credit Facility [Line Items]
Expiration period of warrants	10 years
Conversion of Series E Preferred
Line of Credit Facility [Line Items]
Warrants issued for purchase of stock (in shares)							37,978
Expiration period of warrants	10 years
Convertible Debt \| Conversion of Series E Preferred
Line of Credit Facility [Line Items]
Exercise price (in dollars per share)	$ 11.32
Term Loans
Line of Credit Facility [Line Items]
Borrowing capacity							$ 12,000,000.0		$ 7,500,000.0
Borrowings on loan and security agreement			$ 4,500,000	$ 3,000,000
Interest rate per annum					8.10%
Number of monthly installments					30 months
Final payment fee (as a percent)					6.00%
Final payment fee					$ 700,000
Initial grant date fair value of warrants	$ 100,000		$ 100,000
Number of warrants exercised to purchase common stock (in shares)		16,493
Net shares of common stock issued (in shares)		4,697
Repayments of Lines of Credit					$ 10,000,000
Term Loans \| Conversion of Series D Preferred \| Common stock
Line of Credit Facility [Line Items]
Warrants issued for purchase of stock (in shares)	17,888
Term Loans \| Conversion of Series D Preferred \| Maximum
Line of Credit Facility [Line Items]
Warrants issued for purchase of stock (in shares)	66,668		40,000	26,668
Term Loans \| Conversion of Series E Preferred \| Common stock
Line of Credit Facility [Line Items]
Warrants issued for purchase of stock (in shares)	15,094
Term Loans \| Prepaid after to first anniversary but before second
Line of Credit Facility [Line Items]
Prepayment fee (as a percent)					2.00%
Term Loans \| Prepaid after second anniversary
Line of Credit Facility [Line Items]
Prepayment fee (as a percent)					1.00%
Term Loans \| Convertible Debt \| Conversion of Series D Preferred \| Common stock
Line of Credit Facility [Line Items]
Exercise price (in dollars per share)	$ 16.77
2017 Term Loans
Line of Credit Facility [Line Items]
Interest rate per annum					4.00%
Final payment fee (as a percent)					5.00%
Final payment fee					$ 1,100,000
Outstanding principal balance under Term Loans		$ 12,000,000			21,000,000	12,000,000		$ 21,000,000
Loss on extinguishment of debt					700,000
Debt Issuance Costs, Line of Credit Arrangements, Net					100,000
Interest expense related to the Term Loan					$ 1,200,000	$ 1,300,000	$ 700,000
2017 Term Loans \| Term Loan Prepaid Before the First Anniversary [Member]
Line of Credit Facility [Line Items]
Prepayment fee (as a percent)					3.00%
2017 Term Loans \| Prepaid after to first anniversary but before second
Line of Credit Facility [Line Items]
Prepayment fee (as a percent)					2.00%
2017 Term Loans \| Prepaid after second anniversary
Line of Credit Facility [Line Items]
Prepayment fee (as a percent)					1.00%

Debt - Schedule of Future Minimum Payments on the Term Loans (Details) - 2017 Term Loans $ in Thousands	Dec. 31, 2017 USD ($)
Year ended December 31,
2018	$ 1,065
2019	4,535
2020	9,093
2021	8,665
2022	1,753
Total minimum debt payments	25,111
Less: Amount representing interest	(3,061)
Less: Debt discount and deferred charges	(1,008)
Less: Current portion of loans payable	0
Loans payable, net of current portion	$ 21,042

Debt - Convertible Notes (Narrative) (Details) - USD ($)			12 Months Ended
	Jun. 21, 2016	Jul. 24, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Apr. 30, 2015
Convertible notes
Debt Conversion [Line Items]
Convertible notes interest rate per annum						8.00%
Percentage of convertible notes for which warrants were issued to purchase common stock						20.00%
Exercise price (in dollars per share)		$ 17.55				$ 17.55
Warrants issued for purchase of stock (in shares)		80,813
Expiration period of warrants		3 years
Interest expense related to convertible notes			$ 0	$ 0	$ 300,000
Conversion of Series E Preferred
Debt Conversion [Line Items]
Warrants issued for purchase of stock (in shares)					37,978
Expiration period of warrants	10 years
Conversion of Series E Preferred \| Convertible notes
Debt Conversion [Line Items]
Shares issued on conversion of outstanding convertible notes (in shares)		1,619,550
Maximum \| Conversion of Series E Preferred \| Convertible notes
Debt Conversion [Line Items]
Shares issued on conversion of outstanding convertible notes (in shares)		103,817

Preferred Stock (Details) - $ / shares	Jun. 27, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Temporary Equity Disclosure [Abstract]
Authorized shares of Preferred Stock (in shares)				37,835,623
Preferred Stock, par value (in dollars per share)				$ 0.0001	$ 0.0001
Preferred stock
Preferred stock, shares outstanding		0	0
Preferred stock, shares issued		0	0
Preferred stock automatically converted into common stock upon the closing of the IPO
Preferred stock
Convertible preferred stock automatically converted into common stock (in shares)	10,126,118
Series A Redeemable Convertible Preferred Stock
Preferred stock
Preferred stock, shares outstanding					2,589,868
Preferred stock, shares issued				2,589,868
Series B Preferred
Preferred stock
Preferred stock, shares outstanding					7,437,325
Preferred stock, shares issued				7,437,325
Series C Redeemable Convertible Preferred Stock
Preferred stock
Preferred stock, shares outstanding					5,000,002
Preferred stock, shares issued				5,000,002
Series D Preferred Stock [Member]
Preferred stock
Preferred stock, shares outstanding					8,099,994
Preferred stock, shares issued				8,099,994
Series SRN Redeemable Convertible Preferred Stock
Preferred stock
Preferred stock, shares outstanding					2,111,109
Preferred stock, shares issued				2,111,109
Series E Preferred Stock [Member]
Preferred stock
Preferred stock, shares outstanding					8,888,888
Preferred stock, shares issued				8,888,888

Common Stock - Narrative (Details)					1 Months Ended	12 Months Ended
Common Stock - Narrative (Details)	Aug. 10, 2017 USD ($)	Jun. 23, 2017 $ / shares	Jul. 25, 2016 USD ($) $ / shares shares	Jun. 21, 2016 USD ($) $ / shares shares	Jul. 25, 2016 USD ($) $ / shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($)	Jun. 27, 2016 shares
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction						22,343,254
Common stock, par value (in dollars per share) \| $ / shares						$ 0.0001	$ 0.0001
Proceeds from issuance of common stock \| $						$ 10,000,000	$ 2,743,000	$ 0
Proceeds from Initial Public Offering, net of underwriters' discounts and commissions \| $						$ 0	$ 68,870,000	$ 0
Common stock issued for convertible preferred stock (in shares)									10,126,118
Common stock, shares authorized						200,000,000	200,000,000
Number of votes per share that common stockholders are entitled to						1
Dividends declared or paid on common stock \| $						$ 0
Common stock, shares outstanding						22,343,254	18,438,742
At-The-Market Offering [Member]
Subsidiary, Sale of Stock [Line Items]
Public securities offering, amount authorized \| $	$ 200,000,000.0
Public stock offering, amount authorized \| $						$ 50,000,000
Sale of stock, number of shares issued in transaction						0
2017 PIPE
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction						2,750,000
Common stock, par value (in dollars per share) \| $ / shares						$ 0.0001
Sale of stock, price per share \| $ / shares						$ 16.00
Proceeds from issuance of common stock \| $						$ 47,100,000
Springer Purchase Agreement
Subsidiary, Sale of Stock [Line Items]
Sale of stock, price per share \| $ / shares						$ 17.71
Exercise of common warrants
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction						79,130
Exercise price (in dollars per share) \| $ / shares		$ 17.71
Expiration period of warrants		5 years				5 years
Share price \| $ / shares		$ 0.125
IPO
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction			289,633	5,000,000
Sale of stock, price per share \| $ / shares				$ 14.00
Proceeds from Initial Public Offering, net of underwriters' discounts and commissions \| $				$ 60,800,000	$ 64,500,000
Over-Allotment Option
Subsidiary, Sale of Stock [Line Items]
Sale of stock, price per share \| $ / shares			$ 14.00		$ 14.00
Proceeds from Initial Public Offering, net of underwriters' discounts and commissions \| $			$ 3,700,000
Common stock \| Springer Purchase Agreement
Subsidiary, Sale of Stock [Line Items]
Stock issued (in shares)						338,791

Common Stock - Schedule of Authorized Shares of Common Stock for Future Issuance (Details) - shares	Dec. 31, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Total (in shares)	3,757,059	3,164,965
Shares available for future stock incentive awards
Class of Stock [Line Items]
Total (in shares)	923,440	939,317
Outstanding common stock options
Class of Stock [Line Items]
Total (in shares)	2,657,187	2,128,346
Exercise of common warrants
Class of Stock [Line Items]
Total (in shares)	176,432	97,302

Stock Incentive Plans - Additional Information (Narrative) (Details) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available for future grants	3,757,059	3,164,965
Stock‑based compensation expense \| $	$ 4,081	$ 2,051	$ 1,125
Shares available for future stock incentive awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available for future grants	923,440	939,317
Estimated forfeitures rate	10.00%
Weighted average grant date fair value of stock options (in dollars per share) \| $ / shares	$ 10.97	$ 10.23	$ 5.03
Aggregate intrinsic value of stock options exercised (less than in 2016) \| $	$ 2,600	$ 800	$ 300
Unrecognized compensation expense related to unvested employee stock options \| $	$ 10,200	$ 5,600
Weighted average period for recognition	2 years 10 months 24 days	3 years
Non-employee stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested stock option awards (in shares)	55,073
Unrecognized compensation expense related to unvested non-employee stock options \| $	$ 600	$ 1,100
2008 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for grants	2,213,412
Number of shares available for future grants	18,947
2008 Plan \| Shares available for future stock incentive awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting term	4 years
Award expiration term	10 years
2008 Plan \| Shares available for future stock incentive awards \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Combined voting power of participants (as a percent)	10.00%
2008 Plan \| Shares available for future stock incentive awards \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Combined voting power of participants (as a percent)	10.00%
2016 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for grants	1,947,779
Automatic increase in the number of shares that may be issued (as a percent)	0.04
Number of shares available for future grants	563,300
2016 Plan \| Shares available for future stock incentive awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting term	4 years
Award expiration term	10 years
2016 Plan \| Shares available for future stock incentive awards \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Combined voting power of participants (as a percent)	10.00%
2016 Plan \| Shares available for future stock incentive awards \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Combined voting power of participants (as a percent)	10.00%
Tranche One \| 2008 Plan \| Shares available for future stock incentive awards \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price of incentive stock options (as a percent)	100.00%
Tranche One \| 2016 Plan \| Shares available for future stock incentive awards \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price of incentive stock options (as a percent)	100.00%
Tranche Two \| 2008 Plan \| Shares available for future stock incentive awards \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price of incentive stock options (as a percent)	110.00%
Tranche Two \| 2016 Plan \| Shares available for future stock incentive awards \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price of incentive stock options (as a percent)	110.00%

Stock Incentive Plans - Schedule of Weighted Average Assumptions for Employee Stock Option Grants and Assumptions Used to Estimate the Fair Value of Purchase Rights Granted (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
The ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend yield	0.00%
Employee stock option grants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	2.03%	1.42%	1.79%
Dividend yield	0.00%	0.00%	0.00%
Expected term	5 years 10 months 24 days	6 years 16 days	6 years 6 days
Expected volatility	84.52%	94.97%	79.80%
Weighted-average fair value of common stock	$ 15.32	$ 13.25	$ 7.35
Minimum \| The ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of common stock (in dollars per share)	$ 13.08
Risk-free interest rate	0.79%
Expected term	1 month 2 days
Expected volatility	46.73%
Maximum \| The ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of common stock (in dollars per share)	$ 18.02
Risk-free interest rate	1.19%
Expected term	6 months
Expected volatility	75.78%

Stock Incentive Plans - Schedule of Weighted Average Assumptions for Unvested Non-Employee Stock Options (Details) - Non-employee stock options	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Goods and Nonemployee Services Transaction [Line Items]
Risk‑free interest rate	0.00%	2.06%	1.57%
Expected dividend yield (as a percent)	0.00%	0.00%	0.00%
Expected term	0 days	8 years 9 months 18 days	6 years 5 months 16 days
Expected volatility (as a percent)	0.00%	87.33%	98.18%

Stock Incentive Plans - Summary of Activity Under the 2008 Plan and the 2016 Plan (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Employee Awards
Aggregate intrinsic value
Vested and expected to vest	$ 2,600	$ 800	$ 300
The 2008 Plan and the 2016 Plan \| Employee Awards
Number of Options
Beginning balance (in shares)	1,809,945
Granted (in shares)	882,900
Exercised (in shares)	(203,378)
Forfeited (in shares)	(78,230)
Ending balance (in shares)	2,411,237	1,809,945
Vested (in shares)	1,094,117
Vested and expected to vest (in shares)	2,232,527
Weighted-average exercise price
Beginning balance (in dollars per share)	$ 7.57
Granted (in dollars per share)	15.32
Exercised (in dollars per share)	2.54
Forfeited (in dollars per share)	14.72
Ending balance (in dollars per share)	10.58	$ 7.57
Vested (in dollars per share)	6.80
Vested and expected to vest (in dollars per share)	$ 10.31
Weighted-average remaining contractual term
Outstanding, term	7 years 6 months	7 years 3 months 4 days
Vested, term	5 years 9 months 4 days
Vested and expected to vest, term	7 years 4 months 13 days
Aggregate intrinsic value
Outstanding	$ 4,729	$ 17,459
Vested	4,215
Vested and expected to vest	$ 4,729
The 2008 Plan and the 2016 Plan \| Non‑Employee Awards
Number of Options
Beginning balance (in shares)	318,342
Granted (in shares)	0
Exercised (in shares)	(67,392)
Forfeited (in shares)	(5,000)
Ending balance (in shares)	245,950	318,342
Vested (in shares)	207,502
Vested and expected to vest (in shares)	245,950
Weighted-average exercise price
Beginning balance (in dollars per share)	$ 4.55
Granted (in dollars per share)	0.00
Exercised (in dollars per share)	3.97
Forfeited (in dollars per share)	23.95
Ending balance (in dollars per share)	4.32	$ 4.55
Vested (in dollars per share)	3.82
Vested and expected to vest (in dollars per share)	$ 4.32
Weighted-average remaining contractual term
Outstanding, term	5 years 1 month 6 days	5 years 11 months 16 days
Vested, term	4 years 6 months 11 days
Vested and expected to vest, term	5 years 1 month 6 days
Aggregate intrinsic value
Outstanding	$ 1,351	$ 4,044
Vested	1,243
Vested and expected to vest	$ 1,351

Stock Incentive Plans - Employee Stock Purchase Plan (Narrative) (Details)		3 Months Ended	12 Months Ended
	Jun. 21, 2016 offering_period	Dec. 31, 2016 USD ($) shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($)	Jun. 07, 2016 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)		3,164,965	3,757,059	3,164,965
Stock-based compensation expense (less than in 2017) \| $			$ 4,081,000	$ 2,051,000	$ 1,125,000
ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of offering periods under the ESPP \| offering_period	2
Term of each offering period	6 months
Eligible compensation withheld for use (up to) (as a percent)	25.00%
Purchase price of shares (as a percent)	85.00%
Common stock authorized and reserved for future issuance (in shares)			341,193			357,456
Number of shares of common stock outstanding (as a percent)	0.01
Shares of common stock issued to employees under the ESPP (in shares)			16,263
Stock-based compensation expense (less than in 2017) \| $		$ 0	$ 100,000	$ 0

Stock Incentive Plans - Schedule of Stock-Based Compensation Expense Related to Stock Options and Restricted Common Stock (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total	$ 4,081	$ 2,051	$ 1,125
Stock options and restricted common stock
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total	4,081	2,030	1,125
Stock options and restricted common stock \| Research and development
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total	1,779	1,183	495
Stock options and restricted common stock \| General and administrative
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total	$ 2,302	$ 847	$ 630

Stock Incentive Plans - Restricted Stock (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Proceeds from issuance of common stock	$ 10,000	$ 2,743	$ 0
Restricted shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Issuance of common stock upon exercise of options (in shares)					30,317
Stock issued (in shares)				2,564
Proceeds from issuance of common stock				$ 100

Revenue Arrangements - Spark license agreement (Narrative) (Details)			1 Months Ended	12 Months Ended
	Oct. 31, 2017 USD ($) $ / shares shares	Dec. 02, 2016 USD ($) contract target	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) shares
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Sale of stock, number of shares issued in transaction \| shares				22,343,254
Spark License Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront cash payment		$ 10,000,000.0
Aggregate additional payments due within twelve months		$ 5,000,000.0
Term loan number of payments \| contract		2
Two payments due within 12 months of contract date		$ 2,500,000.0
Number of additional targets \| target		4
Variable fee the Company is eligible to receive for each additional target option elected		$ 2,000,000.0
Election period to exercise additional targets		3 years
Additional development and regulatory milestone payments		$ 65,000,000.0
Commercialization milestone payments		$ 365,000,000.0
Period after first commercial sale when the Company is eligible to receive royalties		10 years
Period for prior written notice to terminate license		90 days
Gross proceeds			$ 5,000,000
Share price (in dollars per share) \| $ / shares			$ 25.35
Premium of average daily VWAP (as a percent)			115.00%
Consecutive calendar days leading up to and ending on the day prior to the Contract Date			30 days
Sale of stock, number of shares issued in transaction \| shares			197,238
Number of shares authorized to be issued pursuant to the Stock Purchase Agreement \| shares				2,758,112
First Acquisition Right
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Aggregate price of shares Spark will have the right to purchase				$ 5,000,000.0
Second Acquisition Right
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Gross proceeds	$ 5,000,000
Share price (in dollars per share) \| $ / shares	$ 24.36
Premium of average daily VWAP (as a percent)	115.00%
Consecutive calendar days leading up to and ending on the day prior to the Contract Date	30 days
Aggregate price of shares Spark will have the right to purchase				5,000,000.0
Sale of stock, number of shares issued in transaction \| shares	205,254
MIT \| Spark License Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
License and option agreement, additional sub-license payment received liability				2,200,000
License and option agreement, payments made relative to calculated premium paid for initial equity investment made under the purchase agreement				$ 400,000

Revenue Arrangements - Accounting Analysis (Narrative) (Details)		12 Months Ended
	Dec. 02, 2016 USD ($) target	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Fair value of initial purchase of common stock combined with the embedded future stock Acquisition Rights		$ 10,000,000	$ 2,743,000
Deferred revenue recorded as a current liability		787,000	1,836,000
Deferred revenue classified as a long term liability		15,919,000	12,439,000
Spark License Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Number of additional target therapy options \| target	4
Fair value of initial purchase of common stock combined with the embedded future stock Acquisition Rights	$ 2,700,000
Total non-contingent payments	15,000,000
Consideration allocated to combined unit of accounting for License and Supply Deliverable	7,100,000
Discount associated with Option Deliverable	$ 5,200,000
Estimated initial term to recognize expected deliveries	4 years
Revenue recognized under agreement (less than in 2017)		100,000	$ 100,000.0	$ 0
Deferred revenue related to agreement		14,700,000
Deferred revenue recorded as a current liability		800,000
Deferred revenue classified as a long term liability		$ 13,900,000

Revenue Arrangements - Spark letter agreement (Details) - USD ($) $ / shares in Units, $ in Thousands				12 Months Ended
	Oct. 31, 2017	Jun. 06, 2017	Dec. 02, 2016	Dec. 31, 2017	Dec. 31, 2016
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Issuance of common stock, license agreement				$ 10,000	$ 2,743
Spark Letter Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Number of shares authorized to be issued pursuant to the Stock Purchase Agreement	205,254	324,362
Aggregate purchase price	$ 5,000	$ 5,000
Aggregate purchase price per share (in dollar per share)	$ 24.36	$ 15.41
Premium of average daily VWAP (as a percent)	115.00%
Issuance of common stock, license agreement			$ 2,700
Sale common stock	$ 2,500
Consecutive calendar days leading up to and ending on the day prior to the Contract Date	30 months

Revenue Arrangements - Sanofi Collaboration Agreement (Narrative) (Details) - Sanofi - USD ($)		1 Months Ended				3 Months Ended	12 Months Ended		62 Months Ended
	Nov. 27, 2012	Jul. 31, 2016	May 31, 2015	Aug. 31, 2013	Nov. 30, 2012	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront payment for initial indication					$ 2,000,000
Additional payment for initial indication				$ 3,000,000
Additional payments received upon exercise of option			$ 2,000,000
Additional payment upon attaining first milestone for celiac disease indication		$ 1,000,000
Aggregate payments under Sanofi Agreement									$ 8,000,000
Initial period to complete research obligations	3 years
Revenue recognized						$ 0	$ 2,800,000	$ 2,100,000

Revenue Arrangements - Termination of the Sanofi Collaboration Agreement (Narrative) (Details) - Sanofi - USD ($) $ in Millions	12 Months Ended	62 Months Ended
	Dec. 31, 2016	Dec. 31, 2017
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Aggregate payments under Sanofi Agreement		$ 8.0
Remaining revenue recognized	$ 2.2

Revenue Arrangements - Other Research and Collaboration Agreements (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Research And Collaboration Agreements
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue recognized (less than in 2016)	$ 0	$ 100,000	$ 300,000

Revenue Arrangements - NIH (Narrative) (Details) - NIH	1 Months Ended		12 Months Ended
Revenue Arrangements - NIH (Narrative) (Details) - NIH	Sep. 30, 2015 USD ($)	May 31, 2014 USD ($)	Dec. 31, 2017 USD ($) grant	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Number of grants \| grant			2
First Grant
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Aggregate amount of grant		$ 8,100,000
Revenue recognized			$ 0	$ 4,100,000	$ 2,400,000
Second Grant
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Aggregate amount of grant	$ 200,000
Revenue recognized			$ 100,000	$ 100,000	$ 100,000

Revenue Arrangements - JDRF (Narrative) (Details) - JDRF - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue recognized		$ 0.1	$ 0.1	$ 0.2
Second Contract
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Aggregate amount of grant	$ 0.4

Revenue Arrangements - Bill and Melinda Gates Foundation (Narrative) (Details) - Bill and Melinda Gates Foundation - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Aggregate amount of grant				$ 1.6	$ 1.2
Extended research term			3 years
Revenue recognized (less than)	$ 0.1	$ 0.5	$ 0.6

Revenue Arrangements - Minpromtorg (Narrative) (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended			30 Months Ended	35 Months Ended
	Feb. 28, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2015	Dec. 31, 2017
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Payments received		$ 0	$ (461)	$ (805)
Minpromtorg
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Contract awarded					$ 4,600
Repayment of funds previously received	$ 200
Penalty fee (as a percent)	10.00%
Penalty fee	$ 200
Period to audit expenditure incurred	3 years
Payments received						$ 1,400
Revenue recognized			$ 500	$ 400

Revenue Arrangements - Skolkovo (Narrative) (Details) - USD ($)		12 Months Ended
	Nov. 28, 2014	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Payments received		$ 0	$ (461,000)	$ (805,000)
Skolkovo
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Aggregate amount of grant	$ 2,700,000.0
Percentage of estimated total cost of research plan	48.50%
Remaining percentage of estimated costs to be contributed by the Company	51.50%
Payments received		$ 1,800,000
Period after completion of project when Company may be required to reimburse the funds received		3 years

Related-Party Transactions (Details) - USD ($) $ / shares in Units, $ in Millions			5 Months Ended	12 Months Ended
	Jun. 23, 2017	Jun. 21, 2016	Aug. 31, 2014	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Apr. 30, 2015
Related Party Transaction [Line Items]
Sale of stock, number of shares issued in transaction				22,343,254
Series D Redeemable Convertible Preferred Stock
Related Party Transaction [Line Items]
Expiration period of warrants		10 years
Conversion of Series E Preferred
Related Party Transaction [Line Items]
Expiration period of warrants		10 years
Landlord \| Series B Preferred
Related Party Transaction [Line Items]
Stock issued (in shares)			49,254
Share price (in dollars per share)			$ 2.03
Total proceeds			$ 0.1
Landlord \| Series D Redeemable Convertible Preferred Stock
Related Party Transaction [Line Items]
Stock issued (in shares)			488,888
Share price (in dollars per share)			$ 4.50
Total proceeds			$ 2.2
Landlord \| Conversion of Series E Preferred
Related Party Transaction [Line Items]
Bridge loan							$ 0.2
Founders
Related Party Transaction [Line Items]
Expenses for consulting services				$ 0.2	$ 0.3	$ 0.3
Springer Purchase Agreement
Related Party Transaction [Line Items]
Sale of stock, price per share				$ 17.71
Springer Purchase Agreement \| Common stock
Related Party Transaction [Line Items]
Stock issued (in shares)				338,791
Stock warrants to purchase common stock
Related Party Transaction [Line Items]
Sale of stock, number of shares issued in transaction				79,130
Exercise price (in dollars per share)	$ 17.71
Expiration period of warrants	5 years			5 years
Share price	$ 0.125

Technology License Agreements - MIT (Narrative) (Details) - MIT - USD ($)		12 Months Ended
	Nov. 25, 2008	Dec. 31, 2017
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Payment amount upon achievement of defined clinical milestones	$ 1,500,000.0
Spark License Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contractual payments defined in the Exclusive Patent License agreement		$ 2,200,000
License and option agreement, payments made relative to calculated premium paid for initial equity investment made under the purchase agreement		$ 400,000
Minimum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of sublicense income	10.00%
Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of sublicense income	30.00%

Technology License Agreements - Shenyang Sunshine Pharmaceutical Co., Ltd (Narrative) (Details) - 3SBio License - USD ($) $ in Millions	1 Months Ended
	May 31, 2014	Dec. 31, 2017	Dec. 31, 2016
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Period after approval to commercialize product	48 months
Aggregate amount of upfront and milestone-based payments		$ 1.0
Additional liability for milestone payments expensed in 2016			$ 2.0
Aggregate amount for future payments upon achievement of clinical and regulatory approval milestones for products containing SVP technology	$ 21.0
Aggregate amount for future payments upon achievement of clinical and regulatory approval milestones for products without SVP technology	$ 41.5
Maximum paid on tiered royalties (percent)	10.00%
Period for prior written notice to terminate license	60 days

Technology License Agreements - Massachusetts Eye and Ear Infirmary and The Schepens Eye Research Institute, Inc. (Narrative) (Details) - MEE	1 Months Ended	6 Months Ended
	May 31, 2016 USD ($) product	Jun. 30, 2016 USD ($)	Dec. 31, 2017 USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
License fees due under agreement		$ 100,000
Accrued License Fees			$ 0
Minimum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Minimum number of products for each target sequence \| product	1
Payment under development milestone method	$ 4,200,000
Payment under sales milestone method	50,000,000
Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Payment under development milestone method	37,000,000
Payment under sales milestone method	$ 70,000,000

Income Taxes (Details) - USD ($)	3 Months Ended	12 Months Ended
Income Taxes (Details) - USD ($)	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2014	Dec. 31, 2015
Income Tax Holiday [Line Items]
Increase in valuation allowance		$ 4,700,000	$ 14,400,000
Research and development tax credit carryforwards		4,092,000	3,052,000
Increase in net operating losses deferred tax assets		44,146,000	$ 39,420,000
Tax Cuts and Jobs Act, reduction of deferred tax assets		23,400,000
Full valuation allowance		23,400,000
Unrecognized Tax Benefits		0
Related interest and penalties accrued		0
Accrued interest related to uncertain tax positions		0
Russia \| Russian Federal Tax Service
Income Tax Holiday [Line Items]
Term of tax holiday granted to Russian subsidiary				10 years
Domestic Tax Authority
Income Tax Holiday [Line Items]
Net operating loss carryforwards		162,900,000
Research and development tax credit carryforwards		2,500,000
State and Local Jurisdiction
Income Tax Holiday [Line Items]
Net operating loss carryforwards		157,900,000
Research and development tax credit carryforwards		2,000,000
Stock Option Tax Credit Carryforward
Income Tax Holiday [Line Items]
Net operating loss carryforwards		$ 800,000
Stock Compensation \| Domestic Tax Authority
Income Tax Holiday [Line Items]
Net operating loss carryforwards					$ 800,000
Stock Compensation \| State and Local Jurisdiction
Income Tax Holiday [Line Items]
Net operating loss carryforwards					$ 800,000
New Accounting Pronouncement, Early Adoption, Effect \| Accounting Standards Update 2016-09
Income Tax Holiday [Line Items]
Increase in valuation allowance	$ 300,000
Increase in net operating losses deferred tax assets	$ 300,000

Income Taxes - Reconciliation of Income Taxes (Details)	12 Months Ended
Income Taxes - Reconciliation of Income Taxes (Details)	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Tax at U.S. statutory rate	34.00%	34.00%	34.00%
State taxes, net of federal benefit	5.80%	5.80%	5.70%
Permanent items	(0.10%)	(1.10%)	(0.70%)
Research tax credits/other	0.50%	1.30%	1.00%
Change in enacted rates	(35.80%)	0.00%	0.00%
Valuation allowance, net	(6.70%)	(39.80%)	(40.00%)
Other	2.30%	(0.20%)	0.00%
Effective income tax rate	0.00%	0.00%	0.00%

Income Taxes - Income Taxes, Deferred Tax Assets and Deferred Tax Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets
Net operating loss carryforwards	$ 44,146	$ 39,420
Research and development credits	4,092	3,052
Stock-based compensation expense	959	904
Deferred rent and other expenses	263	713
Deferred revenue	4,021	5,607
Patent costs/amortization	4,264	4,934
Total deferred tax assets	57,745	54,630
Deferred tax liabilities
Depreciation	(108)	(131)
Debt discount	0	(69)
Unrealized foreign exchange gain	0	(1,525)
Total deferred tax liabilities	(108)	(1,725)
Net deferred tax assets	57,637	52,905
Valuation allowance	(57,637)	(52,905)
Net deferred tax assets	$ 0	$ 0

Defined Contribution Plan (Details) - 401(k) Plan - Defined-contribution savings plan - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Contribution Plan Disclosure [Line Items]
Vesting period	4 years
Employer contribution made	$ 0.2	$ 0.1	$ 0.1

Selected Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Grant and collaboration revenue	$ 17	$ 27	$ 26	$ 137	$ 2,930	$ 1,048	$ 2,017	$ 2,088	$ 207	$ 8,083	$ 6,011
Operating expenses	19,294	13,881	15,897	14,919	16,790	8,516	8,418	9,029	63,991	42,753	31,315
Net loss	(19,544)	(14,676)	(15,967)	(15,134)	(14,083)	(7,728)	(6,923)	(7,476)	(65,321)	(36,210)	(25,174)
Accretion of redeemable convertible preferred stock					0	0	(2,210)	(2,356)	0	(4,566)	(7,335)
Net (loss) income attributable to common stockholders - basic & diluted	$ (19,544)	$ (14,676)	$ (15,967)	$ (15,134)	$ (14,083)	$ (7,728)	$ (9,133)	$ (9,832)	$ (65,321)	$ (40,776)	$ (32,509)
Net loss per share attributable to common stockholders—basic and diluted (in dollars per share)	$ (0.88)	$ (0.66)	$ (0.85)	$ (0.82)	$ (0.77)	$ (0.43)	$ (2.75)	$ (4.52)	$ (3.20)	$ (3.89)	$ (15.13)