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Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2016	Mar. 17, 2017	Jun. 30, 2016
Document and Entity Information
Document Type	10-K
Amendment Flag	false
Document and Period End Date	Dec. 31, 2016
Document Fiscal Year Focus	2016
Document Fiscal Period Focus	FY
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		18,480,138
Entity Public Float			$ 152,365,971
Entity Current Reporting Status	No

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents	$ 58,656	$ 32,337
Short-term deposits and investments	25,485	4,125
Restricted cash	78	133
Accounts receivable	215	824
Prepaid expenses and other current assets	2,382	1,494
Total current assets	86,816	38,913
Property and equipment, net	2,047	2,029
Restricted cash and other deposits	316	316
Other assets	122	1,566
Total assets	89,301	42,824
Current liabilities:
Accounts payable	3,882	2,179
Accrued expenses	3,921	3,378
Loans payable, current portion	4,067	0
Deferred revenue, current portion	1,836	1,313
Contingently repayable grant funding	0	420
Total current liabilities	13,706	7,290
Non‑current liabilities:
Deferred rent and lease incentive	222	105
Loans payable, net of current portion	7,977	11,855
Deferred revenue, net of current portion	12,439	2,295
Other long‑term liabilities	0	290
Total liabilities	34,344	21,835
Commitments and contingencies (Notes 8 and 13)
Total redeemable convertible preferred stock	0	137,482
Stockholders’ equity (deficit):
Preferred stock, $0.0001 par value; 10,000,000 and 0 shares authorized; 0 shares issued and outstanding at December 31, 2016 and December 31, 2015, respectively.	0	0
Common stock, $0.0001 par value; 200,000,000 and 62,164,377 shares authorized at December 31, 2016 and December 31, 2015 respectively; 18,438,742 and 2,180,976 shares issued, 18,438,742 and 2,173,399 shares outstanding as of December 31, 2016 and December 31, 2015, respectively.	1	0
Additional paid-in capital	211,125	1
Receivable from stock option exercises	(75)	0
Accumulated deficit	(151,576)	(111,508)
Accumulated other comprehensive loss	(4,518)	(4,986)
Total stockholders’ equity (deficit)	54,957	(116,493)
Total liabilities, redeemable convertible preferred stock and stockholders’ equity	$ 89,301	$ 42,824

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2016	Dec. 31, 2015
Convertible preferred stock, par value (in dollars per share)	$ 0.0001
Convertible preferred stock, shares authorized		37,835,623
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	0
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	62,164,377
Common stock, shares issued	18,438,742	2,180,976
Common stock, shares outstanding	18,438,742	2,173,399
Series A redeemable convertible preferred stock
Convertible preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Convertible preferred stock, shares authorized	0	2,589,868
Convertible preferred stock, shares issued	0	2,589,868
Series B redeemable convertible preferred stock
Convertible preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Convertible preferred stock, shares authorized	0	7,437,325
Convertible preferred stock, shares issued	0	7,437,325
Series C redeemable convertible preferred stock
Convertible preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Convertible preferred stock, shares authorized	0	5,000,002
Convertible preferred stock, shares issued	0	5,000,002
Series D redeemable convertible preferred stock
Convertible preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Convertible preferred stock, shares authorized	0	8,166,662
Convertible preferred stock, shares issued	0	8,099,994
Series SRN redeemable convertible preferred stock
Convertible preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Convertible preferred stock, shares authorized	0	5,611,112
Convertible preferred stock, shares issued	0	2,111,109
Series E redeemable convertible preferred stock
Convertible preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Convertible preferred stock, shares authorized	0	9,030,654
Convertible preferred stock, shares issued	0	8,888,888

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement of Comprehensive Income [Abstract]
Grant and collaboration revenue	$ 8,083	$ 6,011	$ 3,040
Operating expenses:
Research and development	29,702	22,980	10,486
General and administrative	13,051	8,335	7,953
Total operating expenses	42,753	31,315	18,439
Loss from operations	(34,670)	(25,304)	(15,399)
Investment income	234	171	111
Foreign currency transaction gain (loss), net	(525)	933	3,004
Interest expense	(1,253)	(948)	(552)
Other expense, net	4	(26)	(44)
Net loss	(36,210)	(25,174)	(12,880)
Other comprehensive loss:
Foreign currency translation adjustment	504	(1,110)	(3,281)
Unrealized gain (loss) on securities	(36)	0	0
Comprehensive loss	(35,742)	(26,284)	(16,161)
Net loss	(36,210)	(25,174)	(12,880)
Accretion of redeemable convertible preferred stock	(4,566)	(7,335)	(4,951)
Net effect of extinguishment of Series SRN redeemable convertible preferred stock	0	0	1,459
Net loss attributable to common stockholders	$ (40,776)	$ (32,509)	$ (16,372)
Net loss per share attributable to common stockholders
Basic and diluted (in dollars per share)	$ (3.89)	$ (15.13)	$ (7.84)
Weighted average common shares outstanding
Basic and diluted (in shares)	10,493,939	2,150,422	2,090,677

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Series D redeemable convertible preferred stock	Series E redeemable convertible preferred stock	Preferred stock 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	Preferred stock Series SRN redeemable convertible preferred stock	Preferred stock Series E redeemable convertible preferred stock	Common stock	Additional paid-In Capital	Additional paid-In Capital Series D redeemable convertible preferred stock	Additional paid-In Capital Series E redeemable convertible preferred stock	Stock option receivable	Accumulated deficit	Accumulated other comprehensive income (loss)
Beginning balance (in shares) at Dec. 31, 2013				2,589,868	7,437,325	5,000,002	4,888,889	777,777	0
Beginning balance at Dec. 31, 2013				$ 3,350	$ 19,662	$ 18,381	$ 24,366	$ 4,643	$ 0
Increase (Decrease) in Temporary Equity
Accretion of preferred stock to redemption value	$ 4,951			$ 143	$ 871	$ 889	$ 1,855	$ 1,193
Issuance of redeemable convertible preferred stock, net of issuance costs (in shares)							3,211,105	1,333,332
Issuance of redeemable convertible preferred stock, net of issuance costs	0						$ 14,349	$ 5,790
Net effect of extinguishment of Series SRN redeemable preferred stock	(1,459)							$ 1,459			$ (1,459)
Ending balance (in shares) at Dec. 31, 2014				2,589,868	7,437,325	5,000,002	8,099,994	2,111,109	0
Ending balance at Dec. 31, 2014				$ 3,493	$ 20,533	$ 19,270	$ 40,570	$ 10,167	$ 0
Beginning balance (in shares) at Dec. 31, 2013										2,027,254
Beginning balance at Dec. 31, 2013	(69,463)									$ 0	1			$ 0	$ (68,869)	$ (595)
Increase (Decrease) in Stockholders' Equity
Vesting of restricted common stock (in shares)										58,104
Vesting of restricted common stock	69										69
Issuance of common stock upon exercise of options (in shares)										38,639
Issuance of common stock upon exercise of options	68										68
Stock‑based compensation expense	1,224										1,224
Accretion of preferred stock to redemption value	(4,951)										(2,820)				(2,131)
Issuance of redeemable convertible preferred stock, net of issuance costs	0						14,349	5,790
Net effect of extinguishment and conversion of convertible stock upon listing	1,459							(1,459)			1,459
Currency translation adjustment	(3,281)															(3,281)
Unrealized gains (losses) on securities	0
Net loss	(12,880)														(12,880)
Ending balance (in shares) at Dec. 31, 2014										2,123,997
Ending balance at Dec. 31, 2014	(87,755)									$ 0	1			0	(83,880)	(3,876)
Increase (Decrease) in Temporary Equity
Accretion of preferred stock to redemption value	7,335			$ 151	$ 915	$ 908	$ 2,332	$ 1,915	$ 1,114
Issuance of redeemable convertible preferred stock, net of issuance costs (in shares)									8,888,888
Issuance of redeemable convertible preferred stock, net of issuance costs	0								$ 36,114
Ending balance (in shares) at Dec. 31, 2015				2,589,868	7,437,325	5,000,002	8,099,994	2,111,109	8,888,888
Ending balance at Dec. 31, 2015	137,482			$ 3,644	$ 21,448	$ 20,178	$ 42,902	$ 12,082	$ 37,228
Increase (Decrease) in Stockholders' Equity
Vesting of restricted common stock (in shares)										7,688
Vesting of restricted common stock	23										23
Issuance of common stock upon exercise of options (in shares)										41,714
Issuance of common stock upon exercise of options	86										86
Stock‑based compensation expense	1,125										1,125
Accretion of preferred stock to redemption value	(7,335)										(4,881)				(2,454)
Issuance of redeemable convertible preferred stock, net of issuance costs	0								36,114
Issuance of common stock warrants	3,647										3,647
Currency translation adjustment	(1,110)															(1,110)
Unrealized gains (losses) on securities	0
Net loss	$ (25,174)														(25,174)
Ending balance (in shares) at Dec. 31, 2015	2,173,399									2,173,399
Ending balance at Dec. 31, 2015	$ (116,493)									$ 0	1			0	(111,508)	(4,986)
Increase (Decrease) in Temporary Equity
Accretion of preferred stock to redemption value	4,566			$ 75	$ 449	$ 446	$ 1,131	$ 913	$ 1,552
Net effect of extinguishment of Series SRN redeemable preferred stock	(142,048)									$ (1)	(142,047)
Conversion of convertible stock upon listing (in shares)				(2,589,868)	(7,437,325)	(5,000,002)	(8,099,994)	(2,111,109)	(8,888,888)
Conversion of convertible stock upon listing				$ (3,719)	$ (21,897)	$ (20,624)	$ (44,033)	$ (12,995)	$ (38,780)
Ending balance (in shares) at Dec. 31, 2016				0	0	0	0	0	0
Ending balance at Dec. 31, 2016	0			$ 0	$ 0	$ 0	$ 0	$ 0	$ 0
Increase (Decrease) in Stockholders' Equity
Vesting of restricted common stock (in shares)										8,535
Vesting of restricted common stock	21										21
Issuance of common stock upon exercise of options (in shares)										71,816
Issuance of common stock upon exercise of options	149										224			(75)
Stock‑based compensation expense	2,030										2,030
Accretion of preferred stock to redemption value	(4,566)										(708)				(3,858)
Exercise of common warrants (in shares)										572,003
Exercise of common warrants	0
Net effect of extinguishment and conversion of convertible stock upon listing (in shares)										10,126,118
Net effect of extinguishment and conversion of convertible stock upon listing	142,048									$ 1	142,047
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										64,465
Issuance of common stock, license agreement (in shares)										197,238
Issuance of common stock, license agreement	2,743										2,743
Conversion of preferred stock warrants into warrants for the purchase of common stock		$ 189	$ 113									$ 189	$ 113
Currency translation adjustment	504															504
Unrealized gains (losses) on securities	(36)															(36)
Net loss	$ (36,210)														(36,210)
Ending balance (in shares) at Dec. 31, 2016	18,438,742									18,438,742
Ending balance at Dec. 31, 2016	$ 54,957									$ 1	$ 211,125			$ (75)	$ (151,576)	$ (4,518)

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Issuance of redeemable convertible preferred stock, issuance costs	$ 302	$ 0
Preferred stock \| Series D redeemable convertible preferred stock
Issuance of redeemable convertible preferred stock, issuance costs		100,734
Preferred stock \| Series SRN redeemable convertible preferred stock
Issuance of redeemable convertible preferred stock, issuance costs		$ 209,587
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, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating activities
Net loss	$ (36,210)	$ (25,174)	$ (12,880)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	802	1,044	864
Amortization of premiums and accretion of discounts on investments	193	0	0
Loss on disposal of fixed assets	3	0	0
Stock‑based compensation expense	2,051	1,125	1,224
Non‑cash interest expense	263	198	155
Change in fair value of redeemable convertible preferred stock warrant	12	(83)	38
Changes in operating assets and liabilities:
Accounts receivable	610	(153)	(514)
Prepaid expenses and other assets	(860)	(1,011)	(404)
Restricted cash and other deposits	103	977	(1,779)
Accounts payable	2,686	667	(90)
Deferred revenue	10,409	(507)	(20)
Contingently repayable grant funding	(461)	(805)	305
Accrued expenses and other liabilities	716	1,259	415
Net cash used in operating activities	(19,683)	(22,463)	(12,686)
Investing activities
Maturities of short term government obligations	6,900	0	0
Purchase of short term investments	(28,416)	(3,516)	0
Purchases of property and equipment	(586)	(1,163)	(227)
Proceeds from the sale of property and equipment	2	0	0
Net cash used in investing activities	(22,100)	(4,679)	(227)
Financing activities
Net proceeds from issuance of preferred stock and warrants	0	32,669	20,140
Principle payments on loan payable	0	(2,336)	0
Deferred IPO costs paid	(4,103)	(302)	0
Issuance of convertible note	0	7,092	0
Proceeds from Initial Public Offering, net of underwriters' discounts and commissions	68,870	0	0
Issuance of common stock	2,743	0	0
Proceeds from loans payable, net of issuance costs	0	6,674	4,494
Exercise of stock options	149	109	137
Net cash provided by financing activities	67,659	43,906	24,771
Effect of exchange rate changes on cash	443	(1,019)	(3,323)
Net increase in cash and cash equivalents	26,319	15,745	8,535
Cash and cash equivalents at beginning of period	32,337	16,592	8,057
Cash and cash equivalents at end of period	58,656	32,337	16,592
Cash paid during the year for:
Interest	972	531	366
Supplemental noncash investing and financing activities:
Purchase of property and equipment not yet paid	147	0	0
Venture debt termination fee liability	0	270	270
Issuance of preferred warrants in connection with venture loans	0	137	121
Reclassification of deferred IPO costs from non-current assets to additional paid-in capital	4,404	0	0
Initial public offering costs in accounts payable and accrued liabilities	122	1,246	0
Accrued dividends and accretion of preferred stock to redemption value	4,566	7,335	4,951
Conversion of bridge loans into Series E preferred	0	7,288	0
Unrealized gain on marketable securities	$ (36)	$ 0	$ 0

Nature of the Business and Basis of Presentation	12 Months Ended
Nature of the Business and Basis of Presentation	Dec. 31, 2016
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. On June 21, 2016 the Company completed an IPO of its common stock 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. Liquidity The Company has incurred losses since inception and negative cash flows from operating activities. As of December 31, 2016 and December 31, 2015, the Company had an accumulated deficit of $151.6 million and $111.5 million, respectively. The Company’s cash and cash equivalents as of December 31, 2016 and December 31, 2015, includes $2.4 million and $3.0 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. 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, 2016, 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, 2016

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, income taxes, collectability of accounts receivable, useful lives of long-lived assets, accrued expenses, and accounting for project development. 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 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 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, 2016, the Company maintained approximately $2.5 million in Russian bank accounts, of which $1.6 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, 2016 and December 31, 2015.

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, common stock warrants, and redeemable convertible preferred 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. The carrying amount of loans payable approximates their estimated fair value due to the consistency between the prevailing market rates in effect and the effective interest rate of 11.6% 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.

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 year ended December 31, 2016 or the year ended December 31, 2015.

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, five years for equipment and three years for computer and office 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 through December 31, 2016.

Debt Issuance Costs

Debt issuance costs and fees paid to lenders are recorded as a direct deduction from the face amount of the related debt. Debt issuance costs are accounted for as additional debt discount and 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 exchange translation adjustment			Unrealized gains (losses) on available-for-sale securities			Accumulated other comprehensive income (loss)
Balance as of December 31, 2015	$	(4,986	)	—			$	(4,986	)
Other comprehensive gain (loss) during the period	504			(36		)	468
Balance as of December 31, 2016	$	(4,482	)	$	(36	)	$	(4,518	)

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 our SVP technology. The terms of these agreements contain multiple deliverables which may include (i) licenses, or options to obtain licenses, to our 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, (v) options to acquire licenses for additional therapeutic areas. Payments to us 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.

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, we are at risk as to whether the collaboration partner will choose to exercise the option. Factors that we consider 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 on the basis of 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 in excess of cash actually 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 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. 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. Should such costs be incurred, they would be classified as a component of income tax expense.

Preferred stock

Prior to the completion of the IPO, the Company classified Preferred Stock as temporary equity and initially records it at the original issuance price, net of issuance costs and discounts. The carrying value is accreted up to the redemption value over the earliest redemption period. The carrying value is 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 has issued common stock warrants and redeemable convertible preferred stock warrants to investors and lenders. Common stock warrants are classified as a component of permanent equity because they are freestanding financial instruments that are legally detachable and separately exercisable from other debt and equity instruments, are contingently exercisable, do not embody an obligation for the Company to repurchase its own shares, and permit the holders to receive a fixed number of common shares upon exercise. In addition, such warrants require physical settlement and do not provide any guarantee of value or return. Common stock warrants are initially recorded at their issuance date fair value and are not subsequently re‑measured. These warrants are valued using the Black‑Scholes option pricing model (“Black‑Scholes”).

Redeemable convertible preferred stock warrants are classified as a liability and are initially recorded at their fair value and re-measured on each subsequent balance sheet date while the warrants are outstanding. Changes in fair value are recorded in interest expense, net in the accompanying consolidated statements of operations and comprehensive loss. The redeemable convertible warrants are valued using Black-Scholes.

In connection with the automatic conversion of the Company’s convertible preferred stock, which occurred upon the closing of the IPO on June 27, 2016, the preferred stock warrants became warrants to purchase common stock. The Company performed the final mark to market adjustment on the preferred stock warrant using the fair value of the underlying common shares of $14.00 per share on June 27, 2016 and recorded the change in fair value in other income (expense), net in the consolidated statement of operations and comprehensive loss. The preferred stock warrant liability was then reclassified to additional paid-in-capital as the preferred stock warrants became warrants to purchase common stock.

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 of stock option grants 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 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 giving consideration to 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, 2016 and December 31, 2015, 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”), which amends the guidance for revenue recognition to replace numerous industry- specific requirements. ASU 2014-09 implements a five-step process for customer contract revenue recognition that focuses on transfer of control, as opposed to transfer of risk and rewards. ASU 2014-09 also requires enhanced disclosures regarding the nature, amount, timing, and uncertainty of revenues and cash flows from contracts with customers. Other major provisions include ensuring the time value of money is considered in the transaction price, and allowing estimates of variable consideration to be recognized before contingencies are resolved in certain circumstances. The guidance permits two methods of adoption: retrospectively to each prior reporting period presented (full retrospective method), or retrospectively with the cumulative effect of initially applying the guidance recognized at the date of initial application (modified retrospective method). The amendments in ASU 2014-09 are effective for reporting periods beginning after December 15, 2017. Early adoption is permitted, but not before December 15, 2016. The new standard will be effective for us beginning December 31, 2018, and adoption as of the original effective date of January 1, 2018.

The Company started an initial assessment of the potential changes from adopting ASU 2014-09 and is in the process of evaluating whether the expected effect of these changes is material to its financial statements. The assessment includes identifying and analyzing the impact of the standard by reviewing the Company's current accounting policies and practices to identify potential differences that would result from applying the requirements of the new standard to each revenue contract associated with all of the Company's revenue streams. We plan to adopt the new standard effective January 1, 2018 using the modified retrospective method. The Company continues to monitor additional changes, modifications, clarifications or interpretations being undertaken by the FASB, which may impact the Company’s current conclusions.

In August 2014, the FASB issued ASU No. 2014-15, Presentation of Financial Statements-Going Concern (“ASU 2014-15”). ASU 2014-15 requires management to assess an entity’s ability to continue as a going concern and to provide related disclosures in certain circumstances. The requirement of ASU 2014-15 became effective for the annual financial statement period ending after December 15, 2016. We adopted ASU 2014-15 for the year ended December 31, 2016, and the adoption did not have a material effect on the Company's consolidated financial statements.

In February 2016, FASB issued ASU No. 2016-02, Leases (“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. We are currently evaluating the effect of the adoption of this guidance on our consolidated financial statements.

In March 2016, the FASB issued ASU No. 2016-9, Improvements to Employee Share-Based Payment Accounting (“ASU 2016-9”), which simplifies several aspects of accounting for share-based payment transactions including the income tax consequences, classification of awards as either equity or liabilities, the estimation of forfeitures, shares withheld for taxes and classification on the statement of cash flows. This guidance is effective for annual reporting periods beginning after December 15, 2016, including interim periods within that reporting period, with early adoption permitted. As part of the adoption of this guidance the Company will continue to use estimated forfeitures in its calculation of stock-based compensation expense. We adopted ASU 2016-09 on January 1, 2017 and there was not a material impact on the consolidated financial statements.

In August 2016, the FASB issued ASU No. 2016-15, Statements of Cash Flows - 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. We are currently evaluating the effect of the adoption of this guidance on our 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 are currently evaluating the effect of the adoption of this guidance on our consolidated financial statements.

Available-for-Sale Marketable Securities

12 Months Ended

Dec. 31, 2016

Investments, Debt and Equity Securities [Abstract]

Available-for-Sale Marketable Securities

As of December 31, 2016 and December 31, 2015, the Company’s available-for-sale marketable securities consisted of debt securities issued by the U.S. government and corporate debt securities.

The following tables summarize the Company’s available-for-sale marketable securities by major type of security as of December 31, 2016 and December 31, 2015 (in thousands):



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



	December 31, 2015
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value
U.S. Treasury securities	$	3,516	$	—	$	—	$	3,516
Corporate bonds	—		—		—		—
Total available-for-sale marketable securities	$	3,516	$	—	$	—	$	3,516

All available-for-sale marketable securities are classified in the Company’s Condensed 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, 2016, the Company’s marketable debt securities mature at various dates through August 2017. The fair values and amortized cost of marketable debt securities by contractual maturity were as follows (in thousands):



	December 31, 2016				December 31, 2015
	Fair Value		Amortized Cost		Fair Value		Amortized Cost
Less than one year	$	24,785	$	24,821	$	3,516	$	3,516

As of December 31, 2016 the Company held a total of 6 out of 20 positions that were in an unrealized loss position, none of which had been in an unrealized loss position for 12 months or greater. 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 that, therefore, it had no other-than-temporary impairments on these securities as of December 31, 2016. 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 maturity.

- Definition

The entire disclosure for investments in certain debt and equity securities.

+ References + Details

Net Loss Per Share

12 Months Ended

Dec. 31, 2016

Earnings Per Share [Abstract]

Net Loss Per Share

Because the Company has reported a net loss attributable to common stockholders for all periods presented, basic and diluted net loss per share attributable to common stockholders are the same for those periods. 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,
	2016			2015			2014
Numerator:
Net (loss)	$	(36,210	)	$	(25,174	)	$	(12,880	)
Less: accretion on preferred stock	(4,566		)	(7,335		)	(4,951		)
Net effect of extinguishment of preferred stock	$	—		$	—		$	1,459
Net loss attributable to common stockholders	$	(40,776	)	$	(32,509	)	$	(16,372	)
Denominator:
Weighted‑average common shares outstanding—basic and diluted	10,493,939			2,150,422			2,090,677
Net loss per share attributable to common stockholders—basic and diluted	$	(3.89	)	$	(15.13	)	$	(7.84	)

Potential common shares issuable upon conversion of warrants to purchase common stock and stock options that are excluded from the computation of diluted weighted average shares outstanding are as follows:



	Year Ended December 31,
	2016	2015	2014
Redeemable convertible preferred stock	—	9,890,580	6,471,358
Stock options to purchase common stock	2,128,346	1,569,379	1,143,237
Stock warrants to purchase common stock	97,302	650,618	—
Redeemable convertible preferred stock warrants	—	26,832	17,094
Total	2,225,648	12,137,409	7,631,689

Fair Value Measurements

12 Months Ended

Dec. 31, 2016

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, 2016, 2015 and 2014, 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, 2016
	(level 1)		(level 2)		(level 3)		Total
Money market funds, included in cash equivalents	$	183	$	—	$	—	$	183
Tri-party repurchase agreements, included in cash equivalents	$	—	$	27,000	$	—	$	27,000
US Treasury obligations, included in investments	$	—	$	—	$	—	$	—
Corporate bonds, included in investments	$	24,785	$	—	$	—	$	24,785
Warrants to purchase redeemable convertible preferred stock, included in other long term liabilities	$	—	$	—	$	—	$	—



	December 31, 2015
	(level 1)		(level 2)		(level 3)		Total
US Treasury obligations, included in cash equivalents	$	14,486	$	—	$	—	$	14,486
US Treasury obligations, included in investments	$	3,516	$	—	$	—	$	3,516
Corporate bonds, included in investments	$	—	$	—	$	—	$	—
Warrants to purchase redeemable convertible preferred stock, included in other long term liabilities	$	—	$	—	$	290	$	290



	December 31, 2014
	(level 1)	(level 2)	(level 3)	Total
Warrants to purchase redeemable convertible preferred stock, included in other long term liabilities	—	—	236	236

At December 31, 2016, all cash and cash equivalent investments where held in money market funds and tri-party repurchase agreements. The tri-party repurchase agreements are collateralized by government securities for an amount not less than 102% of their value. All tri-party repurchase agreements have maturities of three months or less at the time of investment.

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

The average maturity for the US Treasury obligations, included in cash equivalents at December 31, 2015 was 34 days and for those included within investments at December 31, 2015 was 106 days. Fair value of US Treasury obligations approximates amortized value.

In July 2015, in connection with the issuance of convertible notes (Note 9), the Company issued warrants to purchase up to 315,198 shares of the Company's Common Stock at an exercise price of $4.50. At the IPO, these warrants were converted to warrants to purchase 80,813 shares of common stock at an exercise price of $17.55 per share. These warrants expire three years from date of issuance. Common stock warrants are classified as permanent equity which are initially recorded at issuance date fair value and are not subsequently re‑measured.

In August 2013 and July 2014, in conjunction with the execution of a loan and security agreement (Note 9), the Company issued warrants to the lenders for the purchase of up to 66,668 shares of the Company’s Series D preferred stock (“Series D Preferred”) at an exercise price of $4.50 per share. At the IPO, these warrants were converted to warrants to purchase 17,888 of common stock at an exercise price of $16.77. These warrants are classified as permanent equity in the accompanying consolidated balance sheets and will expire four years from the date of issuance.

In December 2015, in conjunction with the execution of a loan and security agreement (Note 9), the Company issued warrants to the lenders for the purchase of up to 37,978 shares of the Company’s Series E preferred stock ("Series E Preferred") at an exercise price of $4.50 per share. At the IPO, these warrants were converted to warrants to purchase 15,094 of common stock at an exercise price of $11.32. These warrants are classified as permanent equity in the accompanying consolidated balance sheets and will expire four years from the date of issuance.

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

Level 3 instruments consist of the Company's Series D and Series E Preferred stock warrant liability. The fair values of the outstanding warrants were measured using the Black-Scholes option pricing model. Inputs used to determined estimated fair value of the warrant liabilities include the estimated fair value of the underlying stock at the valuation date, the estimated term of the warrants, risk-free interest rates, expected dividends and the expected volatility of the underlying stock. The significant unobservable inputs used in the fair value measurement of the warrant liabilities were the fair value of the underlying stock at the valuation date and the estimated term of the warrants. Generally, increase (decreases) in the fair value of the underlying stock and estimated term would result in directionally similar impact to the fair value measurement.



	Warrant Liability
Fair value as of December 31, 2014	$	236
Fair value of warrants issued	137
Change in fair value	(83		)
Fair value as of December 31, 2015	290
Fair value of warrants issued	—
Change in fair value	12
Reclassification to additional paid in capital at IPO	(302		)
Fair value as of December 31, 2016	$	—

- 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, 2016

Property, Plant and Equipment [Abstract]

Property and Equipment

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



	December 31,			December 31,
	2016			2015
Laboratory equipment	$	4,713		$	4,028
Computer equipment and software	562			409
Leasehold improvements	222			91
Furniture and fixtures	226			222
Office equipment	64			62
P,P&E—Construction in process	—			144
Total property and equipment	5,787			4,956
Less accumulated depreciation	(3,740		)	(2,927		)
Property and equipment, net	$	2,047		$	2,029

Depreciation expense was $0.8 million, $1.0 million and $0.9 million for the years ended December 31, 2016, 2015 and 2014, respectively.

Accrued Expenses

12 Months Ended

Dec. 31, 2016

Payables and Accruals [Abstract]

Accrued Expenses

Accrued expenses consist of the following (in thousands):



	December 31,		December 31,
	2016		2015
Payroll and employee related expenses	$	1,551	$	669
Legal	196		213
Current portion of deferred rent and lease incentive	29		405
Accrued patent fees	415		219
Accrued external research and development costs	794		1,649
Accrued audit fees	224		135
Accrued grant refund	152		—
Accrued interest	81		34
Other	479		54
Accrued expenses	$	3,921	$	3,378

Commitments and Contingencies

12 Months Ended

Dec. 31, 2016

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 in Watertown, Massachusetts. As part of the 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 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.

The Company subleased a portion of its facility to a tenant with a term that would have expired in March 2017. In March 2015, the tenant terminated the sublease and vacated the space. The sublease amount from the tenant was recorded as a reduction of lease expense. There was no sublease income recorded during the year ended December 31, 2016, and income totaling $0.2 million and $0.7 million for the year ended December 31, 2015 and 2014, respectively.

In April 2015, the Company amended the lease agreement to exchange 13,711 square feet of space for another 15,174 square feet of space within the same building. Rental payments on the prior space ceased as of March 31, 2015 and rental payments on the new space began on October 1, 2015.

In August 2016, the Company signed an amendment to the operating lease, which extends the lease term of the Company's laboratory and office space through March 31, 2020. The lease agreement 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 $0.3 million and $0.5 million for the years ended December 31, 2016 and December 31, 2015, 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, 2016 and $0.4 million at December 31, 2015, respectively.

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

Rent expense, net of sublease payments, for the years ended December 31, 2016, 2015 and 2014 was $1.6 million, $1.1 million and $0.6 million, respectively.

As of December 31, 2016, the future minimum lease payments under the lease agreement, as amended by the lease amendment are as follows (in thousands):



Year ended December 31,
2017	$	1,244
2018	1,291
2019	1,330
2020	335
Total minimum lease payments	$	4,200

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 does not expect significant claims related to these indemnification obligations and, accordingly, has concluded that the fair value of these obligations is negligible, and no related reserves have been established.

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, 2016

Debt Disclosure [Abstract]

Debt

Term Loans

On August 9, 2013, the Company entered into a loan and security agreement with two lenders 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. The amounts borrowed are collectively referred to as “Term Loans.” In December 2015, the Company refinanced its existing debt facility that was originally entered into on August 9, 2013, as amended with Oxford Finance LLC (“Oxford”) and Square 1Bank (“Square 1”), to increase the amount of the borrowing to $12.0 million and to extend the repayment term. 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. As of December 31, 2016 and December 31, 2015, the outstanding principal balance under the Term Loans was $12.0 million.

According to ASC 470‑50‑40, the refinancing and modification of the prior debt in a non‑troubled debt situation must be treated as either an extinguishment or a modification based on whether the present value of the cash flows under the terms of the new debt instrument is different by greater than, or less than, 10% from the present value of the remaining cash flows under the terms of the original instrument. For cash flow changes greater than 10%, the debt modification is accounted for as a debt extinguishment, whereby the original debt is derecognized and the new debt is initially recorded at fair value, with the difference recognized as an extinguishment gain or loss. For cash flow changes of less than 10%, the new loan is considered a modification and no gain or loss is recognized. In considering all cash flow changes, the Company concluded that the refinancing of the debt as of December 31, 2015 is a modification of the debt and not a debt extinguishment, and as a result the debt is initially recorded at its amortizable value net of discounts and deferred costs.

The Term Loans are collateralized by the assets of the Company and bear interest at 8.1% per annum. The monthly payments for the Term Loans are initially interest only through January 2017. Principal repayments for the Term Loans are due over 30 monthly installments beginning on February 1, 2017. The Term Loans may be prepaid at the Company’s option at any time prior to maturity subject to a prepayment fee of 3% if prepaid prior to the first anniversary of the borrowing date, 2% if prepaid after the first anniversary but before the second anniversaries, and 1% if prepaid after the second anniversary.

The Term Loans do not include any financial covenants. The Term Loans require 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 is recorded as a loan discount. In addition, the Term Loans contain 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. As a result, the Company concluded that a material adverse change has not occurred and is unlikely to occur, therefore, no liability has been recorded in connection with the clause.

In connection with the Term Loans, 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 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 four years from the date of issuance.

Additionally, with the refinancing of the Term Loans 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 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 four years from the date of issuance.

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

Term Loan discounts are amortized as additional interest expense over the term of the loans. During the years ended December 31, 2016, 2015 and 2014, the Company recognized $1.2 million, $0.6 million and $0.6 million, respectively, of interest expense related to the Term Loan.

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 prices had been established at the time that the warrants were converted.

Future minimum payments on the Term Loans as of December 31, 2016 are as follows (in thousands):



Year ended December 31,
2017	$	5,319
2018	5,318
2019	3,379
Total minimum debt payments	14,016
Less: Amount representing interest	(1,296		)
Less: Debt discount and deferred charges	(621		)
Less: Current portion of loans payable	(4,122		)
Loans payable, net of current portion	$	7,977

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 year ended December 31, 2016, $0.3 million for the year ended December 31, 2015, and no interest expense for the year ended December 31, 2014.

Preferred Stock	12 Months Ended
Preferred Stock	Dec. 31, 2016
Temporary Equity Disclosure [Abstract]
Preferred Stock	Preferred Stock Prior to the completion of its IPO, the Company had 37,835,623 and 28,804,969 authorized shares of Preferred Stock as of December 31, 2015 and 2014, respectively. The Company issued Preferred Stock with a $0.0001 par value to investors for cash or as settlement for outstanding debt under convertible notes (Note 9). The Company had issued 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. In April 2014 and August 2014, the Company issued an additional 3,211,105 shares of Series D Preferred at $4.50 per share for total net proceeds of $14,349,239. In July 2014, the Company issued an additional 1,333,332 shares of Series SRN Preferred at $4.50 per share for total net proceeds of $5.8 million. In connection with the issuance of the additional shares of Series SRN Preferred, the Series SRN Preferred terms were amended. Significant terms that were amended included a change of the Series SRN Preferred optional and mandatory conversion price (other than a special conversion event, as defined in the certificate of incorporation) to $16.77 per share, the elimination of a time‑based tranche investment requirement, and the removal of a call option for the Company to repurchase the Series SRN Preferred shares. Based upon the qualitative characteristics of the amendments, the Company determined that the changes significantly modified the terms of Series SRN Preferred resulting in an extinguishment of the then outstanding SRN Preferred shares. As a result, the carrying value of Series SRN Preferred of $5.0 million at the date of the amendment was derecognized, and the amended Series SRN Preferred shares were recorded at their fair value of $4.50 per share. The difference of $1.5 million was recorded as additional paid in capital. In August 2015 and September 2015, the Company issued an aggregate of 7,269,338 shares of Series E Preferred at $4.50 per share for total gross proceeds of $32.7 million with issuance costs totaling $0.2 million. In addition, the Company issued 1,619,550 shares of Series E Preferred in connection with the conversion of convertible notes (Note 9). In connection with the Series E Preferred issuances, each Series E Preferred stockholder also received warrants to purchase a number of shares of the Company’s common stock that equal to 25% of the number of Series E Preferred shares issued. The fair value of the issued common stock warrants is accounted for as an issuance discount on the Series E Preferred. The common stock warrants are classified as permanent equity and were recorded as additional paid‑in capital. 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, 2016

Equity [Abstract]

Common Stock

As of December 31, 2016, the Company has 200,000,000 shares of Common Stock authorized for issuance, $0.0001 par value per share, with 18,438,742 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. Common stock voting rights on certain matters are subject to the powers, preferences, and rights of the Senior Preferred.

Dividends

The common stockholders are entitled to receive dividends, if and when declared by the Board of Directors. The Company may not declare or pay any cash dividends to the common stockholders unless dividends are first declared and paid to the holders of Preferred Stock in accordance with their respective terms. Through December 31, 2016, no dividends have been declared or paid on common stock.

Liquidation

After holders of Preferred Stock are satisfied of their liquidation preferences upon any voluntary or involuntary liquidation, dissolution or winding up of the affairs of the Company, the common stockholders are then entitled to receive that portion of the remaining funds to be distributed to all holders of the Company’s stock on an as‑converted basis.

Reserved Shares

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



	Periods ending
	December 31, 2016	December 31, 2015
Conversion of Series A Preferred	—	664,068
Conversion of Series B Preferred	—	1,907,006
Conversion of Series C Preferred	—	1,282,051
Conversion of Series D Preferred	—	2,191,412
Conversion of Series SRN Preferred	—	1,798,433
Conversion of Series E Preferred	—	2,662,885
Exercise of common warrants	97,302	651,618
Shares available for future stock incentive awards	939,317	100,034
Exercise of outstanding common stock options	2,128,346	1,569,379
Total	3,164,965	12,826,886

- Definition

The entire disclosure for shareholders' equity comprised of portions attributable to the parent entity and noncontrolling interest, including other comprehensive income. Includes, but is not limited to, balances of common stock, preferred stock, additional paid-in capital, other capital and retained earnings, accumulated balance for each classification of other comprehensive income and amount of comprehensive income.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 4.E)
-URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74567-122707

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
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Reference 4: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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-Paragraph 1
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Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
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-Paragraph 1
-Subparagraph (SX 210.5-02(31))
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Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Reference 9: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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Reference 10: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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Reference 11: http://www.xbrl.org/2003/role/presentationRef
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Reference 12: http://www.xbrl.org/2003/role/presentationRef
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Reference 13: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21475-112644

Reference 14: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Preferred Stock
-URI http://asc.fasb.org/extlink&oid=6521494

Reference 15: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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+ Details

Stock Incentive Plans

12 Months Ended

Dec. 31, 2016

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

Stock Incentive Plans

The Company maintained 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. As of December 31, 2016, a total of 2,213,412 shares of common stock are authorized for grants under the 2008 Plan with 18,947 shares available for future grants. All stock options granted under the 2008 Plan, and unvested, may be exercised into restricted stock subject to forfeiture provisions upon termination.

The 2008 Plan provided that the exercise price of incentive stock options cannot be less than 100% of the fair market value of the common stock on the grant date for participants who own less than 10% 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 granted under the 2008 Plan vest over periods as determined by the Board, which are generally four years and 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, any cash dividends in the foreseeable future. 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 forfeitures rate of 10% based on historical attrition trends. The Company records stock‑based compensation expense only on the awards that are expected to vest.

As of the S-1 registration date of May 24, 2016, the Company ceased granting awards under the 2008 Plan; however, awards issued under the 2008 Plan remain subject to the terms of 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 21, 2016. The 2016 Plan provides for the granting of incentive and non‑qualified stock option and restricted stock awards as determined by the Board. As of December 31, 2016, a total of 1,210,256 shares of common stock are authorized for grants under the 2016 Plan with 747,294 shares available for future grant.

The 2016 Plan provides that the exercise price of incentive stock options cannot be less than 100% of the fair market value of the common stock on the grant date for participants who own less than 10% 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 granted under the 2016 Plan vest over periods as determined by the Board, which are generally four years and with terms that generally expire ten years from the grant date.

On June 7, 2016, the Company’s stockholders approved the 2016 Employee Stock Purchase Plan (the “ESPP”), which became effective June 21, 2016. A total of 173,076 shares of common stock were reserved for issuance under the ESPP. In addition, 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 (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. As of December 31, 2016, no shares have been issued under the ESPP.

The weighted average assumptions used for employee stock option grants issued for the years ended December 31, 2016, 2015 and 2014 are as follows:



	Year Ended December 31,
	2016			2015			2014
Risk-free interest rate	1.42		%	1.79		%	1.93		%
Expected dividend yield	—			—			—
Expected life	6.04			6.02			5.94
Expected volatility	94.97		%	79.80		%	100.81		%
Weighted-average fair value of common stock	$	13.25		$	7.35		$	8.97

The resulting weighted average grant date fair value of stock options granted to employees during the years ended December 31, 2016, 2015 and 2014 was $10.23, $5.03 and $6.86, respectively. The aggregate intrinsic value of stock options exercised during the years ended December 31, 2016, 2015 and 2014 was $0.8 million, $0.3 million and $0.3 million, respectively.

As of December 31, 2016, 2015 and 2014, total unrecognized compensation expense related to unvested employee stock options was $5.6 million, $2.3 million and $0.9 million, respectively, which is expected to be recognized over a weighted average period of 3.0 years, 3.1 years and 2.3 years, respectively.

The weighted average assumptions used for unvested non‑employee stock options are as follows:



	Year Ended December 31,
	2016		2015		2014
Risk‑free interest rate	2.06	%	1.57	%	1.22	%
Expected dividend yield	—		—		—
Expected life (in years)	8.80		6.46		5.87
Expected volatility	87.33	%	98.18	%	98.25	%

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. Non‑employee stock‑based compensation expense of $0.4 million, $0.4 million and $0.5 million was recorded during the years ended December 31, 2016, 2015 and 2014, respectively.

As of December 31, 2016, 2015 and 2014, total unrecognized compensation expense related to unvested non‑employee stock options was $1.1 million, $0.1 million and $0.4 million, respectively.

The following table summarizes the activity under the 2008 Plan and the 2016 Plan since December 31, 2014:



					Weighted‑average
			Weighted‑average		remaining	Aggregate
	Number of		exercise		contractual term	intrinsic value
	options		price		(in years)	(in thousands)
Employee
Outstanding at December 31, 2014	800,165		$	3.24	6.99	$	4,895
Granted	485,833		$	7.91
Exercised	(20,861	)	$	1.21
Forfeited	(17,977	)	$	6.32
Outstanding at December 31, 2015	1,247,160		$	5.05	7.41	$	3,130
Vested at December 31, 2015	668,465		$	2.61	5.71	$	2,946
Vested and expected to vest at December 31, 2015	1,171,380		$	4.84	7.26	$	3,138

Outstanding at December 31, 2015	1,247,160		$	5.05	7.41	$	3,130
Granted	543,595		$	13.25
Exercised	(57,850	)	$	2.00
Forfeited	(12,164	)	$	7.41
Outstanding at December 31, 2016	1,720,741		$	7.73	7.43	$	16,425
Vested at December 31, 2016	830,833		$	4.05	5.65	$	10,885
Vested and expected to vest at December 31, 2016	1,601,908		$	7.44	7.30	$	15,731
Non‑Employee
Outstanding at December 31, 2014	343,072		$	2.93	6.50	$	2,207
Granted	—		$	—
Exercised	(20,852	)	$	2.91
Forfeited	—		$	—
Outstanding at December 31, 2015	322,220		$	2.93	5.50	$	1,288
Vested at December 31, 2015	310,680		$	2.70	5.40	$	1,288
Vested and expected to vest at December 31, 2015	322,219		$	2.93	5.50	$	1,288

Outstanding at December 31, 2015	322,220		$	2.93	5.50	$	1,288
Granted	102,435		$	8.43
Exercised	(17,049	)	$	0.47
Forfeited	(1	)	$	3.44
Outstanding at December 31, 2016	407,605		$	4.42	5.79	$	5,224
Vested at December 31, 2016	327,879		$	3.44	4.98	$	4,508
Vested and expected to vest at December 31, 2016	406,200		$	4.40	5.77	$	5,215

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 has a repurchase provision whereby the Company has the right to repurchase any unvested shares when/if the shareholders terminate their business 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. The Company records payment received from the granting of restricted stock as a liability which is amortized over the vesting period. The liability became fully amortized as of December 31, 2016.

Total fair value of restricted shares that vested during the years ended December 31, 2016, 2015 and 2014 was less than $0.1 million.

The following table summarizes the restricted stock award activity of the 2008 Plan and the 2016 Plan for the years ended December 31, 2016, 2015 and 2014:



	Year Ended December 31,
	2016				2015				2014
			Weighted‑				Weighted‑				Weighted‑
			average				average				average
			exercise				exercise				exercise
	Shares		price		Shares		price		Shares		price
Unvested at beginning of period	7,574		$	2.77	15,264		$	2.77	70,804		$	1.56
Issued	962		9.36		—		—		2,564		0.62
Vested	(8,536	)	3.51		(7,690	)	2.72		(58,104	)	1.44
Unvested at end of period	—		$	—	7,574		$	2.77	15,264		$	2.77

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 options and restricted common stock in the following expense categories of its consolidated statements of operations and comprehensive loss (in thousands):



	Year Ended December 31,
	2016		2015		2014
Research and development	$	1,183	$	495	$	384
General and administrative	847		630		840
Total	$	2,030	$	1,125	$	1,224

- Definition

The entire disclosure for compensation-related costs for equity-based compensation, which may include disclosure of policies, compensation plan details, allocation of equity compensation, incentive distributions, equity-based arrangements to obtain goods and services, deferred compensation arrangements, employee stock ownership plan details and employee stock purchase plan details.

+ References + Details

Revenue Arrangements

12 Months Ended

Dec. 31, 2016

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 Company’s Synthetic Vaccine Particles (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 will be due within twelve months of the contract dated December 2, 2016 (“Contract Date”). 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 “Purchase Agreement”) with Spark. Pursuant to the 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 Stock 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 Stock Purchase Agreement are subject to customary conditions. Beyond the Initial Closing, the Purchase Agreement provides for 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 will have the right (the “First Acquisition Right”) to purchase a number of shares of Common Stock equal to an aggregate price of $5.0 million.


•	Second Acquisition Right. During the period beginning on October 1, 2017 and ending on November 1, 2017, Spark will have the right (the “Second Acquisition Right”) to purchase a number of shares of Common Stock equal to an aggregate price of $5.0 million.

The First Acquisition Rights and Second Acquisition Rights are collectively referred to herein as the “Acquisition Rights”. The number of shares issued under the Acquisition Rights will be determined based on the calculated purchase price on the date of notification, which is defined as 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 Acquisition Right notification date. 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 execution of the Spark License Agreement and Purchase Agreement, the Company made contractual payments defined in the MIT license agreement (Note 15) totaling $1.5 million for the MIT sub-license provided to Spark, and $0.2 million relative to the calculated premium paid by Spark for the initial equity investment made under the 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 stock purchase agreement and the collaboration and 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 stock 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 consideration allocated to the combined unit of accounting for the License and Supply Deliverable, totaling $7.1 million, will be recognized based on the expected deliveries over an 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).

There were no revenues recognized related to the Spark License Agreement for the year ended December 31, 2016.

The Company determined that each potential future clinical and regulatory milestone was non-substantive.

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

As of December 31, 2016, there was $12.3 million of deferred revenue related to this agreement. A total of $1.8 million was recorded as a current liability and $10.5 million was classified as a long term liability in the accompanying consolidated balance sheet.

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 revenue in the amount of $2.8 million, $2.1 million and $2.1 million for the years ended December 31, 2016, 2015 and 2014, respectively.

Termination of the Sanofi Collaboration Agreement

On November 9, 2016, the Company received written notice from Sanofi that Sanofi has elected to terminate in its entirety the Sanofi Agreement. The termination of the Sanofi Agreement will be effective on May 8, 2017, or the Termination Date, which is 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 has recognized the remaining $2.2 million in revenue associated with the Sanofi Agreement, as the Company has no more performance obligation under the contact.

Upon timely exercise by the Company, all rights granted to Sanofi will terminate and revert 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 will also expire on the Termination Date.

The Company has exercised its right to acquire the development programs under the Sanofi Agreement. The Company will be 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

The Company has entered into other research and collaboration agreements in 2016 and 2015 for which the Company recognized revenue of less than $0.1 million and $0.3 million. There was no revenue recognized for the year ended December 31, 2014.

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 revenue in the amount of $4.1 million, $2.4 million and $0.6 million for the years ended December 31, 2016, 2015 and 2014, 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 revenue in an amount less than $0.1 million for the year ended December 31, 2016 and $0.1 million for the year ended December 31, 2015, related to this grant.

JDRF

The Company had two contracts in effect with JDRF during 2014 and only one of those contracts was in effect during 2015 and 2016. The first contract was a continuation and completion of the 2011 grant for $0.8 million. The Company recognized the remaining $0.2 million of revenue during the year ended December 31, 2014 under this contract.

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

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. For the years ended December 31, 2016, 2015 and 2014, the Company recognized revenue of $0.5 million, $0.6 million and $0.2 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, 2016, the Company received payments totaling approximately $1.4 million.

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.

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 HPV and diseases associated with this infection. The grant covers a 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. The Company has received from Skolkovo $1.8 million through the year ended December 31, 2016.

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, 2016
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 10), 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 10). 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. The Company incurred expenses for consulting services provided by its founders totaling $0.3 million, $0.3 million and $0.3 million during each of the years ended December 31, 2016, 2015 and 2014, respectively.

Technology License Agreements	12 Months Ended
Technology License Agreements	Dec. 31, 2016
Goodwill and Intangible Assets Disclosure [Abstract]
Technology License Agreements	Technology License Agreements MIT On November 25, 2008, the Company entered into an Exclusive Patent License agreement with the Massachusetts Institute of Technology (“MIT”). 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. In connection with the execution of the Spark License Agreement, the Company made contractual payments defined in the Exclusive Patent License agreement totaling $1.5 million for the sub-license provided to Spark, and $0.2 million relative to the calculated premium paid by Spark for the initial equity investment made under the 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, 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 has been 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”) 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 Agreement, 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 December 31, 2016, the Company paid its initial license fee due under the agreement totaling $0.1 million.

Income Taxes

12 Months Ended

Dec. 31, 2016

Income Tax Disclosure [Abstract]

Income Taxes

The Company provides for income taxes under 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, 2016, 2015 and 2014, the Company did not record a current or deferred income tax expense or benefit. A reconciliation of income taxes computed using the U.S. federal statutory rate to that reflected in operations is as follows:



	Year Ended December 31,
	2016		2015		2014
Statutory U.S. federal rate	34.0	%	34	%	34	%
State income taxes - net of federal benefit	5.8	%	5.7	%	5.9	%
Permanent items	(1.1	)%	(0.7	)%	(0.7	)%
Research tax credits/other	1.3	%	1.0	%	1.6	%
Valuation allowance, net	(39.8	)%	(40	)%	(33	)%
Other	(0.2	)%	—	%	(7.8	)%
Net deferred tax assets	—	%	—	%	—	%

The tax effects of temporary differences that give rise to the Company's net deferred tax assets are as follows (in thousands):



	Year Ended December 31,
	2016			2015
Deferred Tax Assets
Net operating loss carryforwards	$	39,420		$	31,958
Research and development credits	3,052			2,305
Stock-based compensations expense	904			570
Deferred rent and other expenses	713			493
Deferred revenue	5,607			1,582
Patent costs/amortization	4,934			3,500
Tenant improvement allowance	—			—
Warrant liability	—			114
Gross deferred tax assets	54,630			40,522

Deferred Tax Liabilities
Depreciation	$	(131	)	$	(197	)
Debt discount	(69		)	(97		)
Unrealized foreign exchange gain	(1,525		)	(1,728		)
Gross deferred tax liabilities	(1,725		)	(2,022		)
Net deferred tax assets	52,905			38,500
Valuation allowance	(52,905		)	(38,500		)
Net deferred tax assets	$	—		$	—

The Company has provided a full valuation allowance against its net deferred tax assets, as the Company believes that it is more likely than not that the 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 $14.4 million and $10.1 million for the years ended December 31, 2016 and 2015, 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. As a result, previously reported deferred tax assets were adjusted for the change in tax rate. There has been no change to the tax holiday status for the subsidiary as of December 31, 2016.

At December 31, 2016, the Company had federal and state net operating loss carryforwards of $102.1 million and $95.6 million, which will expire at various times through 2036. Included in the net operating loss above is approximately $0.8 million related to excess stock option deductions. The Company also has federal and state research and development tax credit carryforwards of $2.1 million and $1.5 million available to reduce future tax liabilities, which will expire at various times through 2036.

Included in the carryforwards above, the Company has federal and state NOLs related to stock compensation in the amount of $0.8 million and $0.8 million, respectively that is not included in deferred tax assets. When the excess stock-based compensation related to the NOL carryover tax assets are realized, the benefit will be credited directly to stockholders’ equity under the Company’s current accounting policy. However, the Company will adopt ASU 2016-09, Improvements to Employee Share-Based Payment Accounting, during the quarter ended March 31, 2017. As a result of the adoption, the net operating losses deferred tax assets will increase by $0.3 million and will be offset by a corresponding increase in the valuation allowance. The adoption of ASU 2016-09 will have no impact to the Company’s income statement, balance sheet, or retained earnings.

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, Income Taxes to uncertain tax positions. As of the adoption date on January 1, 2009 and through December 31, 2016, 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, 2016, 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.

401(k) Savings Plan	12 Months Ended
401(k) Savings Plan	Dec. 31, 2016
Compensation and Retirement Disclosure [Abstract]
401(k) Savings Plan	401(k) Savings Plan The Company maintains a defined‑contribution savings 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 $0.1 million, $0.1 million and $0.1 million for the years ended December 31, 2016, 2015 and 2014, respectively, have been recorded in the consolidated statements of operations and comprehensive loss.

Selected Quarterly Financial Data (Unaudited)

12 Months Ended

Dec. 31, 2016

Quarterly Financial Information Disclosure [Abstract]

Selected Quarterly Financial Data (Unaudited)

The following table summarizes unaudited quarterly financial data for the years ended December 31, 2016, 2015 and 2014 (in thousands, except per share data).



	Three Months Ended (unaudited)
	March 31,			June 30,			September 30,			December 31,
	2016			2016			2016			2016
	(in thousands, except per share data)
Revenue	$	2,088		$	2,017		$	1,048		$	2,930
Operating expenses	9,029			8,418			8,516			16,790
Net (loss) income	$	(7,476	)	$	(6,923	)	$	(7,728	)	$	(14,083	)
Accretion on preferred stock	(2,356		)	(2,210		)	—			—
Net effect of extinguishment of preferred stock	—			—			—			—
Net (loss) income attributable to common stockholders - basic & diluted	$	(9,832	)	$	(9,133	)	$	(7,728	)	$	(14,083	)
Net (loss) per share attributable to common stockholders:
Basic & diluted	$	(4.52	)	$	(2.75	)	$	(0.43	)	$	(0.77	)
Weighted-average common shares outstanding used in net (loss) income per share attributable to common stockholders:
Basic & diluted	2,175,037			3,322,546			18,108,014			18,265,771



	Three Months Ended (unaudited)
	March 31,			June 30,			September 30,			December 31,
	2015			2015			2015			2015
	(in thousands, except per share data)
Revenue	$	1,034		$	1,236		$	1,607		$	2,134
Operating expenses	6,844			7,552			7,678			9,241
Net (loss) income	$	(5,733	)	$	(6,867	)	$	(5,725	)	$	(6,849	)
Accretion on preferred stock	(1,561		)	(1,562		)	(1,836		)	(2,376		)
Net effect of extinguishment of preferred stock	—			—			—			—
Net (loss) income attributable to common stockholders - basic & diluted	$	(7,294	)	$	(8,429	)	$	(7,561	)	$	(9,225	)
Net (loss) per share attributable to common stockholders:
Basic & diluted	$	(3.43	)	$	(3.93	)	$	(3.50	)	$	(4.26	)
Weighted-average common shares outstanding used in net (loss) income per share attributable to common stockholders:
Basic & diluted	2,126,878			2,147,184			2,159,658			2,167,769

Subsequent Events	12 Months Ended
Subsequent Events	Dec. 31, 2016
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events None

Summary of Significant Accounting Policies (Policies)	12 Months Ended
Summary of Significant Accounting Policies (Policies)	Dec. 31, 2016
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, income taxes, collectability of accounts receivable, useful lives of long-lived assets, accrued expenses, and accounting for project development. 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 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 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 and Preferred stock	Preferred stock Prior to the completion of the IPO, the Company classified Preferred Stock as temporary equity and initially records it at the original issuance price, net of issuance costs and discounts. The carrying value is accreted up to the redemption value over the earliest redemption period. The carrying value is 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.
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.
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, common stock warrants, and redeemable convertible preferred 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. The carrying amount of loans payable approximates their estimated fair value due to the consistency between the prevailing market rates in effect and the effective interest rate of 11.6% 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. 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.
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, five years for equipment and three years for computer and office 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.
Debt Issuance Costs	Debt Issuance Costs Debt issuance costs and fees paid to lenders are recorded as a direct deduction from the face amount of the related debt. Debt issuance costs are accounted for as additional debt discount and 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).
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 our SVP technology. The terms of these agreements contain multiple deliverables which may include (i) licenses, or options to obtain licenses, to our 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, (v) options to acquire licenses for additional therapeutic areas. Payments to us 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. 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, we are at risk as to whether the collaboration partner will choose to exercise the option. Factors that we consider 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 on the basis of 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 in excess of cash actually 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 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. 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. Should such costs be incurred, they would be classified as a component of income tax expense.
Warrants	Warrants The Company has issued common stock warrants and redeemable convertible preferred stock warrants to investors and lenders. Common stock warrants are classified as a component of permanent equity because they are freestanding financial instruments that are legally detachable and separately exercisable from other debt and equity instruments, are contingently exercisable, do not embody an obligation for the Company to repurchase its own shares, and permit the holders to receive a fixed number of common shares upon exercise. In addition, such warrants require physical settlement and do not provide any guarantee of value or return. Common stock warrants are initially recorded at their issuance date fair value and are not subsequently re‑measured. These warrants are valued using the Black‑Scholes option pricing model (“Black‑Scholes”). Redeemable convertible preferred stock warrants are classified as a liability and are initially recorded at their fair value and re-measured on each subsequent balance sheet date while the warrants are outstanding. Changes in fair value are recorded in interest expense, net in the accompanying consolidated statements of operations and comprehensive loss. The redeemable convertible warrants are valued using Black-Scholes.
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 of stock option grants 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 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 giving consideration to 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.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (“ASU 2014-09”), which amends the guidance for revenue recognition to replace numerous industry- specific requirements. ASU 2014-09 implements a five-step process for customer contract revenue recognition that focuses on transfer of control, as opposed to transfer of risk and rewards. ASU 2014-09 also requires enhanced disclosures regarding the nature, amount, timing, and uncertainty of revenues and cash flows from contracts with customers. Other major provisions include ensuring the time value of money is considered in the transaction price, and allowing estimates of variable consideration to be recognized before contingencies are resolved in certain circumstances. The guidance permits two methods of adoption: retrospectively to each prior reporting period presented (full retrospective method), or retrospectively with the cumulative effect of initially applying the guidance recognized at the date of initial application (modified retrospective method). The amendments in ASU 2014-09 are effective for reporting periods beginning after December 15, 2017. Early adoption is permitted, but not before December 15, 2016. The new standard will be effective for us beginning December 31, 2018, and adoption as of the original effective date of January 1, 2018. The Company started an initial assessment of the potential changes from adopting ASU 2014-09 and is in the process of evaluating whether the expected effect of these changes is material to its financial statements. The assessment includes identifying and analyzing the impact of the standard by reviewing the Company's current accounting policies and practices to identify potential differences that would result from applying the requirements of the new standard to each revenue contract associated with all of the Company's revenue streams. We plan to adopt the new standard effective January 1, 2018 using the modified retrospective method. The Company continues to monitor additional changes, modifications, clarifications or interpretations being undertaken by the FASB, which may impact the Company’s current conclusions. In August 2014, the FASB issued ASU No. 2014-15, Presentation of Financial Statements-Going Concern (“ASU 2014-15”). ASU 2014-15 requires management to assess an entity’s ability to continue as a going concern and to provide related disclosures in certain circumstances. The requirement of ASU 2014-15 became effective for the annual financial statement period ending after December 15, 2016. We adopted ASU 2014-15 for the year ended December 31, 2016, and the adoption did not have a material effect on the Company's consolidated financial statements. In February 2016, FASB issued ASU No. 2016-02, Leases (“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. We are currently evaluating the effect of the adoption of this guidance on our consolidated financial statements. In March 2016, the FASB issued ASU No. 2016-9, Improvements to Employee Share-Based Payment Accounting (“ASU 2016-9”), which simplifies several aspects of accounting for share-based payment transactions including the income tax consequences, classification of awards as either equity or liabilities, the estimation of forfeitures, shares withheld for taxes and classification on the statement of cash flows. This guidance is effective for annual reporting periods beginning after December 15, 2016, including interim periods within that reporting period, with early adoption permitted. As part of the adoption of this guidance the Company will continue to use estimated forfeitures in its calculation of stock-based compensation expense. We adopted ASU 2016-09 on January 1, 2017 and there was not a material impact on the consolidated financial statements. In August 2016, the FASB issued ASU No. 2016-15, Statements of Cash Flows - 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. We are currently evaluating the effect of the adoption of this guidance on our 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 are currently evaluating the effect of the adoption of this guidance on our consolidated financial statements.

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2016

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 exchange translation adjustment			Unrealized gains (losses) on available-for-sale securities			Accumulated other comprehensive income (loss)
Balance as of December 31, 2015	$	(4,986	)	—			$	(4,986	)
Other comprehensive gain (loss) during the period	504			(36		)	468
Balance as of December 31, 2016	$	(4,482	)	$	(36	)	$	(4,518	)

Available-for-Sale Marketable Securities (Tables)

12 Months Ended

Dec. 31, 2016

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, 2016 and December 31, 2015 (in thousands):



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



	December 31, 2015
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value
U.S. Treasury securities	$	3,516	$	—	$	—	$	3,516
Corporate bonds	—		—		—		—
Total available-for-sale marketable securities	$	3,516	$	—	$	—	$	3,516

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, 2016				December 31, 2015
	Fair Value		Amortized Cost		Fair Value		Amortized Cost
Less than one year	$	24,785	$	24,821	$	3,516	$	3,516

Net Loss Per Share (Tables)

12 Months Ended

Dec. 31, 2016

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,
	2016			2015			2014
Numerator:
Net (loss)	$	(36,210	)	$	(25,174	)	$	(12,880	)
Less: accretion on preferred stock	(4,566		)	(7,335		)	(4,951		)
Net effect of extinguishment of preferred stock	$	—		$	—		$	1,459
Net loss attributable to common stockholders	$	(40,776	)	$	(32,509	)	$	(16,372	)
Denominator:
Weighted‑average common shares outstanding—basic and diluted	10,493,939			2,150,422			2,090,677
Net loss per share attributable to common stockholders—basic and diluted	$	(3.89	)	$	(15.13	)	$	(7.84	)

Schedule of Potential Common Shares Issuable Upon Conversion of Warrants



	Year Ended December 31,
	2016	2015	2014
Redeemable convertible preferred stock	—	9,890,580	6,471,358
Stock options to purchase common stock	2,128,346	1,569,379	1,143,237
Stock warrants to purchase common stock	97,302	650,618	—
Redeemable convertible preferred stock warrants	—	26,832	17,094
Total	2,225,648	12,137,409	7,631,689

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2016

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, 2016
	(level 1)		(level 2)		(level 3)		Total
Money market funds, included in cash equivalents	$	183	$	—	$	—	$	183
Tri-party repurchase agreements, included in cash equivalents	$	—	$	27,000	$	—	$	27,000
US Treasury obligations, included in investments	$	—	$	—	$	—	$	—
Corporate bonds, included in investments	$	24,785	$	—	$	—	$	24,785
Warrants to purchase redeemable convertible preferred stock, included in other long term liabilities	$	—	$	—	$	—	$	—



	December 31, 2015
	(level 1)		(level 2)		(level 3)		Total
US Treasury obligations, included in cash equivalents	$	14,486	$	—	$	—	$	14,486
US Treasury obligations, included in investments	$	3,516	$	—	$	—	$	3,516
Corporate bonds, included in investments	$	—	$	—	$	—	$	—
Warrants to purchase redeemable convertible preferred stock, included in other long term liabilities	$	—	$	—	$	290	$	290



	December 31, 2014
	(level 1)	(level 2)	(level 3)	Total
Warrants to purchase redeemable convertible preferred stock, included in other long term liabilities	—	—	236	236

Schedule of the Fair Values of Outstanding Warrants



	Warrant Liability
Fair value as of December 31, 2014	$	236
Fair value of warrants issued	137
Change in fair value	(83		)
Fair value as of December 31, 2015	290
Fair value of warrants issued	—
Change in fair value	12
Reclassification to additional paid in capital at IPO	(302		)
Fair value as of December 31, 2016	$	—

Property and Equipment (Tables)

12 Months Ended

Dec. 31, 2016

Property, Plant and Equipment [Abstract]

Schedule of Property and Equipment

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



	December 31,			December 31,
	2016			2015
Laboratory equipment	$	4,713		$	4,028
Computer equipment and software	562			409
Leasehold improvements	222			91
Furniture and fixtures	226			222
Office equipment	64			62
P,P&E—Construction in process	—			144
Total property and equipment	5,787			4,956
Less accumulated depreciation	(3,740		)	(2,927		)
Property and equipment, net	$	2,047		$	2,029

Accrued Expenses (Tables)

12 Months Ended

Dec. 31, 2016

Payables and Accruals [Abstract]

Schedule of Accrued Expenses

Accrued expenses consist of the following (in thousands):



	December 31,		December 31,
	2016		2015
Payroll and employee related expenses	$	1,551	$	669
Legal	196		213
Current portion of deferred rent and lease incentive	29		405
Accrued patent fees	415		219
Accrued external research and development costs	794		1,649
Accrued audit fees	224		135
Accrued grant refund	152		—
Accrued interest	81		34
Other	479		54
Accrued expenses	$	3,921	$	3,378

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2016

Commitments and Contingencies Disclosure [Abstract]

Schedule of Future Minimum Lease Payments

As of December 31, 2016, the future minimum lease payments under the lease agreement, as amended by the lease amendment are as follows (in thousands):



Year ended December 31,
2017	$	1,244
2018	1,291
2019	1,330
2020	335
Total minimum lease payments	$	4,200

Debt (Tables)

12 Months Ended

Dec. 31, 2016

Debt Disclosure [Abstract]

Schedule of Future Minimum Payments on the Term Loans

Future minimum payments on the Term Loans as of December 31, 2016 are as follows (in thousands):



Year ended December 31,
2017	$	5,319
2018	5,318
2019	3,379
Total minimum debt payments	14,016
Less: Amount representing interest	(1,296		)
Less: Debt discount and deferred charges	(621		)
Less: Current portion of loans payable	(4,122		)
Loans payable, net of current portion	$	7,977

Common Stock (Tables)

12 Months Ended

Dec. 31, 2016

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:



	Periods ending
	December 31, 2016	December 31, 2015
Conversion of Series A Preferred	—	664,068
Conversion of Series B Preferred	—	1,907,006
Conversion of Series C Preferred	—	1,282,051
Conversion of Series D Preferred	—	2,191,412
Conversion of Series SRN Preferred	—	1,798,433
Conversion of Series E Preferred	—	2,662,885
Exercise of common warrants	97,302	651,618
Shares available for future stock incentive awards	939,317	100,034
Exercise of outstanding common stock options	2,128,346	1,569,379
Total	3,164,965	12,826,886

- 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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
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-URI http://asc.fasb.org/extlink&oid=6928386&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=6928386&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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-URI http://asc.fasb.org/extlink&oid=6928386&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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-Paragraph 1
-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))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 11: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21521-112644

Reference 12: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
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-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21484-112644

Reference 13: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
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-URI http://asc.fasb.org/extlink&oid=6928386&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=6928386&loc=d3e21488-112644

+ Details

Stock Incentive Plans (Tables)

12 Months Ended

Dec. 31, 2016

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

Schedule of Weighted Average Assumptions Used for Stock Option Grants

The weighted average assumptions used for unvested non‑employee stock options are as follows:



	Year Ended December 31,
	2016		2015		2014
Risk‑free interest rate	2.06	%	1.57	%	1.22	%
Expected dividend yield	—		—		—
Expected life (in years)	8.80		6.46		5.87
Expected volatility	87.33	%	98.18	%	98.25	%

The weighted average assumptions used for employee stock option grants issued for the years ended December 31, 2016, 2015 and 2014 are as follows:



	Year Ended December 31,
	2016			2015			2014
Risk-free interest rate	1.42		%	1.79		%	1.93		%
Expected dividend yield	—			—			—
Expected life	6.04			6.02			5.94
Expected volatility	94.97		%	79.80		%	100.81		%
Weighted-average fair value of common stock	$	13.25		$	7.35		$	8.97

Summary of Activity

The following table summarizes the activity under the 2008 Plan and the 2016 Plan since December 31, 2014:



					Weighted‑average
			Weighted‑average		remaining	Aggregate
	Number of		exercise		contractual term	intrinsic value
	options		price		(in years)	(in thousands)
Employee
Outstanding at December 31, 2014	800,165		$	3.24	6.99	$	4,895
Granted	485,833		$	7.91
Exercised	(20,861	)	$	1.21
Forfeited	(17,977	)	$	6.32
Outstanding at December 31, 2015	1,247,160		$	5.05	7.41	$	3,130
Vested at December 31, 2015	668,465		$	2.61	5.71	$	2,946
Vested and expected to vest at December 31, 2015	1,171,380		$	4.84	7.26	$	3,138

Outstanding at December 31, 2015	1,247,160		$	5.05	7.41	$	3,130
Granted	543,595		$	13.25
Exercised	(57,850	)	$	2.00
Forfeited	(12,164	)	$	7.41
Outstanding at December 31, 2016	1,720,741		$	7.73	7.43	$	16,425
Vested at December 31, 2016	830,833		$	4.05	5.65	$	10,885
Vested and expected to vest at December 31, 2016	1,601,908		$	7.44	7.30	$	15,731
Non‑Employee
Outstanding at December 31, 2014	343,072		$	2.93	6.50	$	2,207
Granted	—		$	—
Exercised	(20,852	)	$	2.91
Forfeited	—		$	—
Outstanding at December 31, 2015	322,220		$	2.93	5.50	$	1,288
Vested at December 31, 2015	310,680		$	2.70	5.40	$	1,288
Vested and expected to vest at December 31, 2015	322,219		$	2.93	5.50	$	1,288

Outstanding at December 31, 2015	322,220		$	2.93	5.50	$	1,288
Granted	102,435		$	8.43
Exercised	(17,049	)	$	0.47
Forfeited	(1	)	$	3.44
Outstanding at December 31, 2016	407,605		$	4.42	5.79	$	5,224
Vested at December 31, 2016	327,879		$	3.44	4.98	$	4,508
Vested and expected to vest at December 31, 2016	406,200		$	4.40	5.77	$	5,215

Summary of Restricted Stock Award Activity

The following table summarizes the restricted stock award activity of the 2008 Plan and the 2016 Plan for the years ended December 31, 2016, 2015 and 2014:



	Year Ended December 31,
	2016				2015				2014
			Weighted‑				Weighted‑				Weighted‑
			average				average				average
			exercise				exercise				exercise
	Shares		price		Shares		price		Shares		price
Unvested at beginning of period	7,574		$	2.77	15,264		$	2.77	70,804		$	1.56
Issued	962		9.36		—		—		2,564		0.62
Vested	(8,536	)	3.51		(7,690	)	2.72		(58,104	)	1.44
Unvested at end of period	—		$	—	7,574		$	2.77	15,264		$	2.77

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



	Year Ended December 31,
	2016		2015		2014
Research and development	$	1,183	$	495	$	384
General and administrative	847		630		840
Total	$	2,030	$	1,125	$	1,224

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2016

Income Tax Disclosure [Abstract]

Reconciliation of Income Taxes

A reconciliation of income taxes computed using the U.S. federal statutory rate to that reflected in operations is as follows:



	Year Ended December 31,
	2016		2015		2014
Statutory U.S. federal rate	34.0	%	34	%	34	%
State income taxes - net of federal benefit	5.8	%	5.7	%	5.9	%
Permanent items	(1.1	)%	(0.7	)%	(0.7	)%
Research tax credits/other	1.3	%	1.0	%	1.6	%
Valuation allowance, net	(39.8	)%	(40	)%	(33	)%
Other	(0.2	)%	—	%	(7.8	)%
Net deferred tax assets	—	%	—	%	—	%

Schedule of Tax Effects of Temporary Differences

The tax effects of temporary differences that give rise to the Company's net deferred tax assets are as follows (in thousands):



	Year Ended December 31,
	2016			2015
Deferred Tax Assets
Net operating loss carryforwards	$	39,420		$	31,958
Research and development credits	3,052			2,305
Stock-based compensations expense	904			570
Deferred rent and other expenses	713			493
Deferred revenue	5,607			1,582
Patent costs/amortization	4,934			3,500
Tenant improvement allowance	—			—
Warrant liability	—			114
Gross deferred tax assets	54,630			40,522

Deferred Tax Liabilities
Depreciation	$	(131	)	$	(197	)
Debt discount	(69		)	(97		)
Unrealized foreign exchange gain	(1,525		)	(1,728		)
Gross deferred tax liabilities	(1,725		)	(2,022		)
Net deferred tax assets	52,905			38,500
Valuation allowance	(52,905		)	(38,500		)
Net deferred tax assets	$	—		$	—

Selected Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2016

Quarterly Financial Information Disclosure [Abstract]

Summary of Unaudited Quarterly Financial Data

The following table summarizes unaudited quarterly financial data for the years ended December 31, 2016, 2015 and 2014 (in thousands, except per share data).



	Three Months Ended (unaudited)
	March 31,			June 30,			September 30,			December 31,
	2016			2016			2016			2016
	(in thousands, except per share data)
Revenue	$	2,088		$	2,017		$	1,048		$	2,930
Operating expenses	9,029			8,418			8,516			16,790
Net (loss) income	$	(7,476	)	$	(6,923	)	$	(7,728	)	$	(14,083	)
Accretion on preferred stock	(2,356		)	(2,210		)	—			—
Net effect of extinguishment of preferred stock	—			—			—			—
Net (loss) income attributable to common stockholders - basic & diluted	$	(9,832	)	$	(9,133	)	$	(7,728	)	$	(14,083	)
Net (loss) per share attributable to common stockholders:
Basic & diluted	$	(4.52	)	$	(2.75	)	$	(0.43	)	$	(0.77	)
Weighted-average common shares outstanding used in net (loss) income per share attributable to common stockholders:
Basic & diluted	2,175,037			3,322,546			18,108,014			18,265,771



	Three Months Ended (unaudited)
	March 31,			June 30,			September 30,			December 31,
	2015			2015			2015			2015
	(in thousands, except per share data)
Revenue	$	1,034		$	1,236		$	1,607		$	2,134
Operating expenses	6,844			7,552			7,678			9,241
Net (loss) income	$	(5,733	)	$	(6,867	)	$	(5,725	)	$	(6,849	)
Accretion on preferred stock	(1,561		)	(1,562		)	(1,836		)	(2,376		)
Net effect of extinguishment of preferred stock	—			—			—			—
Net (loss) income attributable to common stockholders - basic & diluted	$	(7,294	)	$	(8,429	)	$	(7,561	)	$	(9,225	)
Net (loss) per share attributable to common stockholders:
Basic & diluted	$	(3.43	)	$	(3.93	)	$	(3.50	)	$	(4.26	)
Weighted-average common shares outstanding used in net (loss) income per share attributable to common stockholders:
Basic & diluted	2,126,878			2,147,184			2,159,658			2,167,769

Nature of the Business and Basis of Presentation - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands			1 Months Ended	12 Months Ended
	Jul. 25, 2016	Jun. 21, 2016	Jul. 25, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Jun. 27, 2016
Subsidiary, Sale of Stock [Line Items]
Net proceeds				$ 68,870	$ 0	$ 0
Common stock issued for convertible preferred stock (in shares)							10,126,118
IPO
Subsidiary, Sale of Stock [Line Items]
Number of shares issued		5,000,000
Price to the public (in dollars per share)		$ 14.00
Net proceeds		$ 60,800	$ 64,500
Over-Allotment Option
Subsidiary, Sale of Stock [Line Items]
Number of shares issued	289,633
Price to the public (in dollars per share)	$ 14.00		$ 14.00
Net proceeds	$ 3,700

Nature of the Business and Basis of Presentation - Liquidity (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Accumulated deficit	$ 151,576	$ 111,508
Cash and Cash Equivalents [Line Items]
Unrestricted cash held by subsidiary	58,656	32,337	$ 16,592	$ 8,057
Unrestricted cash \| Russian subsidiary
Cash and Cash Equivalents [Line Items]
Unrestricted cash held by subsidiary	$ 2,400	$ 3,000

Summary of Significant Accounting Policies - Segment Information (Details)	12 Months Ended
	Dec. 31, 2016 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, conversion ratio	0.2564

Summary of Significant Accounting Policies - Cash Equivalents and Short Term Investments (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash and Cash Equivalents [Line Items]
Realized gains or losses on sale of investments	$ 0	$ 0	$ 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, 2016 USD ($)
Concentration Risk [Line Items]
Cash maintained in Russian bank accounts	$ 2.5
U.S. dollars
Concentration Risk [Line Items]
Cash maintained in Russian bank accounts	$ 1.6

Summary of Significant Accounting Policies - Fair Value of Financial Instruments (Details)	Dec. 31, 2016
Loans payable
Debt Instrument [Line Items]
Interest rate on loans payable	11.60%

Summary of Significant Accounting Policies - Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2016
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) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($)
AOCI Attributable to Parent, Net of Tax
Beginning balance	$ (116,493)
Other comprehensive gain (loss) during the period	468
Ending balance	54,957
Foreign exchange translation adjustment
AOCI Attributable to Parent, Net of Tax
Beginning balance	(4,986)
Other comprehensive gain (loss) during the period	504
Ending balance	(4,482)
Unrealized gains (losses) on available-for-sale securities
AOCI Attributable to Parent, Net of Tax
Beginning balance	0
Other comprehensive gain (loss) during the period	(36)
Ending balance	(36)
Accumulated other comprehensive income (loss)
AOCI Attributable to Parent, Net of Tax
Beginning balance	(4,986)
Ending balance	$ (4,518)

Summary of Significant Accounting Policies - Warrants (Details)	Jun. 27, 2016 $ / shares
Accounting Policies [Abstract]
Fair value of underlying common shares (in dollars per share)	$ 14.00

Available-for-Sale Marketable Securities - Summary of Available-for-Sale Marketable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 24,821	$ 3,516
Gross Unrealized Gains	17	0
Gross Unrealized Losses	(53)	0
Estimated Fair Value	24,785	3,516
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	0	3,516
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	0	3,516
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	24,821	0
Gross Unrealized Gains	17	0
Gross Unrealized Losses	(53)	0
Estimated Fair Value	$ 24,785	$ 0

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, 2016	Dec. 31, 2015
Fair Value
Less than one year	$ 24,785	$ 3,516
Amortized Cost
Less than one year	$ 24,821	$ 3,516

Available-for-Sale Marketable Securities - Narrative (Details)	12 Months Ended
	Dec. 31, 2016 USD ($) position
Investments, Debt and Equity Securities [Abstract]
Number of positions in an unrealized loss position, less than 12 months	6
Number of investment positions	20
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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Numerator:
Net loss	$ (14,083)	$ (7,728)	$ (6,923)	$ (7,476)	$ (6,849)	$ (5,725)	$ (6,867)	$ (5,733)	$ (36,210)	$ (25,174)	$ (12,880)
Less: accretion on preferred stock	0	0	(2,210)	(2,356)	(2,376)	(1,836)	(1,562)	(1,561)	(4,566)	(7,335)	(4,951)
Net effect of extinguishment of Series SRN redeemable convertible preferred stock	0	0	0	0	0	0	0	0	0	0	1,459
Net loss attributable to common stockholders	$ (14,083)	$ (7,728)	$ (9,133)	$ (9,832)	$ (9,225)	$ (7,561)	$ (8,429)	$ (7,294)	$ (40,776)	$ (32,509)	$ (16,372)
Denominator:
Weighted‑average common shares outstanding—basic and diluted (in shares)	18,265,771	18,108,014	3,322,546	2,175,037	2,167,769	2,159,658	2,147,184	2,126,878	10,493,939	2,150,422	2,090,677
Net loss per share attributable to common stockholders—basic and diluted (in dollars per share)	$ (0.77)	$ (0.43)	$ (2.75)	$ (4.52)	$ (4.26)	$ (3.50)	$ (3.93)	$ (3.43)	$ (3.89)	$ (15.13)	$ (7.84)

Net Loss Per Share - Schedule of Potential Common Shares Issuable Upon Conversion of Warrants (Details) - shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Potential common shares
Total (in shares)	2,225,648	12,137,409	7,631,689
Redeemable convertible preferred stock
Potential common shares
Total (in shares)	0	9,890,580	6,471,358
Stock options to purchase common stock
Potential common shares
Total (in shares)	2,128,346	1,569,379	1,143,237
Stock warrants to purchase common stock
Potential common shares
Total (in shares)	97,302	650,618	0
Redeemable convertible preferred stock warrants
Potential common shares
Total (in shares)	0	26,832	17,094

Fair Value Measurements - Summary of Assets Measured at Fair Value on a Recurring Basis (Details) - Recurring - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Warrants to purchase redeemable convertible preferred stock, included in other long term liabilities	$ 0	$ 290	$ 236
(level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Warrants to purchase redeemable convertible preferred stock, included in other long term liabilities	0	0	0
(level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Warrants to purchase redeemable convertible preferred stock, included in other long term liabilities	0	0	0
(level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Warrants to purchase redeemable convertible preferred stock, included in other long term liabilities	0	290	$ 236
US Treasury obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in investments	0	3,516
US Treasury obligations \| (level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in investments	0	3,516
US Treasury obligations \| (level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in investments	0	0
US Treasury obligations \| (level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in investments	0	0
Corporate bonds, included in investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in investments	24,785	0
Corporate bonds, included in investments \| (level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in investments	24,785	0
Corporate bonds, included in investments \| (level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in investments	0	0
Corporate bonds, included in investments \| (level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in investments	0	0
Money market funds, included in cash equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in cash equivalents	183
Money market funds, included in cash equivalents \| (level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in cash equivalents	183
Money market funds, included in cash equivalents \| (level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in cash equivalents	0
Money market funds, included in cash equivalents \| (level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in cash equivalents	0
Tri-party repurchase agreements, included in cash equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in cash equivalents	27,000
Tri-party repurchase agreements, included in cash equivalents \| (level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in cash equivalents	0
Tri-party repurchase agreements, included in cash equivalents \| (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, included in cash equivalents \| (level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in cash equivalents	$ 0
US Treasury obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in cash equivalents		14,486
US Treasury obligations \| (level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in cash equivalents		14,486
US Treasury obligations \| (level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in cash equivalents		0
US Treasury obligations \| (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) - $ / shares				1 Months Ended	12 Months Ended
	Jun. 27, 2016	Jun. 21, 2016	Jul. 24, 2015	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Jul. 31, 2015	Apr. 30, 2015	Jul. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Warrants issued for purchase of stock (in shares)	80,813						315,198
Expiration period of warrants	3 years
Exercise price (in dollars per share)	$ 17.55
Number of warrants exercised to purchase common stock (in shares)				16,493
Net shares of common stock issued (in shares)				4,697
Conversion of Series D Preferred
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Warrants issued for purchase of stock (in shares)									66,668
Expiration period of warrants	4 years	4 years
Exercise price (in dollars per share)									$ 4.50
Conversion of Series E Preferred
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Warrants issued for purchase of stock (in shares)						37,978
Expiration period of warrants	4 years
Exercise price (in dollars per share)						$ 4.50
Convertible Debt \| Conversion of Series D Preferred
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Warrants issued for purchase of stock (in shares)	17,888
Convertible Debt \| Conversion of Series E Preferred
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Warrants issued for purchase of stock (in shares)	15,094
Exercise price (in dollars per share)	$ 11.32
Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Agreements collateralized by government securities (not less than) (as a percent)				102.00%	102.00%
Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Average maturity, term					297 days
US Treasury obligations \| US Treasury obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Average maturity, term						106 days
US Treasury obligations \| US Treasury obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Average maturity, term						34 days
Convertible Notes Payable
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Warrants issued for purchase of stock (in shares)			80,813
Expiration period of warrants			3 years
Exercise price (in dollars per share)			$ 17.55
Convertible Notes Payable \| Common stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Exercise price (in dollars per share)							$ 4.5	$ 17.55
Term Loan
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of warrants exercised to purchase common stock (in shares)				16,493
Net shares of common stock issued (in shares)				4,697
Term Loan \| Common stock \| Conversion of Series D Preferred
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Warrants issued for purchase of stock (in shares)		17,888
Term Loan \| Common stock \| Conversion of Series E Preferred
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Warrants issued for purchase of stock (in shares)		15,094
Term Loan \| Common stock \| Convertible Debt \| Conversion of Series D Preferred
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Exercise price (in dollars per share)		$ 16.77

Fair Value Measurements - Schedule of the Fair Values of Outstanding Warrants (Details) - Warrant Liability - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Fair value at beginning of period	$ 290	$ 236
Fair value of warrants issued	0	137
Change in fair value	12	(83)
Reclassification to additional paid in capital at IPO	(302)
Fair value at end of period	$ 0	$ 290

Property and Equipment - Schedule of Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 5,787	$ 4,956
Less accumulated depreciation	(3,740)	(2,927)
Property and equipment, net	2,047	2,029
Laboratory equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	4,713	4,028
Computer equipment and software
Property, Plant and Equipment [Line Items]
Total property and equipment	562	409
Leasehold improvements
Property, Plant and Equipment [Line Items]
Total property and equipment	222	91
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Total property and equipment	226	222
Office equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	64	62
P,P&E—Construction in process
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 0	$ 144

Property and Equipment - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Property, Plant and Equipment [Abstract]
Depreciation expense	$ 802	$ 1,044	$ 864

Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Payables and Accruals [Abstract]
Payroll and employee related expenses	$ 1,551	$ 669
Legal	196	213
Current portion of deferred rent and lease incentive	29	405
Accrued patent fees	415	219
Accrued external research and development costs	794	1,649
Accrued audit fees	224	135
Accrued grant refund	152	0
Accrued interest	81	34
Other	479	54
Accrued expenses	$ 3,921	$ 3,378

Commitments and Contingencies - Narrative (Details)	12 Months Ended
Commitments and Contingencies - Narrative (Details)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2012 USD ($)	Apr. 30, 2015 ft²
Sublease income	$ 0	$ 200,000	$ 700,000
Deferred rent and lease incentive liability	300,000	500,000
Current portion of deferred rent and lease incentive liability (less than in 2016)	29,000	405,000
Rent expense, net of sublease payments	1,600,000	1,100,000	$ 600,000
Restricted cash and other deposits \| Letter of Credit
Letter of credit	300,000.0
Accrued expenses
Current portion of deferred rent and lease incentive liability (less than in 2016)	$ 100,000	$ 400,000
Laboratory and office space
Area exchanged (in square feet) \| ft²					13,711
Laboratory and office space agreement amendment
Area exchanged (in square feet) \| ft²					15,174
Maximum \| Laboratory and office space
Tenant improvement allowance				$ 700,000.0

Commitments and Contingencies - Schedule of Future Minimum Lease Payments (Details) $ in Thousands	Dec. 31, 2016 USD ($)
Year ended December 31,
2017	$ 1,244
2018	1,291
2019	1,330
2020	335
Total minimum lease payments	$ 4,200

Debt - Term Loans (Narrative) (Details)			1 Months Ended			12 Months Ended
Debt - Term Loans (Narrative) (Details)	Jun. 27, 2016 $ / shares shares	Jun. 21, 2016 USD ($) shares	Dec. 31, 2016 USD ($) shares	Jul. 31, 2014 USD ($) $ / shares shares	Aug. 31, 2013 USD ($) shares	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2014 USD ($)	Jul. 31, 2015 shares	Aug. 09, 2013 USD ($) lender
Line of Credit Facility [Line Items]
Borrowings on loan and security agreement \| $						$ 0	$ 6,674,000	$ 4,494,000
Warrants issued for purchase of stock (in shares)	80,813								315,198
Exercise price (in dollars per share) \| $ / shares	$ 17.55
Expiration period of warrants	3 years
Interest expense related to the Term Loan \| $						1,253,000	$ 948,000	552,000
Number of warrants exercised to purchase common stock (in shares)			16,493
Net shares of common stock issued (in shares)			4,697
Conversion of Series D Preferred
Line of Credit Facility [Line Items]
Warrants issued for purchase of stock (in shares)				66,668
Exercise price (in dollars per share) \| $ / shares				$ 4.50
Expiration period of warrants	4 years	4 years
Conversion of Series E Preferred
Line of Credit Facility [Line Items]
Warrants issued for purchase of stock (in shares)							37,978
Exercise price (in dollars per share) \| $ / shares							$ 4.50
Expiration period of warrants	4 years
Term Loans
Line of Credit Facility [Line Items]
Number of lenders \| lender										2
Borrowing capacity \| $							$ 12,000,000.0			$ 7,500,000.0
Borrowings on loan and security agreement \| $				$ 4,500,000	$ 3,000,000
Outstanding principal balance under Term Loans \| $			$ 12,000,000			$ 12,000,000	12,000,000
Interest rate per annum						8.10%
Number of monthly installments						30 months
Final payment fee (as a percent)						6.00%
Total final payment fee \| $			$ 700,000			$ 700,000
Initial grant date fair value of warrants \| $		$ 100,000		$ 100,000
Interest expense related to the Term Loan \| $						$ 1,200,000	$ 600,000	$ 600,000
Number of warrants exercised to purchase common stock (in shares)			16,493
Net shares of common stock issued (in shares)			4,697
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 prior to first anniversary
Line of Credit Facility [Line Items]
Prepayment fee (as a percent)			3.00%			3.00%
Term Loans \| Prepaid prior to first anniversary but before second
Line of Credit Facility [Line Items]
Prepayment fee (as a percent)			2.00%			2.00%
Term Loans \| Prepaid after second anniversary
Line of Credit Facility [Line Items]
Prepayment fee (as a percent)			1.00%			1.00%

Debt - Schedule of Future Minimum Payments on the Term Loans (Details) - Term Loan $ in Thousands	Dec. 31, 2016 USD ($)
Year ended December 31,
2017	$ 5,319
2018	5,318
2019	3,379
Total minimum debt payments	14,016
Less: Amount representing interest	(1,296)
Less: Debt discount and deferred charges	(621)
Less: Current portion of loans payable	(4,122)
Loans payable, net of current portion	$ 7,977

Debt - Convertible Notes (Narrative) (Details) - USD ($)			12 Months Ended
	Jun. 27, 2016	Jul. 24, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Jul. 31, 2015	Apr. 30, 2015
Debt Instrument [Line Items]
Exercise price (in dollars per share)	$ 17.55
Warrants issued to convertible note holders (in shares)	80,813					315,198
Term of warrants	3 years
Conversion of Series E Preferred
Debt Instrument [Line Items]
Exercise price (in dollars per share)				$ 4.50
Warrants issued to convertible note holders (in shares)				37,978
Term of warrants	4 years
Convertible notes
Debt Instrument [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
Warrants issued to convertible note holders (in shares)		80,813
Term of warrants		3 years
Interest expense related to convertible notes			$ 0	$ 300,000	$ 0
Convertible notes \| Conversion of Series E Preferred
Debt Instrument [Line Items]
Shares issued on conversion of outstanding convertible notes (in shares)		1,619,550
Convertible notes \| Common stock
Debt Instrument [Line Items]
Exercise price (in dollars per share)						$ 4.5	$ 17.55
Convertible notes \| Common stock \| Conversion of Series E Preferred \| Maximum
Debt Instrument [Line Items]
Shares issued on conversion of outstanding convertible notes (in shares)		103,817

Preferred Stock (Details) - USD ($)			1 Months Ended	2 Months Ended	5 Months Ended	12 Months Ended
Preferred Stock (Details) - USD ($)	Jun. 27, 2016	Jul. 24, 2015	Jul. 31, 2014	Sep. 30, 2015	Aug. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Temporary Equity Disclosure [Abstract]
Authorized shares of Preferred Stock (in shares)							37,835,623	28,804,969
Convertible preferred stock, par value (in dollars per share)						$ 0.0001
Preferred stock
Carrying value of Series SRN Preferred						$ 0	$ 137,482,000
Issuance costs						$ 4,103,000	$ 302,000	$ 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
Temporary Equity Disclosure [Abstract]
Authorized shares of Preferred Stock (in shares)						0	2,589,868
Convertible preferred stock, par value (in dollars per share)						$ 0.0001	$ 0.0001
Preferred stock
Convertible preferred stock, shares issued						0	2,589,868
Series B redeemable convertible preferred stock
Temporary Equity Disclosure [Abstract]
Authorized shares of Preferred Stock (in shares)						0	7,437,325
Convertible preferred stock, par value (in dollars per share)						$ 0.0001	$ 0.0001
Preferred stock
Convertible preferred stock, shares issued						0	7,437,325
Series C redeemable convertible preferred stock
Temporary Equity Disclosure [Abstract]
Authorized shares of Preferred Stock (in shares)						0	5,000,002
Convertible preferred stock, par value (in dollars per share)						$ 0.0001	$ 0.0001
Preferred stock
Convertible preferred stock, shares issued						0	5,000,002
Series D redeemable convertible preferred stock
Temporary Equity Disclosure [Abstract]
Authorized shares of Preferred Stock (in shares)						0	8,166,662
Convertible preferred stock, par value (in dollars per share)						$ 0.0001	$ 0.0001
Preferred stock
Convertible preferred stock, shares issued						0	8,099,994
Number of shares issued					3,211,105
Price per share (in dollars per share)					$ 4.50
Total net proceeds from issuance of Series D Preferred					$ 14,349,239
Series SRN redeemable convertible preferred stock
Temporary Equity Disclosure [Abstract]
Authorized shares of Preferred Stock (in shares)						0	5,611,112
Convertible preferred stock, par value (in dollars per share)						$ 0.0001	$ 0.0001
Preferred stock
Convertible preferred stock, shares issued						0	2,111,109
Number of shares issued			1,333,332
Price per share (in dollars per share)			$ 4.50
Total net proceeds from issuance of Series D Preferred			$ 5,800,000
Conversion price (in dollars per share)			$ 16.77
Carrying value of Series SRN Preferred			$ 5,000,000
Difference recorded as additional paid in capital			$ 1,500,000
Series E redeemable convertible preferred stock
Temporary Equity Disclosure [Abstract]
Authorized shares of Preferred Stock (in shares)						0	9,030,654
Convertible preferred stock, par value (in dollars per share)						$ 0.0001	$ 0.0001
Preferred stock
Convertible preferred stock, shares issued						0	8,888,888
Number of shares issued				7,269,338
Price per share (in dollars per share)				$ 4.50
Total gross proceeds from issuance of Series E Preferred				$ 32,700,000
Issuance costs				$ 200,000
Warrants to purchase common stock, percentage of Series E Preferred shares issued		25.00%
Series E redeemable convertible preferred stock \| Convertible notes
Preferred stock
Number of shares issued		1,619,550

Common Stock - Narrative (Details)	12 Months Ended
Common Stock - Narrative (Details)	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 $ / shares shares
Equity [Abstract]
Common stock, shares authorized	200,000,000	62,164,377
Common stock, par value (in dollars per share) \| $ / shares	$ 0.0001	$ 0.0001
Common stock, shares issued	18,438,742	2,180,976
Common stock, shares outstanding	18,438,742	2,173,399
Number of votes per share that common stockholders are entitled to	1
Dividends declared or paid on common stock \| $	$ 0

Common Stock - Schedule of Authorized Shares of Common Stock for Future Issuance (Details) - shares	Dec. 31, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Total (in shares)	3,164,965	12,826,886
Shares available for future stock incentive awards
Class of Stock [Line Items]
Total (in shares)	939,317	100,034
Exercise of outstanding common stock options
Class of Stock [Line Items]
Total (in shares)	2,128,346	1,569,379
Exercise of common warrants
Class of Stock [Line Items]
Total (in shares)	97,302	651,618
Conversion of Series A Preferred
Class of Stock [Line Items]
Total (in shares)	0	664,068
Conversion of Series B Preferred
Class of Stock [Line Items]
Total (in shares)	0	1,907,006
Conversion of Series C Preferred
Class of Stock [Line Items]
Total (in shares)	0	1,282,051
Conversion of Series D Preferred
Class of Stock [Line Items]
Total (in shares)	0	2,191,412
Conversion of Series SRN Preferred
Class of Stock [Line Items]
Total (in shares)	0	1,798,433
Conversion of Series E Preferred
Class of Stock [Line Items]
Total (in shares)	0	2,662,885

Stock Incentive Plans - Additional Information (Narrative) (Details) $ / shares in Units, $ in Millions		12 Months Ended
	Jun. 21, 2016 shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2014 USD ($) $ / shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available for future grants		3,164,965	12,826,886
Unrecognized compensation expense related to unvested non-employee stock options \| $		$ 1.1	$ 0.1	$ 0.4
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		939,317	100,034
Weighted average grant date fair value of stock options (in dollars per share) \| $ / shares		$ 10.23	$ 5.03	$ 6.86
Aggregate intrinsic value of stock options \| $		$ 0.8	$ 0.3	$ 0.3
Unrecognized compensation expense related to unvested employee stock options \| $		$ 5.6	$ 2.3	$ 0.9
Weighted average period for recognition		3 years 7 days	3 years 1 month 6 days	2 years 3 months 18 days
Non-employee stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Non-employee stock-based compensation expense \| $		$ 0.4	$ 0.4	$ 0.5
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
Estimated forfeitures rate		10.00%
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		1,210,256
Number of shares available for future grants		747,294
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%
ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available for future grants	173,076
Automatic increase in the number of shares that may be issued (as a percent)	0.01
Shares issued under ESPP (in shares)		0
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 (Details) - Shares available for future stock incentive awards - $ / shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	1.42%	1.79%	1.93%
Expected dividend yield (as a percent)	0.00%	0.00%	0.00%
Expected life	6 years 16 days	6 years 6 days	5 years 11 months 9 days
Expected volatility (as a percent)	94.97%	79.80%	100.81%
Weighted-average fair value of common stock (in dollars per share)	$ 13.25	$ 7.35	$ 8.97

Stock Incentive Plans - Schedule of Weighted Average Assumptions for Unvested Non Employee Stock Options (Details) - Non-employee stock options	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Goods and Nonemployee Services Transaction [Line Items]
Risk‑free interest rate	2.06%	1.57%	1.22%
Expected dividend yield (as a percent)	0.00%	0.00%	0.00%
Expected life	8 years 9 months 18 days	6 years 5 months 16 days	5 years 10 months 13 days
Expected volatility (as a percent)	87.33%	98.18%	98.25%

Stock Incentive Plans - Summary of Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Employee
Aggregate intrinsic value
Vested and expected to vest	$ 800	$ 300	$ 300
The 2008 Plan and the 2016 Plan \| Employee
Number of Options
Beginning balance (in shares)	1,247,160	800,165
Granted (in shares)	543,595	485,833
Exercised (in shares)	(57,850)	(20,861)
Forfeited (in shares)	(12,164)	(17,977)
Ending balance (in shares)	1,720,741	1,247,160	800,165
Vested (in shares)	830,833	668,465
Vested and expected to vest (in shares)	1,601,908	1,171,380
Weighted-average exercise price
Beginning balance (in dollars per share)	$ 5.05	$ 3.24
Granted (in dollars per share)	13.25	7.91
Exercised (in dollars per share)	2.00	1.21
Forfeited (in dollars per share)	7.41	6.32
Ending balance (in dollars per share)	7.73	5.05	$ 3.24
Vested (in dollars per share)	4.05	2.61
Vested and expected to vest (in dollars per share)	$ 7.44	$ 4.84
Weighted-average remaining contractual term
Outstanding, term	7 years 5 months 6 days	7 years 4 months 28 days	6 years 11 months 27 days
Vested, term	5 years 7 months 26 days	5 years 8 months 16 days
Vested and expected to vest, term	7 years 3 months 20 days	7 years 3 months 4 days
Aggregate intrinsic value
Outstanding	$ 16,425	$ 3,130	$ 4,895
Vested	10,885	2,946
Vested and expected to vest	$ 15,731	$ 3,138
The 2008 Plan and the 2016 Plan \| Non‑Employee
Number of Options
Beginning balance (in shares)	322,220	343,072
Granted (in shares)	102,435	0
Exercised (in shares)	(17,049)	(20,852)
Forfeited (in shares)	(1)	0
Ending balance (in shares)	407,605	322,220	343,072
Vested (in shares)	327,879	310,680
Vested and expected to vest (in shares)	406,200	322,219
Weighted-average exercise price
Beginning balance (in dollars per share)	$ 2.93	$ 2.93
Granted (in dollars per share)	8.43	0.00
Exercised (in dollars per share)	0.47	2.91
Forfeited (in dollars per share)	3.44	0.00
Ending balance (in dollars per share)	4.42	2.93	$ 2.93
Vested (in dollars per share)	3.44	2.70
Vested and expected to vest (in dollars per share)	$ 4.40	$ 2.93
Weighted-average remaining contractual term
Outstanding, term	5 years 9 months 15 days	5 years 6 months	6 years 6 months
Vested, term	4 years 11 months 23 days	5 years 4 months 24 days
Vested and expected to vest, term	5 years 9 months 7 days	5 years 6 months
Aggregate intrinsic value
Outstanding	$ 5,224	$ 1,288	$ 2,207
Vested	4,508	1,288
Vested and expected to vest	$ 5,215	$ 1,288

Stock Incentive Plans - Restricted Stock (Narrative) (Details) - USD ($)	12 Months Ended				24 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cumulative payments received	$ 2,743,000	$ 0	$ 0
Restricted shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares issued upon the early exercise of stock options				30,317
Number of shares issued			2,564
Cumulative payments received					$ 100,000
Total fair value of restricted shares that vested (less than)	100,000	$ 100,000	$ 100,000
Unrecognized compensation cost related to unvested restricted stock	$ 0

Stock Incentive Plans - Summary of Restricted Stock Award Activity (Details) - Restricted shares - $ / shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Shares
Beginning balance (in shares)	7,574	15,264	70,804
Issued (in shares)	962	0	2,564
Vested (in shares)	(8,536)	(7,690)	(58,104)
Ending balance (in shares)	0	7,574	15,264
Weighted-average exercise price
Beginning balance (in dollars per share)	$ 2.77	$ 2.77	$ 1.56
Issued (in dollars per share)	9.36	0.00	0.62
Vested (in dollars per share)	3.51	2.72	1.44
Ending balance (in dollars per share)	$ 0.00	$ 2.77	$ 2.77

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, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total	$ 2,051	$ 1,125	$ 1,224
Stock options and restricted common stock
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total	2,030	1,125	1,224
Stock options and restricted common stock \| Research and development
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total	1,183	495	384
Stock options and restricted common stock \| General and administrative
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total	$ 847	$ 630	$ 840

Revenue Arrangements - Spark license agreement (Narrative) (Details)		1 Months Ended	12 Months Ended
	Dec. 02, 2016 USD ($) target	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Aggregate gross proceeds			$ 2,743,000	$ 0	$ 0
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
Number of additional targets \| target	4
Variable fee the Company is eligible to receive for each additional target option elected	$ 2,000,000.0
Period for Spark to exercise options for additional licenses	3 years
Additional development and regulatory milestone payments, assuming successful development and commercialization	$ 65,000,000.0
Commercialization milestone payments, assuming successful development and commercialization	$ 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
Number of shares issued \| shares		197,238
Aggregate gross proceeds		$ 5,000,000
Share price (in dollars per share) \| $ / shares		$ 25.35	$ 25.35
Premium of VWAP (as a percent)		115.00%	115.00%
Consecutive calendar days leading up to and ending on the day prior to the Contract Date		30 days
Number of shares authorized to be issued pursuant to the Stock Purchase Agreement \| shares		2,758,112	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	$ 5,000,000.0
Second 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	$ 5,000,000.0
MIT \| Spark License Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contractual payments made for MIT sub-license provided to Spark	$ 1,500,000
Payment relative to calculated premium paid by Spark for initial equity investment	$ 200,000

Revenue Arrangements - Accounting Analysis (Narrative) (Details)		12 Months Ended
	Dec. 02, 2016 USD ($) target	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Equity recorded from initial stock purchase		$ 2,743,000
Deferred revenue recorded as a current liability		1,836,000	$ 1,313,000
Deferred revenue classified as a long term liability		12,439,000	2,295,000
Spark License Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Number of additional target therapy options \| target	4
Equity recorded from initial stock purchase	$ 2,700,000
Total non-contingent payments	15,000,000
Consideration allocated to combined unit of accounting for License Deliverables and Supply Deliverable	7,100,000
Discount associated with Option Deliverable	$ 5,200,000
Initial estimated performance period	4 years
Revenue recognized under agreement		0	$ 0	$ 0
Deferred revenue related to agreement		12,300,000
Deferred revenue recorded as a current liability		1,800,000
Deferred revenue classified as a long term liability		$ 10,500,000

Revenue Arrangements - Sanofi Collaboration Agreement (Narrative) (Details) - Sanofi - USD ($) $ in Millions		1 Months Ended				12 Months Ended			50 Months Ended
	Nov. 27, 2012	Jul. 31, 2016	May 31, 2015	Aug. 31, 2013	Nov. 30, 2012	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2016
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront payment for first indication					$ 2.0
Additional payment for initial indication				$ 3.0
Additional payments received upon exercise of option			$ 2.0
Additional payment upon attaining first milestone for celiac disease indication		$ 1.0
Aggregate payments under Sanofi Agreement									$ 8.0
Initial period to complete research obligations	3 years
Revenue						$ 2.8	$ 2.1	$ 2.1

Revenue Arrangements - Termination of the Sanofi Collaboration Agreement (Narrative) (Details) - Sanofi - USD ($) $ in Millions	12 Months Ended	50 Months Ended
	Dec. 31, 2016	Dec. 31, 2016
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, 2016	Dec. 31, 2015	Dec. 31, 2014
Other Research And Collaboration Agreements
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue	$ 100,000	$ 300,000	$ 0

Revenue Arrangements - NIH (Narrative) (Details) - NIH $ in Millions	1 Months Ended		12 Months Ended
	Sep. 30, 2015 USD ($)	May 31, 2014 USD ($)	Dec. 31, 2016 USD ($) grant	Dec. 31, 2015 USD ($)	Dec. 31, 2014 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		$ 8.1
Revenue recognized			$ 4.1	$ 2.4	$ 0.6
Second Grant
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Aggregate amount of grant	$ 0.2
Revenue recognized			$ 0.1	$ 0.1

Revenue Arrangements - JDRF (Narrative) (Details) - JDRF $ in Millions	1 Months Ended	12 Months Ended
	Sep. 30, 2014 USD ($)	Dec. 31, 2016 USD ($) contract	Dec. 31, 2015 USD ($) contract	Dec. 31, 2014 USD ($) contract
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Number of contracts in effect \| contract		1	1	2
First Contract
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Aggregate amount of grant				$ 0.8
Revenue recognized				0.2
Second Contract
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Aggregate amount of grant	$ 0.4
Revenue recognized		$ 0.1	$ 0.2	$ 0.1

Revenue Arrangements - Bill and Melinda Gates Foundation (Narrative) (Details) - Bill and Melinda Gates Foundation - USD ($) $ in Millions	12 Months Ended				24 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2014
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Aggregate amount of grant				$ 1.2	$ 1.6
Extended research term			3 years
Revenue recognized	$ 0.5	$ 0.6	$ 0.2

Revenue Arrangements - Minpromtorg (Narrative) (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended			23 Months Ended	30 Months Ended
	Feb. 28, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Payments received		$ (461)	$ (805)	$ 305
Minpromtorg
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Contract awarded						$ 4,600
Repayment of funds	$ 200
Penalty fee, as percent of contract value	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			25 Months Ended
	Nov. 28, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2016
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Payments received		$ (461,000)	$ (805,000)	$ 305,000
Skolkovo
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Aggregate amount of grant	$ 2,700,000.0
Percentage of estimated cost of research plan	48.50%
Remaining percentage 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	2 Months Ended	5 Months Ended	12 Months Ended
	Sep. 30, 2015	Aug. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Apr. 30, 2015
Series D redeemable convertible preferred stock
Related Party Transaction [Line Items]
Issued (in shares)		3,211,105
Share price (in dollars per share)		$ 4.50
Conversion of Series E Preferred
Related Party Transaction [Line Items]
Issued (in shares)	7,269,338
Share price (in dollars per share)	$ 4.50
Landlord \| Series B redeemable convertible preferred stock
Related Party Transaction [Line Items]
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]
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
Founder
Related Party Transaction [Line Items]
Expenses for consulting services			$ 0.3	$ 0.3	$ 0.3

Technology License Agreements - MIT (Narrative) (Details) - MIT - USD ($)	Dec. 02, 2016	Nov. 25, 2008
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Payment amount upon achievement of defined clinical milestones		$ 1,500,000.0
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%
Spark License Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contractual payments defined in the Exclusive Patent License agreement	$ 1,500,000
Payment relative to calculated premium paid by Spark for initial equity investment	$ 200,000

Technology License Agreements - Shenyang Sunshine Pharmaceutical Co., Ltd (Narrative) (Details) - 3SBio License - USD ($) $ in Millions	1 Months Ended
	May 31, 2014	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
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	8 Months Ended
	May 31, 2016 USD ($) product	Dec. 31, 2016 USD ($)
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	$ 4,175,000
Payment under sales milestone method	50,000,000
Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Payment under development milestone method	37,025,000
Payment under sales milestone method	$ 70,000,000
Initial license fee paid		$ 100,000

Income Taxes - Reconciliation of Income Taxes (Details)	12 Months Ended
Income Taxes - Reconciliation of Income Taxes (Details)	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Statutory U.S. federal rate	34.00%	34.00%	34.00%
State income taxes - net of federal benefit	5.80%	5.70%	5.90%
Permanent items	(1.10%)	(0.70%)	(0.70%)
Research tax credits/other	1.30%	1.00%	1.60%
Valuation allowance, net	(39.80%)	(40.00%)	(33.00%)
Other	(0.20%)	0.00%	(7.80%)
Net deferred tax assets	0.00%	0.00%	0.00%

Income Taxes - Schedule of Tax Effects of Temporary Differences (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Deferred Tax Assets
Net operating loss carryforwards	$ 39,420	$ 31,958
Research and development credits	3,052	2,305
Stock-based compensations expense	904	570
Deferred rent and other expenses	713	493
Deferred revenue	5,607	1,582
Patent costs/amortization	4,934	3,500
Tenant improvement allowance	0	0
Warrant liability	0	114
Gross deferred tax assets	54,630	40,522
Deferred Tax Liabilities
Depreciation	(131)	(197)
Debt discount	(69)	(97)
Unrealized foreign exchange gain	(1,525)	(1,728)
Gross deferred tax liabilities	(1,725)	(2,022)
Net deferred tax assets	52,905	38,500
Valuation allowance	(52,905)	(38,500)
Net deferred tax assets	$ 0	$ 0

Income Taxes - Narrative (Details) - USD ($)	3 Months Ended	12 Months Ended
Income Taxes - Narrative (Details) - USD ($)	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Holiday [Line Items]
Increase in valuation allowance		$ 14,400,000	$ 10,100,000
Excess stock option deductions included in net operating loss carryforwards		800,000
Research and development tax credit carryforwards		3,052,000	2,305,000
Increase in net operating losses deferred tax assets		39,420,000	31,958,000
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 in Russia				10 years
Federal
Income Tax Holiday [Line Items]
Net operating loss carryforwards		102,100,000
Research and development tax credit carryforwards		2,100,000
State
Income Tax Holiday [Line Items]
Net operating loss carryforwards		95,600,000
Research and development tax credit carryforwards		1,500,000
Stock compensation \| Federal
Income Tax Holiday [Line Items]
Net operating loss carryforwards		$ 800,000
Stock compensation \| State
Income Tax Holiday [Line Items]
Net operating loss carryforwards			$ 800,000
Scenario, Forecast \| ASU 2016-09 \| New Accounting Pronouncement, Early Adoption, Effect
Income Tax Holiday [Line Items]
Increase in valuation allowance	$ 300,000
Increase in net operating losses deferred tax assets	$ 300,000

401(k) Savings Plan (Details) - 401(k) Plan - Defined-contribution savings plan - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Contribution Plan Disclosure [Line Items]
Vesting period	4 years
Employer contribution made	$ 0.1	$ 0.1	$ 0.1

Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 2,930	$ 1,048	$ 2,017	$ 2,088	$ 2,134	$ 1,607	$ 1,236	$ 1,034	$ 8,083	$ 6,011	$ 3,040
Operating expenses	16,790	8,516	8,418	9,029	9,241	7,678	7,552	6,844	42,753	31,315	18,439
Net (loss) income	(14,083)	(7,728)	(6,923)	(7,476)	(6,849)	(5,725)	(6,867)	(5,733)	(36,210)	(25,174)	(12,880)
Accretion on preferred stock	0	0	(2,210)	(2,356)	(2,376)	(1,836)	(1,562)	(1,561)	(4,566)	(7,335)	(4,951)
Net effect of extinguishment of preferred stock	0	0	0	0	0	0	0	0	0	0	1,459
Net loss attributable to common stockholders	$ (14,083)	$ (7,728)	$ (9,133)	$ (9,832)	$ (9,225)	$ (7,561)	$ (8,429)	$ (7,294)	$ (40,776)	$ (32,509)	$ (16,372)
Net (loss) per share attributable to common stockholders:
Basic & diluted (in dollars per share)	$ (0.77)	$ (0.43)	$ (2.75)	$ (4.52)	$ (4.26)	$ (3.50)	$ (3.93)	$ (3.43)	$ (3.89)	$ (15.13)	$ (7.84)
Weighted-average common shares outstanding used in net (loss) income per share attributable to common stockholders:
Basic & diluted (in shares)	18,265,771	18,108,014	3,322,546	2,175,037	2,167,769	2,159,658	2,147,184	2,126,878	10,493,939	2,150,422	2,090,677