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Document and Entity Information - shares	3 Months Ended
Document and Entity Information - shares	Mar. 31, 2018	May 04, 2018
Document and Entity Information
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q1
Entity Registrant Name	SELECTA BIOSCIENCES INC
Entity Central Index Key	0001453687
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		22,349,840

Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Current assets:
Cash, cash equivalents, and restricted cash	$ 58,304	$ 70,698
Short-term deposits and investments	24,839	25,940
Accounts receivable	52	63
Prepaid expenses and other current assets	1,789	1,979
Total current assets	84,984	98,680
Property and equipment, net	2,257	2,091
Restricted cash and other assets	2,696	329
Total assets	89,937	101,100
Current liabilities:
Accounts payable	1,789	1,606
Accrued expenses	9,062	8,580
Deferred revenue, current portion	959	787
Total current liabilities	11,810	10,973
Non‑current liabilities:
Deferred rent and lease incentive	129	151
Loans payable, net of current portion	21,127	21,042
Deferred revenue, net of current portion	13,917	15,919
Other long‑term liabilities	1,372	1,201
Total liabilities	48,355	49,286
Commitments and contingencies (Notes 8 and 12)
Stockholders’ equity:
Preferred stock, $0.0001 par value; 10,000,000 shares authorized; no shares issued and outstanding at March 31, 2018 and December 31, 2017, respectively	0	0
Common stock, $0.0001 par value; 200,000,000 shares authorized; 22,349,840 and 22,343,254 shares issued and outstanding as of March 31, 2018 and December 31, 2017, respectively	3	3
Additional paid-in capital	274,334	273,128
Accumulated deficit	(228,357)	(216,897)
Accumulated other comprehensive loss	(4,398)	(4,420)
Total stockholders’ equity	41,582	51,814
Total liabilities and stockholders’ equity	$ 89,937	$ 101,100

Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	22,349,840	22,343,254
Common stock, shares outstanding	22,349,840	22,343,254

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement of Comprehensive Income [Abstract]
Grant and collaboration revenue	$ 0	$ 137
Operating expenses:
Research and development	11,139	11,044
General and administrative	4,674	3,875
Total operating expenses	15,813	14,919
Loss from operations	(15,813)	(14,782)
Investment income	288	113
Foreign currency transaction gain (loss), net	(13)	(165)
Interest expense	(350)	(300)
Net loss	(15,888)	(15,134)
Other comprehensive loss:
Foreign currency translation adjustment	19	123
Unrealized gain on securities	3	15
Total comprehensive loss	(15,866)	(14,996)
Net loss attributable to common stockholders	$ (15,888)	$ (15,134)
Net loss per share attributable to common stockholders:
Net loss per share attributable to common stockholders—basic and diluted (in dollars per share)	$ (0.71)	$ (0.82)
Weighted average common shares outstanding:
Basic and diluted (in shares)	22,345,523	18,474,227

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Cash flows from operating activities
Net loss	$ (15,888)	$ (15,134)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	169	175
Amortization of premiums and accretion of discounts on investments	(32)	89
Stock‑based compensation expense	1,153	693
Non‑cash interest expense	177	144
Changes in operating assets and liabilities:
Accounts receivable	11	152
Prepaid expenses and other current assets	417	(316)
Accounts payable	180	(2,589)
Deferred revenue	0	57
Accrued expenses and other liabilities	404	1,413
Net cash used in operating activities	(13,409)	(15,316)
Cash flows from investing activities
Receipts from the maturity of investments	15,045	0
Purchase of short-term investments	(13,908)	(16,474)
Purchases of property and equipment	(192)	(58)
Net cash provided by (used in) investing activities	945	(16,532)
Cash flows from financing activities
Repayments of long-term debt	0	(728)
Proceeds from exercise of stock options	0	436
Proceeds from issuance of common stock under Employee Stock Purchase Plan	51	0
Net cash provided by (used in) financing activities	51	(292)
Effect of exchange rate changes on cash	19	124
Net decrease in cash, cash equivalents, and restricted cash	(12,394)	(32,016)
Cash, cash equivalents, and restricted cash at beginning of period	71,027	59,050
Cash, cash equivalents, and restricted cash at end of period	58,633	27,034
Supplement cash flow information
Cash paid for interest	261	241
Noncash investing activities
Purchase of property and equipment not yet paid	142	125
Unrealized gain on marketable securities	$ 3	$ 15

Nature of the Business and Basis of Presentation	3 Months Ended
Nature of the Business and Basis of Presentation	Mar. 31, 2018
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. Unaudited Interim Financial Information The accompanying unaudited consolidated financial statements for the three months ended March 31, 2018 and 2017 have been prepared by the Company, pursuant to the rules and regulations of the Securities and Exchange Commission (the “SEC”) for interim financial statements. Certain information and footnote disclosures normally included in financial statements prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) have been condensed or omitted pursuant to such rules and regulations. However, the Company believes that the disclosures are adequate to make the information presented not misleading. These consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements and the notes thereto for the year ended December 31, 2017 included in the Company’s Annual Report on Form 10-K that was filed with the SEC on March 15, 2018 (the "Annual Report on Form 10-K"). The unaudited interim financial statements have been prepared on the same basis as the audited consolidated financial statements. In the opinion of management, the accompanying unaudited interim consolidated financial statements contain all adjustments which are necessary for a fair statement of the Company’s financial position as of March 31, 2018 and consolidated results of operations and cash flows for the three months ended March 31, 2018. Such adjustments are of a normal and recurring nature. The results of operations for the three months ended March 31, 2018 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2018. Liquidity The Company has incurred losses since inception and negative cash flows from operating activities. As of March 31, 2018 and December 31, 2017, the Company had an accumulated deficit of $228.4 million and $216.9 million, respectively. The Company’s cash and cash equivalents as of March 31, 2018 and December 31, 2017, includes $1.0 million and $1.2 million of unrestricted cash held by its Russian subsidiary. The future success of the Company is dependent upon its ability to obtain additional capital through issuances of equity and debt securities and from collaboration and grant agreements in order to further the development of its technology and product candidates, and ultimately upon its ability to attain profitable operations. There can be no assurance that the Company will be able to obtain the necessary financing to successfully develop and market its product candidates or attain profitability. Based on the current operating plan, the Company expects that its cash, cash equivalents, short-term investments and short-term restricted cash as of March 31, 2018, will fund its operating expenses and capital expenditure requirements into mid-2019. The Company has based this estimate on assumptions that may prove to be wrong, and the Company could use its capital resources sooner than it currently expects. Additionally, the process of testing product candidates in clinical trials is costly, and the timing of progress in these trials is uncertain. Because the Company’s product candidates are in various stages of clinical and preclinical development and the outcome of these efforts is uncertain, the Company cannot estimate the actual amounts necessary to successfully complete the development and commercialization of its product candidates or whether, or when, it may achieve profitability. Guarantees and Indemnifications As permitted under Delaware law, the Company indemnifies its officers, directors, consultants and employees for certain events or occurrences that happen by reason of the relationship with, or position held at, the Company. Through March 31, 2018, 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

3 Months Ended

Mar. 31, 2018

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

Principles of Consolidation

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

Foreign Currency

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

Use of Estimates

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

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.

Cash Equivalents, Short-term Investments and Restricted Cash

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 records its investments at fair value in the accompanying consolidated balance sheets. Changes in fair value are recognized through net income. 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.

The Company has restricted cash balances relating to (i) a secured letter of credit in connection with its Headquarters Lease (as defined below), and (ii) funds held for reimbursement to a grant donor. The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the consolidated balance sheet that sum to the total of the same such amounts shown in the consolidated statement of cash flows:



	March 31, 2018		March 31, 2017
Cash and cash equivalents	$	58,228	$	26,637
Restricted cash	76		81
Restricted cash included in other assets	329		316
Total cash, cash equivalents, and restricted cash shown in the consolidated statement of cash flows	$	58,633	$	27,034

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 March 31, 2018, the Company maintained approximately $1.1 million in Russian bank accounts, of which $1.0 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 March 31, 2018 and December 31, 2017.

Fair Value of Financial Instruments

The Company’s financial instruments consist mainly of cash equivalents, short‑term investments, restricted cash, accounts receivable, accounts payable, loans payable, and common stock warrants. The carrying amounts of cash equivalents, short-term investments, restricted cash, accounts receivable, and accounts payable approximate their estimated fair value due to their short-term maturities. At March 31, 2018, the carrying amount of the Company's loan payable approximates its estimated fair value due to the consistency between the prevailing market rates in effect for similar loans and the effective interest rate of 6.77% for the Company's loan outstanding.

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

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

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

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

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

Fair value is a market‑based measure considered from the perspective of a market participant rather than an entity‑specific measure. Therefore, even when market assumptions are not readily available, the Company’s own assumptions are set to reflect those that market participants would use in pricing the asset or liability at the measurement date. The Company uses prices and inputs that are current as of the measurement date, including during periods of market dislocation. In periods of market dislocation, the observability of prices and inputs may change for many instruments. This condition could cause an instrument to be reclassified within levels in the fair value hierarchy. There were no transfers within the fair value hierarchy during the three months ended March 31, 2018 or the year ended December 31, 2017.

Property and Equipment

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

Impairment of Long‑Lived Assets

The Company periodically evaluates its long‑lived assets for potential impairment. Impairment is assessed when there is evidence that events or changes in circumstances indicate that the carrying amount of an asset may not be recovered. Recoverability of these assets is assessed based on undiscounted expected future cash flows from the assets, considering a number of factors, including past operating results, budgets and economic projections, market trends, and product development cycles. Impairment in the carrying value of each asset is assessed when the undiscounted expected future cash flows derived from the asset are less than their carrying value. The Company did not recognize any impairment charges as of March 31, 2018 and December 31, 2017.

Debt Issuance Costs

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

Accumulated Other Comprehensive Income (Loss)

Comprehensive income (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 income (loss) consists of: (i) all components of net loss and (ii) all components of comprehensive loss other than net loss, referred to as other comprehensive loss. Other comprehensive loss is comprised of foreign currency translation adjustments and the unrealized gains and losses on available-for-sale securities.

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



	Foreign currency translation adjustment			Unrealized gains (losses) on available-for-sale securities			Accumulated other comprehensive income (loss)
Balance at December 31, 2017	$	(4,404	)	$	(16	)	$	(4,420	)
Other comprehensive income during the period	19			3			22
Balance at March 31, 2018	$	(4,385	)	$	(13	)	$	(4,398	)

Revenue Recognition

The Company currently generates its revenue through grants, collaboration and license agreements with strategic collaborators for the development and commercialization of product candidates. Prior to January 1, 2018, the Company recognized revenue in accordance with ASC Topic 605, Revenue Recognition ("ASC 605"). Under ASC 605, the Company recognized revenue when all of the following criteria were 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.

Under ASC 605, amounts received prior to satisfying the revenue recognition criteria were 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 were classified as deferred revenue, current portion. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date were classified as deferred revenue, net of current portion.

In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers ("ASU 2014-09"). ASU 2014-09 and its related amendments (collectively referred to as ASC 606) requires that an entity recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. The Company adopted ASC 606, Revenue from Contracts with Customers, on January 1, 2018, using the modified retrospective method for all contracts not completed as of the date of adoption. The reported results for 2018 reflect the application of ASC 606 guidance while the reported results for 2017 were prepared under the guidance of ASC 605, which is also referred to herein as "legacy GAAP" or the "previous guidance". Refer to Note 12 to our consolidated financial statements included elsewhere in this Quarterly Report on Form 10-Q for the impact of adoption of ASU 2014-09 on the Company’s consolidated financial position, results of operations, equity and cash flows as of the adoption date and for the three months ended March 31, 2018. The adoption of ASC 606 represents a change in accounting principle that will more closely align revenue recognition with the delivery of the Company's services and will provide financial statement readers with enhanced disclosures.

Revenue is recognized when a customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of ASC 606, the Company performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract, including whether they are distinct in the context of the contract; (iii) determine the transaction price, including the constraint on variable consideration; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the Company satisfies each performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception, once the contract is determined to be within the scope of ASC 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations, and assesses whether each promised good or service is distinct. If a promised good or service is not distinct, it is combined with other performance obligations. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied. For example, certain performance obligations associated with Spark (see Note 12 to our consolidated financial statements included elsewhere in this Quarterly Report on Form 10-Q) will be satisfied over time, and revenue will be recognized using the output method, based on the proportion of actual deliveries to the total expected deliveries over the initial term.

Collaboration and Grant Revenue

In determining the appropriate amount of revenue to be recognized as it fulfills its obligations under the agreements, the Company performs the five steps above. As part of the accounting for the arrangement, the Company must develop assumptions that require judgment to determine the stand-alone selling price for each performance obligation identified in the contract. The Company uses key assumptions to determine the stand-alone selling price, which may include market conditions, reimbursement rates for personnel costs, development timelines and probabilities of regulatory success.

The Company capitalizes the incremental costs of obtaining a contract with a customer if it expects to recover those costs. Such incremental costs would not have been incurred if the contract with a customer had not been obtained.

Licenses of intellectual property: If the license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, the Company recognizes revenues from non-refundable, up-front fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. If not distinct, the license is combined with other performance obligations in the contract. For licenses that are combined with other performance obligations, the Company assesses the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. Optional licenses are evaluated to determine if they are issued at a discount, and therefore, represent material rights and accounted for as separate performance obligations.

Milestone Payments: At the inception of each arrangement that includes developmental and regulatory milestone payments, the Company evaluates whether the achievement of each milestone specifically relates to the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service within a performance obligation. If the achievement of a milestone is considered a direct result of the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service and the receipt of the payment is based upon the achievement of the milestone, the associated milestone value is allocated to that distinct good or service. If the milestone payment is not specifically related to the Company’s effort to satisfy a performance obligation or transfer a distinct good or service, the amount is allocated to all performance obligations using the relative standalone selling price method. The Company also evaluates the milestone to determine whether they are considered probable of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price to be allocated, otherwise, such amounts are constrained and excluded from the transaction price. At the end of each subsequent reporting period, the Company re-evaluates the probability of achievement of such development milestones and any related constraint, and if necessary, adjusts its estimate of the transaction price. Any such adjustments to the transaction price are allocated to the performance obligations on the same basis as at contract inception. Amounts allocated to a satisfied performance obligation shall be recognized as revenue, or as a reduction of revenue, in the period in which the transaction price changes.

Manufacturing Supply Services: Arrangements that include a promise for future supply of drug substance or drug product for either clinical development or commercial supply at the customer’s discretion are evaluated to determine if they are distinct and optional. For optional services that are distinct, the Company assesses if they are priced at a discount, and therefore, provide a material right to the licensee to be accounted for as separate performance obligations.

Royalties: For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company will recognize revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied) in accordance with the royalty recognition constraint.

Research and Development Costs

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

Clinical Trial Costs

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

Income Taxes

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

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

Warrants

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

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

Stock‑Based Compensation

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

Net Loss Per Share

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

Deferred Rent

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

Contingent Liabilities

The Company accounts for its contingent liabilities in accordance with ASC No. 450, Contingencies. A provision is recorded when it is both probable that a liability has been incurred and the amount of the loss can be reasonably estimated. With respect to legal matters, provisions are reviewed and adjusted to reflect the impact of negotiations, estimated settlements, legal rulings, advice of legal counsel and other information and events pertaining to a particular matter. As of March 31, 2018 and December 31, 2017, 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 November 2015, the FASB issued ASU No. 2015-17, Income Taxes (Topic 740), Balance Sheet Classification of Deferred Taxes. The new standard requires that deferred tax assets and liabilities be classified as non-current in a classified statement of financial position. The Company adopted this standard on January 1, 2017, and the adoption did not have an impact on its consolidated financial statements due to the full valuation allowance position.

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

In February 2016, FASB issued ASU No. 2016-02, Leases (Topic 842) (“ASU 2016-02”). ASU 2016-02 requires a lessee to separate the lease components from the non-lease components in a contract and recognize in the statement of financial position a liability to make lease payments (the lease liability) and a right-of-use asset representing its right to use the underlying asset for the lease term. It also aligns lease accounting for lessors with the revenue recognition guidance in ASU 2014-09. ASU 2016-02 is effective for fiscal years beginning after December 15, 2018, including interim periods within those fiscal years. The Company believes it has less than five lease contracts that will need to be evaluated under ASU 2016-02, and thus does not expect the adoption of ASU 2016-02 to have a material effect on its consolidated financial statements.

In August 2016, the FASB issued ASU No. 2016-15, Statements of Cash Flows (Topic 230) - Classifications of Certain Cash Receipts and Cash Payments ("ASU 2016-15"), to clarify how companies present and classify certain cash receipts and cash payments in the statement of cash flows. This guidance is effective for annual reporting periods beginning after December 15, 2017, including interim periods within that reporting period, with early adoption permitted. The adoption of ASU 2016-15 will have a retrospective impact to its third quarter due to the recognition of debt extinguishment costs during the three and nine months ended September 30, 2017. The Company will classify debt extinguishment costs as cash outflows for financing activities in accordance with ASU 2016-15.

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. The Company adopted ASU 2016-18 during the first quarter ended March 31, 2018. The adoption of ASU 2016-18 resulted in the Company's cash, cash equivalents and restricted cash being included in the beginning and ending amounts for the periods shown on the statement of cash flows and was applied retroactively and reflected in the balances presented for any prior periods. The Company believes that the adoption of this guidance did not have a significant impact on its condensed consolidated financial statements.

Effective January 1, 2018, the Company adopted ASU 2017-09, Compensation-Stock Compensation (Topic 718): Scope of Modification Accounting ("ASU 2017-09"). ASU 2017-09 specifies the types of changes to the terms or conditions of a share-based payment award that require an entity to apply modification accounting in accordance with Topic 718. The adoption of ASU 2017-09 did not have a material impact on the Company's consolidated financial statements.

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

Available-for-Sale Marketable Securities

3 Months Ended

Mar. 31, 2018

Investments, Debt and Equity Securities [Abstract]

Available-for-Sale Marketable Securities

As of March 31, 2018 and December 31, 2017, the Company’s available-for-sale marketable securities consisted of U.S. government and agency securities, commercial paper and corporate bonds.

The following tables summarize the Company’s available-for-sale marketable securities by major type of security as of March 31, 2018 and December 31, 2017 (in thousands):



	March 31, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
U.S. government and agency securities	$	22,854	$	—	$	(13	)	$	22,841
Commercial paper	1,998		—		—			1,998
Total available-for-sale marketable securities	$	24,852	$	—	$	(13	)	$	24,839



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

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

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



	March 31, 2018				December 31, 2017
	Fair Value		Amortized Cost		Fair Value		Amortized Cost
Less than one year	$	24,839	$	24,852	$	25,940	$	25,956

As of March 31, 2018, the Company held a total of 10 U.S. government and agency security positions, all of which were in an unrealized loss position, none of which had been in an unrealized loss position for 12 months or greater. The aggregate fair value of U.S. government and agency security investments in an unrealized loss position is $22.8 million. During the three months ended March 31, 2018, the Company recognized an unrealized loss of less than $0.1 million through net loss relating to changes in the fair value of its investment securities.

Based on the Company’s review of these securities, the Company believes that the cost basis is recoverable and it had no other-than-temporary impairments on these securities as of March 31, 2018. The Company does not intend to sell these debt securities and the Company believes it is not more-likely-than-not that it will be required to sell these securities before the recovery of their amortized cost basis, which may be at maturity.

Net Loss Per Share

3 Months Ended

Mar. 31, 2018

Earnings Per Share [Abstract]

Net Loss Per Share

The Company has reported a net loss attributable to common stockholders for the three months ended March 31, 2018 and 2017, and for this reason basic and diluted net loss per share attributable to common stockholders are the same for all periods presented. The following table sets forth the computation of basic and diluted net loss per share (in thousands, except share and per‑share data):



	Three Months Ended March 31,
	2018			2017
Numerator:
Net loss attributable to common stockholders	$	(15,888	)	$	(15,134	)
Denominator:
Weighted‑average common shares outstanding—basic and diluted	22,345,523			18,474,227
Net loss per share attributable to common stockholders—basic and diluted	$	(0.71	)	$	(0.82	)

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



	March 31,
	2018	2017
Stock options to purchase common stock	2,658,035	2,025,664
Stock warrants to purchase common stock	176,432	97,302
Total	2,834,467	2,122,966

Fair Value Measurements

3 Months Ended

Mar. 31, 2018

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 March 31, 2018 and December 31, 2017, 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):



	March 31, 2018
	(Level 1)		(Level 2)		(Level 3)		Total
Cash equivalents:
Money market funds	$	22,402	$	—	$	—	$	22,402
Commercial paper	—		5,494		—		5,494
Corporate bonds	2,000		—		—		2,000
Total cash equivalents	$	24,402	$	5,494	$	—	$	29,896
Short-term investments:
U.S. government and agency securities	$	—	$	22,841	$	—	$	22,841
Commercial paper	—		1,998		—		1,998
Total short-term investments	$	—	$	24,839	$	—	$	24,839



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

At March 31, 2018 and December 31, 2017, cash and cash equivalent investments were held in money market funds, U.S. government and agency securities, corporate bonds and commercial paper maturing within 90 days from the date of purchase.

The average maturity date for U.S. government and agency securities, included in investments at March 31, 2018 and December 31, 2017 was 181 days and 168 days, respectively. Fair value of U.S. government and agency securities approximated amortized value at each reporting date.

The average maturity date for commercial paper, included in investments at March 31, 2018 was 95 days. Fair value of commercial paper approximated amortized value at March 31, 2018.

The average maturity date for corporate bonds, included in investments at December 31, 2017 was 179 days. Fair value of corporate bonds approximated amortized value at December 31, 2017.

- 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

3 Months Ended

Mar. 31, 2018

Property, Plant and Equipment [Abstract]

Property and Equipment

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



	March 31,			December 31,
	2018			2017
Laboratory equipment	$	5,191		$	5,138
Computer equipment and software	492			480
Leasehold improvements	278			278
Furniture and fixtures	253			235
Office equipment	118			118
Construction in process	258			—
Total property and equipment	6,590			6,249
Less accumulated depreciation	(4,333		)	(4,158		)
Property and equipment, net	$	2,257		$	2,091

Depreciation expense was $0.2 million and $0.2 million for the three months ended March 31, 2018 and 2017, respectively.

Accrued Expenses

3 Months Ended

Mar. 31, 2018

Payables and Accruals [Abstract]

Accrued Expenses

Accrued expenses consist of the following (in thousands):



	March 31,		December 31,
	2018		2017
Payroll and employee related expenses	$	1,441	$	1,902
Current portion of deferred rent and lease incentive	83		72
Collaboration and licensing	859		1,020
Accrued patent fees	408		268
Accrued external research and development costs	4,377		3,578
Accrued professional and consulting services	913		859
Accrued grant refund	175		175
Accrued interest	92		89
Other	714		617
Accrued expenses	$	9,062	$	8,580

Commitments and Contingencies

3 Months Ended

Mar. 31, 2018

Commitments and Contingencies Disclosure [Abstract]

Commitments and Contingencies

Operating Leases

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

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

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

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

As of both March 31, 2018 and December 31, 2017, deferred rent and lease incentive liability totaled $0.2 million. 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 both March 31, 2018 and December 31, 2017.

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

Rent expense for each of the three months ended March 31, 2018 and 2017 was $0.5 million.

As of March 31, 2018, the aggregate future minimum lease payments related to these leases are as follows (in thousands):



Year ended December 31,
2018 (Remainder)	$	1,085
2019	1,482
2020	375
Total minimum lease payments	$	2,942

Other

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

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

Debt

3 Months Ended

Mar. 31, 2018

Debt Disclosure [Abstract]

Debt

2015 Term Loan

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

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

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

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

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

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

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

2017 Term Loan

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

On the Funding Date, the Company entered into a payoff letter with SVB, pursuant to which SVB utilized $10.0 million of the 2017 Term Loan to pay off all outstanding obligations under the 2015 Term Loan. The Company recognized a loss on extinguishment of debt in the amount of $0.7 million during the three months ended September 30, 2017.

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

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

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

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

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

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

As of March 31, 2018 and December 31, 2017, the outstanding principal balance under the Term Loans was $21.0 million.

Future minimum payments on the 2017 Term Loan as of March 31, 2018 are as follows (in thousands):



2018 (Remainder)	$	842
2019	4,587
2020	9,128
2021	8,678
2022	1,753
Total minimum debt payments	$	24,988
Less: Amount representing interest	(2,938		)
Less: Debt discount and deferred charges	(923		)
Less: Current portion of loans payable	—
Loans payable, net of current portion	$	21,127

During the three months ended March 31, 2018 and 2017, the Company recognized $0.3 million and $0.3 million, respectively, of interest expense related to the Term Loans.

Stockholders' Equity

3 Months Ended

Mar. 31, 2018

Equity [Abstract]

Stockholders' Equity

2017 Shelf Registration Statement

On August 10, 2017, the Company filed a universal shelf registration statement on Form S-3 with the SEC to sell up to $200 million of equity and/or debt securities and entered into a sales agreement with Jefferies LLC, as sales agent, pursuant to which the Company may, from time to time, issue and sell common stock with an aggregate value of up to $50 million in an at-the-market, or ATM, offering. The shelf registration statement was declared effective by the SEC on August 28, 2017. As of March 31, 2018, no securities have been issued from this shelf registration statement.

PIPE Financing

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

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

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

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

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

Initial Public Offering

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

As of March 31, 2018, the Company has 200,000,000 shares of common stock authorized for issuance, $0.0001 par value per share, with 22,349,840 shares issued and outstanding. The voting, dividend and liquidation rights of the common stockholders are subject to and qualified by the rights, powers and preferences of the preferred stock. The common stock has the following characteristics:

Voting

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

Dividends

The common stockholders are entitled to receive dividends, if and when declared by the Board of Directors. Through March 31, 2018, no dividends have been declared or paid on common stock.

Liquidation

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

Reserved Shares

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



	March 31, 2018	December 31, 2017
Exercise of common warrants	176,432	176,432
Shares available for future stock incentive awards	1,911,551	923,440
Outstanding common stock options	2,658,035	2,657,187
Total	4,746,018	3,757,059

Stock Incentive Plans

3 Months Ended

Mar. 31, 2018

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

Stock Incentive Plans

Stock Options

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

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

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

On June 7, 2016, the Company’s stockholders approved the 2016 Incentive Award Plan (the “2016 Plan”), which became effective June 20, 2016. The 2016 Plan provides for the granting of incentive and non‑qualified stock option, restricted stock and other stock and cash based awards as determined by the Board. At inception of the 2016 Plan, a total of 1,210,256 shares of common stock were authorized for future grant under the 2016 Plan. The number of shares of common stock that may be issued under the 2016 Plan automatically increases on the first day of each calendar year, beginning in 2017 and ending in and including 2026, by an amount equal to the lesser of: (i) 4% of the number of shares of the Company’s common stock outstanding on the last day of the applicable preceding calendar year and (ii) such smaller number of shares as is determined by the Board. During the three months ended March 31, 2018 and 2017, the number of shares of common stock that may be issued under the 2016 Plan was increased by 893,730 shares and 737,550 shares, respectively. As of March 31, 2018, 2,841,536 shares remain available for future issuance under the 2016 Plan.

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

The fair value of each option award was estimated on the grant date using Black‑Scholes. Expected volatilities are based on historical volatilities from guideline companies because the Company's common stock has not traded for a period that is at least equal to the expected term of its stock option awards. The Company uses the “simplified” method to estimate the expected life of options granted and are expected to be outstanding. The risk‑free interest rate used is the rate for a U.S. Treasury zero coupon issue with a remaining life consistent with the options expected life on the grant date. The Company has not paid, and does not expect to pay in the foreseeable future, any cash dividends. Forfeitures are estimated at the time of grant and are adjusted, if necessary, in subsequent periods if actual forfeitures differ from those estimates. The Company has estimated a forfeiture rate of 10% based on historical attrition trends. The Company records stock‑based compensation expense only on awards that are expected to vest.

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



	Three Months Ended March 31,
	2018			2017
Risk-free interest rate	2.65		%	2.07		%
Dividend yield	—			—
Expected term	6.64			5.92
Expected volatility	84.81		%	84.41		%
Weighted-average fair value of common stock	$	9.53		$	10.89

The weighted average grant date fair value of stock options granted to employees during the three months ended March 31, 2018 and 2017 was $7.07 and $7.77, respectively. There were no employee stock option exercises during the three months ended March 31, 2018. The aggregate intrinsic value of stock options exercised during the three months ended March 31, 2017 was $0.6 million.

As of March 31, 2018 and December 31, 2017, total unrecognized compensation expense related to unvested employee stock options was $8.7 million and $10.2 million, respectively, which is expected to be recognized over a weighted average period of 2.7 years and 2.9 years, respectively.

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

One of the Company's executive officers resigned effective December 31, 2017, and immediately entered into a consulting arrangement with the Company during the period January 1, 2018 to December 31, 2018. In connection with his resignation, (i) all of his unvested stock option awards (62,546 shares in aggregate) were forfeited and expired as of December 31, 2017; (ii) his right to exercise his vested, exercisable stock option awards (126,457 shares in aggregate) as of December 31, 2017 was extended to March 31, 2019, resulting in a modification; and (iii) during the first quarter of 2018, he was granted a stock option award to purchase 20,000 shares of the Company's common stock. For accounting purposes, this has been treated as a cancellation and replacement award, subject to modification accounting. All granted stock awards are presented in the employee awards stock option table below.

There were no stock option awards granted to non-employees during the three months ended March 31 2018 and 2017. 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.

As of March 31, 2018 and December 31, 2017, total unrecognized compensation expense related to unvested non‑employee stock options was $0.4 million and $0.6 million, respectively.

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



					Weighted‑average
					remaining	Aggregate
	Number of		Weighted-average		contractual term	intrinsic value
	options		exercise price ($)		(in years)	(in thousands)
Employee Awards
Outstanding at December 31, 2017	2,411,237		$	10.58	7.50	$	4,729
Granted	52,200		$	9.53
Exercised	—		$	—
Forfeited	(51,352	)	$	16.37
Outstanding at March 31, 2018	2,412,085		$	10.43	7.33	$	5,193
Vested at March 31, 2018	1,172,413		$	7.12	5.67	$	4,645
Vested and expected to vest at March 31, 2018	2,255,453		$	10.22	7.21	$	5,144

Non‑Employee Awards
Outstanding at December 31, 2017	245,950		$	4.32	5.10	$	1,351
Granted	—		$	—
Exercised	—		$	—
Forfeited	—		$	—
Outstanding at March 31, 2018	245,950		$	4.32	4.86	$	1,444
Vested at March 31, 2018	212,308		$	3.89	4.37	$	1,338
Vested and expected to vest at March 31, 2018	245,950		$	4.32	4.86	$	1,444

Employee Stock Purchase Plan

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

Under the ESPP, the Company has set two six-month offering periods during each calendar year, one beginning March 1^st and the other beginning September 1^st of each calendar year, during which employees may elect to have up to 25% of their eligible compensation deducted on each payday on an after-tax basis for use in purchasing the Company's common stock on the last trading day of each offering period, subject to limits imposed by the Internal Revenue Code. The purchase price of the shares may not be less than 85% of the fair market value on the first or last trading day of the offering period, whichever is lower. The first ESPP offer period began on March 1, 2017.

At inception of the ESPP, a total of 173,076 shares of common stock were authorized and reserved for future issuance under the ESPP. The number of shares of common stock that may be issued under the ESPP will automatically increase on the first day of each calendar year, beginning in 2017 and ending in and including 2026, by an amount equal to the lesser of: (i) 1% of the number of shares of the Company’s common stock outstanding on the last day of the applicable preceding calendar year and (ii) such smaller number of shares as is determined by the Company’s Board of Directors. During the three months ended March 31, 2018 and 2017, the number of shares of common stock that may be issued under the ESPP was increased by 223,432 shares and 184,387 shares, respectively. During the three months ended March 31, 2018, the Company issued 6,586 shares of common stock under the ESPP. As of March 31, 2018, 558,046 shares remain available for future issuance under the ESPP.

For the three months ended March 31, 2018 and 2017, the Company recognized less than $0.1 million of stock-based compensation expense under the ESPP.

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



	Three Months Ended March 31,
	2018		2017
Research and development	$	511	$	335
General and administrative	642		358
Total stock-based compensation expense	$	1,153	$	693

Revenue from Contracts with Customers

3 Months Ended

Mar. 31, 2018

Revenue from Contract with Customer [Abstract]

Revenue from Contracts with Customers

The Company adopted ASC 606 on January 1, 2018, using the modified retrospective method for all contracts not completed as of the date of adoption. The reported results for 2018 reflect the application of ASC 606 guidance while the reported results for 2017 were prepared under the guidance of ASC 605, Revenue Recognition (ASC 605), which is also referred to herein as "legacy GAAP" or the "previous guidance". The adoption of ASC 606 represents a change in accounting principle that will more closely align revenue recognition with the delivery of the Company's services and will provide financial statement readers with enhanced disclosures.

Financial Statement Impact of Adopting ASC 606

The cumulative effect of applying the new guidance to all contracts with customers that were not completed as of January 1, 2018, was recorded as an adjustment to accumulated deficit as of the adoption date. As a result of applying the modified retrospective method to adopt the new revenue guidance, the following adjustments were made to the Company's accounts on the condensed consolidated balance sheet as of January 1, 2018:



				Adjustments
	As Reported at			Due to			Balance at
Consolidated Balance Sheet	December 31, 2017			ASU 2014-09			January 1, 2018
Assets
Cash, cash equivalents, and restricted cash	$	70,698		$	—		$	70,698
Short-term deposits and investments	25,940			—			25,940
Accounts receivable	63			—			63
Prepaid expenses and other current assets	1,979			231			2,210
Total current assets	98,680			231			98,911
Property and equipment, net	2,091			—			2,091
Restricted cash and other assets	329			2,367			2,696
Total assets	$	101,100		$	2,598		$	103,698
Liabilities and stockholders’ equity
Current liabilities:
Accounts payable	1,606			—			1,606
Accrued expenses	8,580			—			8,580
Deferred revenue, current portion	787			—			787
Total current liabilities	10,973			—			10,973
Non‑current liabilities:
Deferred rent and lease incentive	151			—			151
Loans payable, net of current portion	21,042			—			21,042
Deferred revenue, net of current portion	15,919			(1,830		)	14,089
Other long‑term liabilities	1,201			—			1,201
Total liabilities	49,286			(1,830		)	47,456
Stockholders’ equity:
Common stock	3			—			3
Additional paid-in capital	273,128			—			273,128
Accumulated deficit	(216,897		)	4,428			(212,469		)
Accumulated other comprehensive loss	(4,420		)	—			(4,420		)
Total stockholders’ equity	51,814			4,428			56,242
Total liabilities and stockholders’ equity	$	101,100		$	2,598		$	103,698

In connection with the adoption of ASC 606, the Company identified three collaboration/grant arrangements that required analysis to quantify the impact of adoption to its opening accumulated deficit balance as of January 1, 2018: Spark Therapeutics, Inc., Skolkovo Foundation and National Institutes of Health.

Spark Therapeutics, Inc.

Spark License Agreement

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

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

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

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

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

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


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

•

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

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

In connection with the Spark License Agreement and Spark Purchase Agreement, the Company has made contractual payments defined in the MIT license agreement (Note 14) totaling $2.2 million for the MIT sub-license provided to Spark, and $0.4 million relative to the calculated premium paid by Spark for the equity investments made under the Spark Purchase Agreement.

Transition of Revenue Recognition from ASC Topic 605 to ASC Topic 606

The terms of the Spark Purchase Agreement and the Spark License Agreement were negotiated at the same time between the parties and the terms of the Spark Purchase Agreement are referenced in the Spark License Agreement in multiple sections. The pricing and terms of the agreements are unique and must be considered in contemplation with each other. There are provisions within the Spark License Agreement that link to the Spark Purchase Agreement related to provisions that constitute a material breach of the license agreement. Therefore, the Company concluded that the two agreements must be combined and evaluated as a single agreement. While the Spark Purchase Agreement and the Spark License agreement are considered to be a single agreement, the Company determined that the purchase of common stock and future acquisition rights are not within the scope of ASC 606. The Company 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 as of the effective date. The remaining $2.3 million of cash received in exchange for the stock and acquisition rights is included in allocable consideration, as this represents the premium paid by Spark on the purchase of common stock, and should be allocated to the remaining performance obligations.

The Company identified the following promises at the inception of the agreement: (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 Obligation, (2) options to research, develop and commercialize gene therapies utilizing the SVP technology for up to four additional target therapy options, or the Option Obligation, (3) manufactured supply of pre-clinical and clinical SVP, or the Supply Obligation at a discount, and (4) option to purchase manufactured supply of commercial SVP, or the Commercial Supply Obligation at fair value. In consideration for these promises, the Company received an upfront payment of $15.0 million. In addition, the Company is eligible to receive additional payments of up to $35.0 million based on the achievement by Spark of future specified development milestones, up to $30.0 million based on the achievement by Spark of future specified regulatory milestones, up to $110.0 million based on the achievement by Spark of future specified commercial milestones, and up to $255.0 million based on the achievement by Spark of future specified sales milestones. The Company will also be eligible to receive tiered royalty payments that reach low double-digits based on future net sales for the duration of the royalty term.

The Company determined that the License Obligation was not capable of being distinct from the Supply Obligation. This is because Spark cannot derive benefit from the license without the simultaneous transfer of the pre-clinical and clinical supply. Therefore, the License Obligation and Supply Obligation are combined as a single performance obligation (the “Combined License and Supply Obligation”). The Company also determined that the Option Obligation, which includes the related Supply Obligation, provides the customer with a material right and is considered a performance obligation in the arrangement since it was priced at an incremental discount. The Company determined that the optional Commercial Supply Obligation does not provide the customer with a material right and is not considered to be a performance obligation because Spark can derive benefit from the Combined License and Supply Obligation without the delivery of the Commercial Supply Obligation and is not at an incremental discount. Therefore, the Company determined that the Spark agreement contains five distinct performance obligations: the Combined License and Supply Obligation, and the four separate Option Obligations.

In determining the transaction price, the Company considered the future development milestones, regulatory milestones, commercial milestones, sales milestone, and sales royalties all represent variable consideration. Each of these variable consideration items was evaluated under the most likely amount method to determine whether such amounts were probable of occurrence, or whether such amounts should be constrained until they become probable. As part of its evaluation of the constraint, the Company considered numerous factors, including that receipt of such milestones is outside the control of the Company. Separately, any consideration related to sales-based milestones as well as royalties on net sales upon commercialization by Spark, will be recognized when the related sales occur as they were determined to relate predominantly to the intellectual property granted to Spark and, therefore, have also been excluded from the transaction price in accordance with the royalty recognition constraint. As of the adoption date and as of March 31, 2018, all milestones were constrained. The Company will re-evaluate the transaction price in each reporting period, as uncertain events are resolved, or as other changes in circumstances occur.

As a result, the Company determined that the up-front payment of $12.3 million ($15.0 million, less fair value of the equity totaling $2.7 million as discussed above) was included in the transaction price and was allocated to the performance obligations based on the Company’s best estimate of their relative stand-alone selling prices. The Company allocated $7.1 million to the Combined License and Supply Obligation and $5.2 million to the discount on the Option Obligation ($1.3 million for each option) using the relative standalone selling price method to each obligation. The standalone selling price for the Combined License and Supply Obligation was determined using a discounted cash flow model. The standalone selling price for the Option Obligation was determined based on the fair value of the license minus the strike price of the option (the probability of exercise was included in the valuation) as well as the estimated discount of the Supply Obligation.

The estimated proceeds to be received from the sale of the Supply Obligation was also included in the transaction price for the Combined License and Supply Obligation. The total consideration allocated to the Combined License and Supply Obligation will be recognized using the output method, based on the proportion of actual deliveries to the total expected deliveries over the initial term which is estimated to be approximately four years. The discount associated with the Option Obligation, along with the proceeds to be received upon exercise and estimated sale of the Supply Obligation, will be recognized when each of the options are exercised, over the related expected deliveries of its supply. If the options expire without exercise, the related deferred revenue associated with each option will be recognized upon expiration (December 1, 2019).

The Company estimated that revenue recognized under ASC 605 is consistent with the revenue recognized in accordance with ASC 606 from contract inception. However, upon adoption of ASC 606, the Company recognized other assets of $2.6 million related to the incremental costs relating to sublicense fees, and a premium, paid to MIT that would not have been incurred if the contract with Spark had not been obtained.

The Company recognized revenues of zero and less than $0.1 million related to the Spark License Agreement during the three months ended March 31, 2018 and 2017, respectively.

As of March 31, 2018, there was a contract liability of $14.7 million representing deferred revenue associated with this agreement. A total of $1.0 million is presented as current and $13.7 million is presented as noncurrent in the accompanying consolidated balance sheet. As of December 31, 2017, there was $14.7 million of deferred revenue related to this agreement. A total of $0.8 million was presented as current and $13.9 million was presented as noncurrent in the accompanying consolidated balance sheet.

Spark Letter Agreement

On June 6, 2017, the Company and Spark entered into a letter agreement (the “Letter Agreement”), pursuant to which the parties agreed that Spark would make the May 2017 License Payment by June 6, 2017. The May 2017 License Payment was received, and recorded as a liability as of June 30, 2017, of which some or all may potentially constitute the reimbursement described below. The parties also agreed that Spark would be deemed to have delivered notice on May 31, 2017 exercising its right to purchase the shares pursuant to the First Acquisition Right. The Letter Agreement further outlines a cost reimbursement arrangement, pursuant to which the Company agreed to reimburse Spark for all costs and expenses, including the cost of materials provided by the Company, associated with the preclinical research and toxicology studies being performed by Spark for any licensed products for a specified amount of time (the “Reimbursement Period”), up to an agreed upon cap of $2.5 million.

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

Under ASC 606, the Company determined that the Letter Agreement resulted in a modification to the original agreement. The amount received totaling $2.5 million and the reimbursements pursuant to the Letter Agreement totaling $2.5 million were both included in the transaction price, and a contract liability was recorded for the amount expected to be repaid. As repayments are made, the underlying contract liability will be reduced. To the extent that an amount is expected to be applied towards the clinical supply obligation, the analysis of variable consideration will be updated accordingly.

The Company utilized the practical expedient relating to contract modifications that occurred prior to January 1, 2018. The practical expedient allows the Company to reflect the aggregate effect of all modifications that occurred before January 1, 2018 when identifying the satisfied and unsatisfied performance obligations, determining the transaction price and allocating the transaction price to the satisfied and unsatisfied performance obligations for the modified contract at transition.

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

Skolkovo Foundation

The Company receives grant funding from the Russia-based Development Fund of New Technologies Development and Commercialization Center ("Skolkovo"). On November 28, 2014, the Company executed a grant awarded by Skolkovo for the development of a therapeutic vaccine using nanoparticles to treat chronic infection caused by human papillomavirus (HPV) and diseases associated with this infection. The grant covers the period from August 1, 2014 through July 21, 2017. The grant provides for up to $2.7 million that covers 48.5% of the estimated total cost of the research plan with the remaining 51.5% of estimated costs to be contributed by the Company. From grant inception through March 31, 2018, the Company has received $2.0 million from Skolkovo.

At any time during the term of the grant agreement, but not more than once per quarter, Skolkovo has the right to request information related to the project and to conduct an audit of the expenses incurred by the Company. In the event the project or the expenses do not meet predefined requirements, the Company may be required to reimburse the funds received up to three years after the completion of the project. As a result, in accordance with ASC 605, the Company determined that the grant funding was not fixed or determinable and the entire amount received through December 31, 2017 was recorded as deferred revenue in the consolidated balance sheet until the completion of the Skolkovo audit or the expiration of the audit term.

The Company assessed this arrangement in accordance with ASC 606 and concluded that the contract counterparty, Skolkovo, is a customer. Furthermore, the Company determined that the agreement with Skolkovo has commercial substance, and that the grant should be accounted for as revenue. The Company identified the research and development services being provided to Skolkovo as the only performance obligation in the agreement. Based on the guidance in ASC 606, the Company concluded that the entire $2.0 million of grant funds received from Skolkovo is variable consideration. Although the Company believes it has an enforceable right to the amounts received, there is risk that an audit could result in the Company needing to refund certain amounts back to Skolkovo, resulting in variability in the transaction price. The Company utilized the “expected value” approach in determining the amount that can be recognized. The Company estimated that it will be entitled to revenue of $1.8 million from the Skolkovo grant, and recorded this amount as a cumulative-effect adjustment through accumulated deficit upon adoption of ASC 606. The remainder of $0.2 million has been recorded as a contract liability upon adoption of ASC 606.

National Institutes of Health

On May 14, 2014, the National Institute on Drug Abuse (“NIDA”), part of the National Institutes of Health ("NIH"), provided the Company with a Notice of Award (“NoA”), for a grant relating to the preclinical development and clinical proof of concept for a synthetic nanoparticle (the “Project”). The grant will help advance the Company’s development of the nanoparticle from a preclinical through early clinical stage and will provide support for one or more clinical trials. Under the terms of the grant, NIDA agreed to provide up to $8.1 million in funding to be used to offset the qualifying expenses incurred by the Company. The grant was to be funded over the course of three years, according to budget forecasts prepared by the Company.

In accordance with ASC 605, revenue associated with this grant was recognized as the related research and development work was performed, and expenses incurred. Revenue under this grant arrangement was recognized using a proportional performance method. Amounts received prior to satisfying the ASC 605 revenue recognition criteria (if any) was recorded as deferred revenue on the balance sheet. Through December 31, 2017, the Company received and recognized under ASC 605 approximately $7.2 million of grant revenue. The remaining $0.9 million available under the grant requires a change to the overhead rate, which is subject to final reconciliation and approval from NIDA and, therefore, was not recognized under ASC 605.

The Company assessed this arrangement in accordance with ASC 606 and concluded that the contract counterparty, NIDA, is a customer. Furthermore, the Company determined that the agreement with NIDA has commercial substance, and that the grant should be accounted for as revenue. The Company identified the research and development services being provided to NIDA as the only performance obligation in the agreement.

Upon adoption of ASC 606, the remaining $0.9 million was not recognized because the change to the overhead rate requires approval from NIDA, and therefore, represents a contract modification. Revenue will be recognized if and when the price change is approved, at which time it will be an enforceable right under contract. The Company expects approval in 2018.

Impact of New Revenue Guidance on Financial Statement Line Items

The following table compares the reported condensed consolidated balance sheet and statement of operations, as of and for the three months ended March 31, 2018, to the pro-forma amounts had the previous guidance been in effect:



	As of March 31, 2018
Consolidated Balance Sheet	As reported			Pro forma as if the previous accounting was in effect
Assets
Cash, cash equivalents, and restricted cash	$	58,304		$	58,304
Short-term deposits and investments	24,839			24,839
Accounts receivable	52			52
Prepaid expenses and other current assets	1,789			1,558
Total current assets	84,984			84,753
Property and equipment, net	2,257			2,257
Restricted cash and other assets	2,696			329
Total assets	$	89,937		$	87,339
Liabilities and stockholders’ equity
Current liabilities:
Accounts payable	$	1,789		1,789
Accrued expenses	9,062			9,062
Deferred revenue, current portion	959			959
Total current liabilities	11,810			11,810
Non‑current liabilities:
Deferred rent and lease incentive	129			129
Loans payable, net of current portion	21,127			21,127
Deferred revenue, net of current portion	13,917			15,747
Other long‑term liabilities	1,372			1,372
Total liabilities	48,355			50,185
Stockholders’ equity:
Common stock	3			3
Additional paid-in capital	274,334			274,334
Accumulated deficit	(228,357		)	(232,785		)
Accumulated other comprehensive loss	(4,398		)	(4,398		)
Total stockholders’ equity	41,582			37,154
Total liabilities and stockholders’ equity	$	89,937		$	87,339

Total reported assets were $2.6 million more than the pro-forma balance sheet, which assumes the previous guidance remained in effect as of March 31, 2018. The higher reported assets were a result of the recognition of other assets of $2.6 million relating to incremental costs of obtaining the contract with Spark.

Total reported liabilities were $1.8 million less than the pro-forma balance sheet, which assumes the previous guidance remained in effect as of March 31, 2018. The lower reported liabilities were due to reductions recognized to deferred revenue balances, as follows: less than $0.1 million relating to Spark, and $1.8 million related to Skolkovo.

There was no difference in the amounts reported in the statement of operations under ASC 606 during the three months ended March 31, 2018, and the pro-forma statement of operations, which assumes the previous guidance remained in effect as of March 31, 2018.

Transaction Price Allocated to Future Performance Obligations

Remaining performance obligations represent the transaction price of contracts for which work has not been performed (or has been partially performed) and excludes unexercised contract options. As of March 31, 2018, the aggregate amount of the transaction price allocated to remaining performance obligations was $8.6 million. The Company expects to recognize revenue on approximately 9.0% of the remaining performance obligations over the next 12 months.

Contract Balances from Contracts with Customers

The following table presents changes in the Company’s contract liabilities during the three months ended March 31, 2018 (in thousands):



	Balance at						Balance at
	Beginning of Period		Additions		Deductions		End of Period
Three Months Ended March 31, 2018
Contract liabilities:
Deferred revenue	$	14,876	$	—	$	—	$	14,876
Other liabilities (i)	2,231		—		—		2,231
Total contract liabilities	$	17,107	$	—	$	—	$	17,107

(i) The current portion of other liabilities in the amount of $0.8 million is presented in the accrued expenses line of the consolidated balance sheet. The noncurrent portion of other liabilities in the amount of $1.4 million is presented in the other long-term liabilities line of the consolidated balance sheet.

The timing of revenue recognition and cash collections results in contract liabilities on the consolidated balance sheets. When consideration is received, or such consideration is unconditionally due, from a customer prior to transferring goods or services to the customer under the terms of a contract, a contract liability is recorded. Contract liabilities are recognized as revenue after control of the products or services is transferred to the customer and all revenue recognition criteria have been met.

Related-Party Transactions	3 Months Ended
Related-Party Transactions	Mar. 31, 2018
Related Party Transactions [Abstract]
Related-Party Transactions	Related‑Party Transactions In connection with the 2017 PIPE, the Company sold to Timothy Springer, Ph.D., a member of its Board of Directors, an aggregate of 338,791 shares of common stock at a purchase price equal to $17.71 per share, which was equal to the most recent consolidated closing bid price on the Nasdaq Global Market on June 23, 2017. In addition, the Company sold warrants to Dr. Springer to purchase up to 79,130 shares of common stock (“Warrant Shares”), exercisable at $17.71 per Warrant Share, and with a term of five years. The purchase price for each warrant was equal to $0.125 for each Warrant Share, consistent with Nasdaq Global Market requirements for an “at the market” offering. The Company incurred expenses for consulting services provided by its founders totaling less than $0.1 million during each of the three months ended March 31, 2018 and 2017.

Technology License Agreements	3 Months Ended
Technology License Agreements	Mar. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Technology License Agreements	Technology License Agreements Massachusetts Institute of Technology On November 25, 2008, the Company entered into an Exclusive Patent License agreement with the Massachusetts Institute of Technology (“MIT”), which is referred to as the Exclusive Patent License. The Company received an exclusive royalty‑bearing license to utilize patents held by MIT in exchange for upfront consideration and annual license maintenance fees. Such fees are expensed as incurred and have not been material to any period presented. In the event the Company sublicenses the MIT patents to a third party, it will be required to remit to MIT a percentage (ranging from 10% to 30%) of sublicense income. In addition, the Company is obligated to pay MIT a certain amount upon the achievement of defined clinical milestones, up to a total of $1.5 million. On December 18, 2008, the Company entered into a patent‑cross‑license agreement with BIND Therapeutics, Inc. whereby each party receives a license for the use of the other patents in their respective fields of use. In exchange for this license, the Company paid a one‑time expense in 2008. As of December 31, 2017, and in connection with the execution of the Spark License Agreement, the Company has made contractual payments pursuant to the Exclusive Patent License totaling $2.2 million for the sublicense granted to Spark, and $0.4 million relative to the calculated premium paid by Spark for the equity investments made under the Spark Purchase Agreement. The Company made no additional payments during the three months ended March 31, 2018. Shenyang Sunshine Pharmaceutical Co., Ltd In May 2014, the Company entered into a license agreement with Shenyang Sunshine Pharmaceutical Co., Ltd. (“3SBio”), which is referred to as the 3SBio License. Pursuant to the 3SBio License, which was amended and restated in May 2017, the Company was granted an exclusive license to certain pegsiticase‑related patents and related “know‑how” owned or in‑licensed by 3SBio for the worldwide (except for Greater China and Japan) development and commercialization of products based thereupon for human therapeutic, diagnostic and prophylactic use. The Company was also granted a worldwide (except for Greater China) exclusive license to develop, commercialize and manufacture or have manufactured products combining the Company’s proprietary SVP technology with pegsiticase or related compounds supplied by 3SBio (or otherwise supplied if the Company’s rights to manufacture are in effect) for human therapeutic, diagnostic and prophylactic use. The Company was also granted a co‑exclusive license to manufacture and have manufactured pegsiticase and related compounds for preclinical and clinical use or, if the 3SBio License is terminated for 3SBio’s material breach, for any use under the 3SBio License. Otherwise, the Company is obligated to obtain all of its supply of such compounds for Phase 3 clinical trials and commercial use from 3SBio under the terms of supply agreements to be negotiated. Pursuant to the 3SBio License, the Company is required to use commercially reasonable efforts to develop and commercialize a product containing pegsiticase or a related compound. If the Company does not commercialize any such product in a particular country in Asia, Africa or South America within 48 months after approval of any such product in the United States or a major European country, then 3SBio will have the right to do so, but only until the Company commercializes a product combining the Company’s SVP technology with any such compound in such country. The Company has paid to 3SBio an aggregate of $1.0 million in upfront and milestone‑based payments under the 3SBio License. An additional liability totaling $2.0 million for milestone payments was expensed in 2016 and is included within accounts payable on the balance sheet as of December 31, 2016. The Company is required to make future payments to 3SBio contingent upon the occurrence of events related to the achievement of clinical and regulatory approval milestones of up to an aggregate of $21.0 million for products containing the Company’s SVP technology, and up to an aggregate of $41.5 million for products without the Company’s SVP technology. The Company is also required to pay 3SBio tiered royalties on annual worldwide net sales (on a country‑by‑country and product‑by‑product basis) related to the pegsiticase component of products at percentages ranging from the low‑to‑mid single digits for products containing the Company’s SVP technology, and a range of no more than ten percent points from the mid‑single digits to low double‑digits for products without the Company’s SVP technology. The Company will pay these royalties to 3SBio, subject to specified reductions, on a country‑by‑country and product‑by‑product basis until the later of (i) the date that all of the patent rights for that product have expired in that country, or (ii) a specified number of years from the first commercial sale of such product in such country. The 3SBio License expires on the date of expiration of all of the Company’s royalty payment obligations unless earlier terminated by either party for an uncured material default or for the other party’s bankruptcy. Any such termination by 3SBio for material default may be on a country‑by‑country or product‑by‑product basis in certain circumstances. The Company may also terminate the 3SBio License on a country‑by‑country or product‑by‑product basis for any reason effective upon 60 days’ prior written notice to 3SBio or, with respect to a given product, immediately upon written notice to 3SBio if the Company identifies a safety or efficacy concern related to such product. Massachusetts Eye and Ear Infirmary and The Schepens Eye Research Institute, Inc. In May 2016, the Company entered into a license agreement with the Massachusetts Eye and Ear Infirmary and The Schepens Eye Research Institute, Inc. (collectively, “MEE”), which is referred to as the MEE License. Under the MEE License, the Company was granted an exclusive commercial worldwide license, with the right to grant sublicenses through multiple tiers, to make, have made, use, offer to sell, sell and import certain products and to practice certain processes, the sale, use or practice of which are covered by patents and proprietary know‑how owned or controlled by MEE, for use of Anc80 gene therapy vectors for gene augmentation therapies expressing certain target sequences. MEE also granted the Company exclusive options to exclusively license certain of their intellectual property rights relating to several additional target sequences and variations thereof each linked to a specified disease. During a defined option period, the Company may exercise this right for up to a designated number of target sequences. If the Company exercises its options, under certain circumstances, the Company may substitute alternative target sequences for previously selected target sequences. The Company agreed to use commercially reasonable efforts to develop and commercialize licensed products pursuant to a development plan, and to market and sell at least one product for each target sequence for which the Company exercised its option as soon as reasonably practicable. Subject to certain exceptions, following commercial launch, the Company must use commercially reasonable efforts to market, sell, and maintain public availability of licensed products in a certain number of specified major markets. Pursuant to the MEE License, the Company agreed to pay MEE a license fee in the low six figures, annual license maintenance fees ranging from the mid‑twenty thousands to mid‑seventy thousands and an option maintenance fee in the low five figures for each exercisable option. The Company also agreed to reimburse MEE for a specified percentage of the past patent expenses for the patents licensed to the Company. The Company also agreed to pay development milestones on a licensed product‑by‑licensed product basis, totaling up to an aggregate of between $4.2 million to $37.0 million and sales milestones on a licensed product‑by‑licensed product basis, totaling up to an aggregate of between $50.0 million to $70.0 million; tiered royalties on a licensed product‑by‑licensed product and country‑by‑country basis equal to a percentage of net sales ranging from mid‑single digits to mid‑teens, subject to the prevalence of the targeted disease and certain reductions; and a percentage, in a range expected to be in the mid‑teens depending on timing, of any sublicense income the Company receives from sublicensing its rights granted thereunder, subject to certain reductions and exclusions. Upon exercise of each option, the Company agreed to pay MEE an option exercise fee ranging from low‑six figures to mid‑six figures, depending on the prevalence of the targeted disease. The MEE License will continue until the expiration of the last to expire of the patent rights licensed thereunder. The Company may terminate the MEE License in whole or in part upon prior written notice. MEE may terminate the MEE License on a target sequence‑by‑target sequence basis if the Company fails to make any scheduled payments in respect of such target sequence or if the Company materially breaches a diligence obligation in respect of such target sequence, in each case if the Company fails to cure within a specified time period. MEE may terminate the MEE License in its entirety if the Company materially breaches certain of its obligations related to diligence, representations and warranties, and maintenance of insurance; if the Company challenges the validity or enforceability of any patents licensed thereunder; if any of the Company’s executive officers are convicted of a felony relating to manufacture, use, sale or importation of licensed products; or upon the Company’s insolvency or bankruptcy. As of June 30, 2016, the Company had paid a total of $0.1 million in license fees due under the MEE License. No additional license fees are due under the MEE License as of March 31, 2018.

Income Taxes	3 Months Ended
Income Taxes	Mar. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The Company provides for income taxes under ASC 740, Income Taxes ("ASC 740"). Under ASC 740, the Company provides deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the Company’s financial statement carrying amounts and the tax bases of assets and liabilities using enacted tax rates expected to be in effect in the years in which the differences are expected to reverse. For the three months ended March 31, 2018 and 2017, the Company did not record a current or deferred income tax expense or benefit. The Company provided a full valuation allowance against its net deferred tax assets because the Company believes it is more likely than not that its deferred tax assets will not be realized. Realization of future tax benefits is dependent on many factors, including the Company’s ability to generate taxable income within the net operating loss carryforward period. The Company has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets and concluded that it is more likely than not that the Company will not realize the benefit of its deferred tax assets. In 2014, the Company's Russian subsidiary was granted a 10 year tax holiday in Russia. The Company's foreign operations continue to benefit from the tax holiday, which is set to expire on December 31, 2023. On December 22, 2017, the President of the United States signed into law the Tax Cuts and Jobs Act ("Tax Reform Act"). The legislation significantly changes U.S. tax law by, among other things, lowering corporate income tax rates, implementing a territorial tax system, expanding the tax base and imposing a tax on deemed repatriated earnings of foreign subsidiaries. The Tax Reform Act permanently reduces the U.S. corporate federal income tax rate from a maximum of 35% to a flat 21% rate, effective January 1, 2018. The Company recognized the impact of the Tax Reform Act in these consolidated financial statements and related disclosures. Due to the complexities involved in accounting for the enactment of the Tax Reform Act, the SEC staff issued Staff Accounting Bulletin No. 118 ("SAB 118"), which allows a registrant to record provisional amounts during a measurement period not to extend beyond one year of the enactment date. In accordance with SAB 118, the Company recorded provisional amounts reflecting the impact of the Tax Reform Act in these consolidated financial statements and related disclosures. The impact of the remeasurement of the Company’s U.S. deferred tax assets and liabilities to 21% resulted in the reduction of deferred tax assets of approximately $23.4 million, which is offset by a full valuation allowance, thus there is no net effect. The Company recorded no tax expense related to the deemed repatriation tax because its foreign entity, Selecta (RUS) is a foreign disregarded entity, which is not subject to the repatriation tax. The Company's preliminary estimate of the Tax Reform Act and the remeasurement of its deferred tax assets and liabilities is subject to the finalization of management’s analysis related to certain matters, such as developing interpretations of the provisions of the Tax Reform Act, changes to certain estimates and the filing of its tax returns. U.S. Treasury regulations, administrative interpretations or court decisions interpreting the Tax Reform Act may require further adjustments and changes in the Company's estimates. The final determination of the Tax Reform Act and the remeasurement of the Company's deferred assets and liabilities will be completed as additional information becomes available, but no later than one year from the enactment of the Tax Reform Act. Utilization of the net operating loss and research and development credit carryforwards may be subject to a substantial annual limitation under Section 382 of the Internal Revenue Code of 1986 due to ownership change limitations that have occurred previously, or that could occur in the future. These ownership changes may limit the amount of net operating loss and research and development credit carryforwards that can be utilized annually to offset future taxable income and tax, respectively. The Company applies ASC 740 to uncertain tax positions. As of the adoption date of January 1, 2010 and through March 31, 2018, 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 March 31, 2018, the Company had no accrued interest related to uncertain tax positions. The statute of limitations for assessment by the Internal Revenue Service and Massachusetts tax authorities is open for tax years since inception. The Company files income tax returns in the United States and Massachusetts. There are currently no federal, state or foreign audits in progress.

Defined Contribution Plan	3 Months Ended
Defined Contribution Plan	Mar. 31, 2018
Retirement Benefits [Abstract]
Defined Contribution Plan	Defined Contribution Plan The Company maintains a defined contribution plan under Section 401(k) of the Internal Revenue Code (the “401(k) Plan”). The 401(k) Plan covers all employees who meet defined minimum age and service requirements, and allows participants to defer a portion of their annual compensation on a pretax basis. The 401(k) Plan provides for matching contributions on a portion of participant contributions pursuant to the 401(k) Plan’s matching formula. All matching contributions vest ratably over 4 years and participant contributions vest immediately. Contributions by the Company totaled $0.1 million for each of the three months ended March 31, 2018 and 2017, and have been recorded in the consolidated statements of operations and comprehensive loss.

Summary of Significant Accounting Policies (Policies)	3 Months Ended
Summary of Significant Accounting Policies (Policies)	Mar. 31, 2018
Accounting Policies [Abstract]
Principles of Consolidation	Principles of Consolidation The consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries, Selecta RUS, LLC (“Selecta (RUS)”), a Russian limited liability corporation, and Selecta Biosciences Security Corporation, a Massachusetts Security Corporation. All significant intercompany accounts and transactions have been eliminated.
Foreign Currency	Foreign Currency The functional currency of Selecta (RUS) is the Russian ruble. Assets and liabilities of Selecta (RUS) are translated at period-end exchange rates, while revenues and expenses are translated at average exchange rates for the period. Translation gains and losses are reflected in accumulated other comprehensive loss within stockholders’ deficit. Foreign currency transaction gains or losses are reflected in the consolidated statements of operations and comprehensive loss.
Use of Estimates	Use of Estimates The preparation of consolidated financial statements in conformity with U.S. GAAP requires the Company’s management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. The Company’s management considers many factors in selecting appropriate financial accounting policies and controls, and bases its estimates on historical experience and other market-specific or other relevant assumptions that it believes to be reasonable under the circumstances. In preparing these consolidated financial statements, management used significant estimates in the following areas, among others: revenue recognition, the fair value of common stock and other equity instruments, accounting for stock-based compensation and estimating accrued research and development expenses. The Company assesses the above estimates on an ongoing basis; however, actual results could materially differ from those estimates.
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.
Cash Equivalents and Short Term Investments	Cash Equivalents, Short-term Investments and Restricted Cash 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 records its investments at fair value in the accompanying consolidated balance sheets. Changes in fair value are recognized through net income. Premiums or discounts from par value are amortized to investment income over the life of the underlying investment. The Company, as part of its cash management strategy, may invest in reverse repurchase agreements. All reverse repurchase agreements are tri-party and have maturities of three months or less at the time of investment. These agreements are collateralized by U.S. treasury securities for an amount no less than 102% of their value. Although available to be sold to meet operating needs or otherwise, securities are generally held through maturity. The cost of securities sold is determined based on the specific identification method for purposes of recording realized gains and losses. During the reporting periods, there were no realized gains or losses on sales of investments, and no investments were adjusted for other than temporary declines in fair value.
Concentrations of Credit Risk and Off-Balance Sheet Risk	Concentrations of Credit Risk and Off‑Balance Sheet Risk Financial instruments that potentially subject the Company to concentration of credit risk consist primarily of cash, cash equivalents, and accounts receivable. Cash and cash equivalents are deposited with federally insured financial institutions in the United States and may, at times, exceed federally insured limits. Management believes that the financial institutions that hold the Company’s deposits are financially credit worthy and, accordingly, minimal risk exists with respect to those balances. Generally, these deposits may be redeemed upon demand and therefore bear minimal interest rate risk. As an integral part of operating its Russian subsidiary, the Company also maintains cash in Russian bank accounts in denominations of both Russian rubles and U.S. dollars. As of March 31, 2018, the Company maintained approximately $1.1 million in Russian bank accounts, of which $1.0 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 March 31, 2018 and December 31, 2017.
Fair Value of Financial Instruments	Fair Value of Financial Instruments The Company’s financial instruments consist mainly of cash equivalents, short‑term investments, restricted cash, accounts receivable, accounts payable, loans payable, and common stock warrants. The carrying amounts of cash equivalents, short-term investments, restricted cash, accounts receivable, and accounts payable approximate their estimated fair value due to their short-term maturities. At March 31, 2018, the carrying amount of the Company's loan payable approximates its estimated fair value due to the consistency between the prevailing market rates in effect for similar loans and the effective interest rate of 6.77% for the Company's loan outstanding. Accounting standards define fair value as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. A three‑level hierarchy is used to prioritize the inputs to valuation techniques used to measure fair value. The hierarchy gives the highest priority to unadjusted quoted prices in active markets for identical assets or liabilities (Level 1 measurements), and the lowest priority to unobservable inputs (Level 3 measurements). The three levels of the fair value hierarchy are described below: Level 1—Level 1 inputs are quoted prices in active markets for identical assets or liabilities that the reporting entity has the ability to access at the measurement date. Level 2—Level 2 inputs are inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly. If the asset or liability has a specified (contractual) term, a Level 2 input must be observable for substantially the full term of the asset or liability. Level 3—Level 3 inputs are unobservable inputs for the asset or liability in which there is little, if any, market activity for the asset or liability at the measurement date. To the extent that a valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. The fair value of the Company's loan payable was determined using Level 3 inputs. Fair value is a market‑based measure considered from the perspective of a market participant rather than an entity‑specific measure. Therefore, even when market assumptions are not readily available, the Company’s own assumptions are set to reflect those that market participants would use in pricing the asset or liability at the measurement date. The Company uses prices and inputs that are current as of the measurement date, including during periods of market dislocation. In periods of market dislocation, the observability of prices and inputs may change for many instruments. This condition could cause an instrument to be reclassified within levels in the fair value hierarchy. There were no transfers within the fair value hierarchy during the three months ended March 31, 2018 or the year ended December 31, 2017.
Property and Equipment	Property and Equipment Property and equipment are recorded at cost and depreciated using the straight‑line method over the estimated useful lives of the respective assets, generally seven years for furniture and fixtures, five years for laboratory equipment and office equipment and three years for computer equipment. Leasehold improvements are amortized over their useful life or the life of the lease, whichever is shorter. Major additions and betterments are capitalized. Maintenance and repairs, which do not improve or extend the life of the respective assets, are charged to operations as incurred. Costs incurred for construction in progress are recorded as assets and are not amortized until the construction is substantially complete and the assets are ready for their intended use.
Impairment of Long-Lived Assets	Impairment of Long‑Lived Assets The Company periodically evaluates its long‑lived assets for potential impairment. Impairment is assessed when there is evidence that events or changes in circumstances indicate that the carrying amount of an asset may not be recovered. Recoverability of these assets is assessed based on undiscounted expected future cash flows from the assets, considering a number of factors, including past operating results, budgets and economic projections, market trends, and product development cycles. Impairment in the carrying value of each asset is assessed when the undiscounted expected future cash flows derived from the asset are less than their carrying value. The Company did not recognize any impairment charges as of March 31, 2018 and December 31, 2017.
Debt Issuance Costs	Debt Issuance Costs Debt issuance costs and fees paid to lenders are classified as a debt discount, and are recorded as a direct deduction from the face amount of the related debt. Issuance costs paid to third parties that are the direct result of the debt issuance are capitalized as a direct deduction from the face amount of the related debt. Debt issuance costs are amortized over the term of the related debt using the interest method and recorded as interest expense. Costs and fees paid to third parties are expensed as incurred.
Accumulated Other Comprehensive Income (Loss)	Accumulated Other Comprehensive Income (Loss) Comprehensive income (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 income (loss) consists of: (i) all components of net loss and (ii) all components of comprehensive loss other than net loss, referred to as other comprehensive loss. Other comprehensive loss is comprised of foreign currency translation adjustments and the unrealized gains and losses on available-for-sale securities.
Revenue Recognition	Revenue Recognition The Company currently generates its revenue through grants, collaboration and license agreements with strategic collaborators for the development and commercialization of product candidates. Prior to January 1, 2018, the Company recognized revenue in accordance with ASC Topic 605, Revenue Recognition ("ASC 605"). Under ASC 605, the Company recognized revenue when all of the following criteria were 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. Under ASC 605, amounts received prior to satisfying the revenue recognition criteria were 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 were classified as deferred revenue, current portion. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date were classified as deferred revenue, net of current portion. In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers ("ASU 2014-09"). ASU 2014-09 and its related amendments (collectively referred to as ASC 606) requires that an entity recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. The Company adopted ASC 606, Revenue from Contracts with Customers, on January 1, 2018, using the modified retrospective method for all contracts not completed as of the date of adoption. The reported results for 2018 reflect the application of ASC 606 guidance while the reported results for 2017 were prepared under the guidance of ASC 605, which is also referred to herein as "legacy GAAP" or the "previous guidance". Refer to Note 12 to our consolidated financial statements included elsewhere in this Quarterly Report on Form 10-Q for the impact of adoption of ASU 2014-09 on the Company’s consolidated financial position, results of operations, equity and cash flows as of the adoption date and for the three months ended March 31, 2018. The adoption of ASC 606 represents a change in accounting principle that will more closely align revenue recognition with the delivery of the Company's services and will provide financial statement readers with enhanced disclosures. Revenue is recognized when a customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of ASC 606, the Company performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract, including whether they are distinct in the context of the contract; (iii) determine the transaction price, including the constraint on variable consideration; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the Company satisfies each performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception, once the contract is determined to be within the scope of ASC 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations, and assesses whether each promised good or service is distinct. If a promised good or service is not distinct, it is combined with other performance obligations. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied. For example, certain performance obligations associated with Spark (see Note 12 to our consolidated financial statements included elsewhere in this Quarterly Report on Form 10-Q) will be satisfied over time, and revenue will be recognized using the output method, based on the proportion of actual deliveries to the total expected deliveries over the initial term. Collaboration and Grant Revenue The Company currently generates its revenue through grants, collaboration and license agreements with strategic collaborators for the development and commercialization of product candidates. The grants, collaboration and license agreements are within the scope of ASC 606. In determining the appropriate amount of revenue to be recognized as it fulfills its obligations under the agreements, the Company performs the five steps above. As part of the accounting for the arrangement, the Company must develop assumptions that require judgment to determine the stand-alone selling price for each performance obligation identified in the contract. The Company uses key assumptions to determine the stand-alone selling price, which may include market conditions, reimbursement rates for personnel costs, development timelines and probabilities of regulatory success. The Company capitalizes the incremental costs of obtaining a contract with a customer if it expects to recover those costs. Such incremental costs would not have been incurred if the contract with a customer had not been obtained. Licenses of intellectual property: If the license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, the Company recognizes revenues from non-refundable, up-front fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. If not distinct, the license is combined with other performance obligations in the contract. For licenses that are combined with other performance obligations, the Company assesses the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. Optional licenses are evaluated to determine if they are issued at a discount, and therefore, represent material rights and accounted for as separate performance obligations. Milestone Payments: At the inception of each arrangement that includes developmental and regulatory milestone payments, the Company evaluates whether the achievement of each milestone specifically relates to the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service within a performance obligation. If the achievement of a milestone is considered a direct result of the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service and the receipt of the payment is based upon the achievement of the milestone, the associated milestone value is allocated to that distinct good or service. If the milestone payment is not specifically related to the Company’s effort to satisfy a performance obligation or transfer a distinct good or service, the amount is allocated to all performance obligations using the relative standalone selling price method. The Company also evaluates the milestone to determine whether they are considered probable of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price to be allocated, otherwise, such amounts are constrained and excluded from the transaction price. At the end of each subsequent reporting period, the Company re-evaluates the probability of achievement of such development milestones and any related constraint, and if necessary, adjusts its estimate of the transaction price. Any such adjustments to the transaction price are allocated to the performance obligations on the same basis as at contract inception. Amounts allocated to a satisfied performance obligation shall be recognized as revenue, or as a reduction of revenue, in the period in which the transaction price changes. Manufacturing Supply Services: Arrangements that include a promise for future supply of drug substance or drug product for either clinical development or commercial supply at the customer’s discretion are evaluated to determine if they are distinct and optional. For optional services that are distinct, the Company assesses if they are priced at a discount, and therefore, provide a material right to the licensee to be accounted for as separate performance obligations. Royalties: For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company will recognize revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied) in accordance with the royalty recognition constraint.
Research and Development Costs	Research and Development Costs Costs incurred in the research and development of the Company’s products are expensed as incurred. Research and development expenses include costs incurred in performing research and development activities, including salaries and benefits, facilities cost, overhead costs, contract services, supplies and other outside costs. Nonrefundable advance payments for goods and services that will be used in future research and development activities are expensed when the activity has been performed or when the goods have been received rather than when the payment is made.
Clinical Trial Costs	Clinical Trial Costs Clinical trial costs are a component of research and development expenses. The Company accrues and expenses clinical trial activities performed by third parties based on an evaluation of the progress to completion of specific tasks using data such as patient enrollment, clinical site activation, and other information provided to the Company by its vendors.
Income Taxes	Income Taxes The Company provides deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the Company’s financial statement carrying amounts and the tax basis of assets and liabilities using enacted tax rates expected to be in effect in the years in which the differences are expected to reverse. A valuation allowance is provided to reduce the deferred tax assets to the amount that will more-likely-than-not be realized. The Company determines whether it is more likely than not that a tax position will be sustained upon examination. If it is not more-likely-than-not that a position will be sustained, none of the benefit attributable to the position is recognized. The tax benefit to be recognized for any tax position that meets the more‑likely‑than‑not recognition threshold is calculated as the largest amount that is more than 50% likely of being realized upon resolution of the contingency. The Company accounts for interest and penalties related to uncertain tax positions as part of its provision for income taxes. To date, the Company has not incurred interest and penalties related to uncertain tax positions.
Warrants	Warrants The Company determines the accounting classification of warrants that are issued, as either liability or equity, by first assessing whether the warrants meet liability classification in accordance with ASC 480-10, Accounting for Certain Financial Instruments with Characteristics of both Liabilities and Equity, and then in accordance with ASC 815-40, Accounting for Derivative Financial Instruments Indexed to, and Potentially Settled in, a Company’s Own Stock. Under ASC 480, warrants are considered liability classified if the warrants are mandatorily redeemable, obligate the issuer to settle the warrants or the underlying shares by paying cash or other assets, or warrants that must or may require settlement by issuing variable number of shares. If warrants do not meet liability classification under ASC 480-10, the Company assesses the requirements under ASC 815-40, which states that contracts that require or may require the issuer to settle the contract for cash are liabilities recorded at fair value, irrespective of the likelihood of the transaction occurring that triggers the net cash settlement feature. If the warrants do not require liability classification under ASC 815-40, in order to conclude equity classification, the Company assesses whether the warrants are indexed to its common stock and whether the warrants are classified as equity under ASC 815-40 or other applicable GAAP. After all relevant assessments are made, the Company concludes whether the warrants are classified as liability or equity. Liability classified warrants are required to be accounted for at fair value both on the date of issuance and on subsequent accounting period ending dates, with all changes in fair value after the issuance date recorded in the statements of operations as a gain or loss. Equity classified warrants are accounted for at fair value on the issuance date with no changes in fair value recognized after the issuance date.
Stock-Based Compensation	Stock‑Based Compensation The Company accounts for all stock‑based compensation granted to employees and non‑employees using a fair value method. Stock‑based compensation awarded to employees is measured at the grant date fair value and is recognized over the requisite service period of the awards, usually the vesting period, on a straight‑line basis, net of estimated forfeitures. Stock‑based compensation awarded to non‑employees are subject to revaluation over their vesting terms. The Company reduces recorded stock‑based compensation for estimated forfeitures. To the extent that actual forfeitures differ from the Company’s estimates, the differences are recorded as a cumulative adjustment in the period the estimates were adjusted. Stock‑based compensation expense recognized in the consolidated financial statements is based on awards that are ultimately expected to vest.
Net Loss Per Share	Net Loss Per Share The Company has reported losses since inception and has computed basic net loss per share attributable to common stockholders by dividing net loss attributable to common stockholders by the weighted average number of common shares outstanding for the period. The Company has computed diluted net loss per common share after considering all potentially dilutive common shares, including stock options, convertible preferred stock, and warrants outstanding during the period except where the effect of including such securities would be antidilutive. Because the Company has reported net losses since inception, these potential common shares have been anti‑dilutive and basic and diluted loss per share have been the same.
Deferred Rent	Deferred Rent Rent expense and lease incentives from operating leases are recognized on a straight‑line basis over the lease term. The difference between rent expense recognized and rental payments is recorded as deferred rent in the accompanying consolidated balance sheets.
Contingent Liabilities	Contingent Liabilities The Company accounts for its contingent liabilities in accordance with ASC No. 450, Contingencies. A provision is recorded when it is both probable that a liability has been incurred and the amount of the loss can be reasonably estimated. With respect to legal matters, provisions are reviewed and adjusted to reflect the impact of negotiations, estimated settlements, legal rulings, advice of legal counsel and other information and events pertaining to a particular matter. As of March 31, 2018 and December 31, 2017, the Company was not a party to any litigation that could have a material adverse effect on the Company’s business, financial position, results of operations or cash flows.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In November 2015, the FASB issued ASU No. 2015-17, Income Taxes (Topic 740), Balance Sheet Classification of Deferred Taxes. The new standard requires that deferred tax assets and liabilities be classified as non-current in a classified statement of financial position. The Company adopted this standard on January 1, 2017, and the adoption did not have an impact on its consolidated financial statements due to the full valuation allowance position. In January 2016, the FASB issued ASU No. 2016-01, Financial Instruments-Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities (“ASU 2016-01”). ASU 2016-01 supersedes the guidance to classify equity securities with readily determinable fair values into different categories (that is, trading or available-for-sale) and requires equity securities (including other ownership interests, such as partnerships, unincorporated joint ventures, and limited liability companies) to be measured at fair value with changes in fair value recognized through net income. The amendments allow equity investments that do not have readily determinable fair values to be remeasured at fair value either upon the occurrence of an observable price change or upon identification of an impairment. The amendments also require enhanced disclosures about those investments. ASU 2016-01 is effective for annual reporting beginning after December 15, 2017, including interim periods within the year of adoption, and calls for prospective application, with early application permitted. The Company adopted ASU 2016-01 during the first quarter of 2018. Because the Company held no equity investments upon adoption, there was no impact on the consolidated financial statements. In February 2016, FASB issued ASU No. 2016-02, Leases (Topic 842) (“ASU 2016-02”). ASU 2016-02 requires a lessee to separate the lease components from the non-lease components in a contract and recognize in the statement of financial position a liability to make lease payments (the lease liability) and a right-of-use asset representing its right to use the underlying asset for the lease term. It also aligns lease accounting for lessors with the revenue recognition guidance in ASU 2014-09. ASU 2016-02 is effective for fiscal years beginning after December 15, 2018, including interim periods within those fiscal years. The Company believes it has less than five lease contracts that will need to be evaluated under ASU 2016-02, and thus does not expect the adoption of ASU 2016-02 to have a material effect on its consolidated financial statements. In August 2016, the FASB issued ASU No. 2016-15, Statements of Cash Flows (Topic 230) - Classifications of Certain Cash Receipts and Cash Payments ("ASU 2016-15"), to clarify how companies present and classify certain cash receipts and cash payments in the statement of cash flows. This guidance is effective for annual reporting periods beginning after December 15, 2017, including interim periods within that reporting period, with early adoption permitted. The adoption of ASU 2016-15 will have a retrospective impact to its third quarter due to the recognition of debt extinguishment costs during the three and nine months ended September 30, 2017. The Company will classify debt extinguishment costs as cash outflows for financing activities in accordance with ASU 2016-15. 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. The Company adopted ASU 2016-18 during the first quarter ended March 31, 2018. The adoption of ASU 2016-18 resulted in the Company's cash, cash equivalents and restricted cash being included in the beginning and ending amounts for the periods shown on the statement of cash flows and was applied retroactively and reflected in the balances presented for any prior periods. The Company believes that the adoption of this guidance did not have a significant impact on its condensed consolidated financial statements. Effective January 1, 2018, the Company adopted ASU 2017-09, Compensation-Stock Compensation (Topic 718): Scope of Modification Accounting ("ASU 2017-09"). ASU 2017-09 specifies the types of changes to the terms or conditions of a share-based payment award that require an entity to apply modification accounting in accordance with Topic 718. The adoption of ASU 2017-09 did not have a material impact on the Company's consolidated financial statements. In January 2017, the FASB issued ASU No. 2017-01, “Business Combinations (Topic 805): Clarifying the Definition of a Business” (ASU 2017-01), which clarifies the definition of a business to assist entities with evaluating whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses. This ASU is effective for public entities for fiscal years beginning after December 15, 2017, with early adoption permitted. The adoption of ASU 2017-01 did not have a material impact on the Company's consolidated financial statements.

Summary of Significant Accounting Policies (Tables)

3 Months Ended

Mar. 31, 2018

Accounting Policies [Abstract]

Schedule of Cash and Cash Equivalents

The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the consolidated balance sheet that sum to the total of the same such amounts shown in the consolidated statement of cash flows:



	March 31, 2018		March 31, 2017
Cash and cash equivalents	$	58,228	$	26,637
Restricted cash	76		81
Restricted cash included in other assets	329		316
Total cash, cash equivalents, and restricted cash shown in the consolidated statement of cash flows	$	58,633	$	27,034

Schedule of Restricted Cash and Cash Equivalents



	March 31, 2018		March 31, 2017
Cash and cash equivalents	$	58,228	$	26,637
Restricted cash	76		81
Restricted cash included in other assets	329		316
Total cash, cash equivalents, and restricted cash shown in the consolidated statement of cash flows	$	58,633	$	27,034

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

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



	Foreign currency translation adjustment			Unrealized gains (losses) on available-for-sale securities			Accumulated other comprehensive income (loss)
Balance at December 31, 2017	$	(4,404	)	$	(16	)	$	(4,420	)
Other comprehensive income during the period	19			3			22
Balance at March 31, 2018	$	(4,385	)	$	(13	)	$	(4,398	)

Available-for-Sale Marketable Securities (Tables)

3 Months Ended

Mar. 31, 2018

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 March 31, 2018 and December 31, 2017 (in thousands):



	March 31, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
U.S. government and agency securities	$	22,854	$	—	$	(13	)	$	22,841
Commercial paper	1,998		—		—			1,998
Total available-for-sale marketable securities	$	24,852	$	—	$	(13	)	$	24,839



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

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):



	March 31, 2018				December 31, 2017
	Fair Value		Amortized Cost		Fair Value		Amortized Cost
Less than one year	$	24,839	$	24,852	$	25,940	$	25,956

Net Loss Per Share (Tables)

3 Months Ended

Mar. 31, 2018

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):



	Three Months Ended March 31,
	2018			2017
Numerator:
Net loss attributable to common stockholders	$	(15,888	)	$	(15,134	)
Denominator:
Weighted‑average common shares outstanding—basic and diluted	22,345,523			18,474,227
Net loss per share attributable to common stockholders—basic and diluted	$	(0.71	)	$	(0.82	)

Schedule of Potential Common Shares Issuable Upon Conversion of Warrants

Potential dilutive common share equivalents consist of the following:



	March 31,
	2018	2017
Stock options to purchase common stock	2,658,035	2,025,664
Stock warrants to purchase common stock	176,432	97,302
Total	2,834,467	2,122,966

Fair Value Measurements (Tables)

3 Months Ended

Mar. 31, 2018

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):



	March 31, 2018
	(Level 1)		(Level 2)		(Level 3)		Total
Cash equivalents:
Money market funds	$	22,402	$	—	$	—	$	22,402
Commercial paper	—		5,494		—		5,494
Corporate bonds	2,000		—		—		2,000
Total cash equivalents	$	24,402	$	5,494	$	—	$	29,896
Short-term investments:
U.S. government and agency securities	$	—	$	22,841	$	—	$	22,841
Commercial paper	—		1,998		—		1,998
Total short-term investments	$	—	$	24,839	$	—	$	24,839



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

Property and Equipment (Tables)

3 Months Ended

Mar. 31, 2018

Property, Plant and Equipment [Abstract]

Schedule of Property and Equipment

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



	March 31,			December 31,
	2018			2017
Laboratory equipment	$	5,191		$	5,138
Computer equipment and software	492			480
Leasehold improvements	278			278
Furniture and fixtures	253			235
Office equipment	118			118
Construction in process	258			—
Total property and equipment	6,590			6,249
Less accumulated depreciation	(4,333		)	(4,158		)
Property and equipment, net	$	2,257		$	2,091

Accrued Expenses (Tables)

3 Months Ended

Mar. 31, 2018

Payables and Accruals [Abstract]

Schedule of Accrued Expenses

Accrued expenses consist of the following (in thousands):



	March 31,		December 31,
	2018		2017
Payroll and employee related expenses	$	1,441	$	1,902
Current portion of deferred rent and lease incentive	83		72
Collaboration and licensing	859		1,020
Accrued patent fees	408		268
Accrued external research and development costs	4,377		3,578
Accrued professional and consulting services	913		859
Accrued grant refund	175		175
Accrued interest	92		89
Other	714		617
Accrued expenses	$	9,062	$	8,580

Commitments and Contingencies (Tables)

3 Months Ended

Mar. 31, 2018

Commitments and Contingencies Disclosure [Abstract]

Schedule of Future Minimum Lease Payments

As of March 31, 2018, the aggregate future minimum lease payments related to these leases are as follows (in thousands):



Year ended December 31,
2018 (Remainder)	$	1,085
2019	1,482
2020	375
Total minimum lease payments	$	2,942

Debt (Tables)

3 Months Ended

Mar. 31, 2018

Debt Disclosure [Abstract]

Schedule of Future Minimum Payments on the Term Loans

Future minimum payments on the 2017 Term Loan as of March 31, 2018 are as follows (in thousands):



2018 (Remainder)	$	842
2019	4,587
2020	9,128
2021	8,678
2022	1,753
Total minimum debt payments	$	24,988
Less: Amount representing interest	(2,938		)
Less: Debt discount and deferred charges	(923		)
Less: Current portion of loans payable	—
Loans payable, net of current portion	$	21,127

Stockholders' Equity (Tables)

3 Months Ended

Mar. 31, 2018

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:



	March 31, 2018	December 31, 2017
Exercise of common warrants	176,432	176,432
Shares available for future stock incentive awards	1,911,551	923,440
Outstanding common stock options	2,658,035	2,657,187
Total	4,746,018	3,757,059

- 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
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21506-112644

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

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

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

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

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

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21475-112644

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(28))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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
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-Name Accounting Standards Codification
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-SubTopic 10
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-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=84159872&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=84159872&loc=d3e21484-112644

Reference 13: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21538-112644

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

+ Details

Stock Incentive Plans (Tables)

3 Months Ended

Mar. 31, 2018

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

Schedule of Weighted Average Assumptions Used for Stock Option Grants



	Three Months Ended March 31,
	2018			2017
Risk-free interest rate	2.65		%	2.07		%
Dividend yield	—			—
Expected term	6.64			5.92
Expected volatility	84.81		%	84.41		%
Weighted-average fair value of common stock	$	9.53		$	10.89

Summary of Activity Under the 2008 Plan and the 2016 Plan

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



					Weighted‑average
					remaining	Aggregate
	Number of		Weighted-average		contractual term	intrinsic value
	options		exercise price ($)		(in years)	(in thousands)
Employee Awards
Outstanding at December 31, 2017	2,411,237		$	10.58	7.50	$	4,729
Granted	52,200		$	9.53
Exercised	—		$	—
Forfeited	(51,352	)	$	16.37
Outstanding at March 31, 2018	2,412,085		$	10.43	7.33	$	5,193
Vested at March 31, 2018	1,172,413		$	7.12	5.67	$	4,645
Vested and expected to vest at March 31, 2018	2,255,453		$	10.22	7.21	$	5,144

Non‑Employee Awards
Outstanding at December 31, 2017	245,950		$	4.32	5.10	$	1,351
Granted	—		$	—
Exercised	—		$	—
Forfeited	—		$	—
Outstanding at March 31, 2018	245,950		$	4.32	4.86	$	1,444
Vested at March 31, 2018	212,308		$	3.89	4.37	$	1,338
Vested and expected to vest at March 31, 2018	245,950		$	4.32	4.86	$	1,444

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



	Three Months Ended March 31,
	2018		2017
Research and development	$	511	$	335
General and administrative	642		358
Total stock-based compensation expense	$	1,153	$	693

Revenue from Contracts with Customers (Tables)

3 Months Ended

Mar. 31, 2018

Revenue from Contract with Customer [Abstract]

Schedule of Impact of New Accounting Pronouncements



	As of March 31, 2018
Consolidated Balance Sheet	As reported			Pro forma as if the previous accounting was in effect
Assets
Cash, cash equivalents, and restricted cash	$	58,304		$	58,304
Short-term deposits and investments	24,839			24,839
Accounts receivable	52			52
Prepaid expenses and other current assets	1,789			1,558
Total current assets	84,984			84,753
Property and equipment, net	2,257			2,257
Restricted cash and other assets	2,696			329
Total assets	$	89,937		$	87,339
Liabilities and stockholders’ equity
Current liabilities:
Accounts payable	$	1,789		1,789
Accrued expenses	9,062			9,062
Deferred revenue, current portion	959			959
Total current liabilities	11,810			11,810
Non‑current liabilities:
Deferred rent and lease incentive	129			129
Loans payable, net of current portion	21,127			21,127
Deferred revenue, net of current portion	13,917			15,747
Other long‑term liabilities	1,372			1,372
Total liabilities	48,355			50,185
Stockholders’ equity:
Common stock	3			3
Additional paid-in capital	274,334			274,334
Accumulated deficit	(228,357		)	(232,785		)
Accumulated other comprehensive loss	(4,398		)	(4,398		)
Total stockholders’ equity	41,582			37,154
Total liabilities and stockholders’ equity	$	89,937		$	87,339

As a result of applying the modified retrospective method to adopt the new revenue guidance, the following adjustments were made to the Company's accounts on the condensed consolidated balance sheet as of January 1, 2018:



				Adjustments
	As Reported at			Due to			Balance at
Consolidated Balance Sheet	December 31, 2017			ASU 2014-09			January 1, 2018
Assets
Cash, cash equivalents, and restricted cash	$	70,698		$	—		$	70,698
Short-term deposits and investments	25,940			—			25,940
Accounts receivable	63			—			63
Prepaid expenses and other current assets	1,979			231			2,210
Total current assets	98,680			231			98,911
Property and equipment, net	2,091			—			2,091
Restricted cash and other assets	329			2,367			2,696
Total assets	$	101,100		$	2,598		$	103,698
Liabilities and stockholders’ equity
Current liabilities:
Accounts payable	1,606			—			1,606
Accrued expenses	8,580			—			8,580
Deferred revenue, current portion	787			—			787
Total current liabilities	10,973			—			10,973
Non‑current liabilities:
Deferred rent and lease incentive	151			—			151
Loans payable, net of current portion	21,042			—			21,042
Deferred revenue, net of current portion	15,919			(1,830		)	14,089
Other long‑term liabilities	1,201			—			1,201
Total liabilities	49,286			(1,830		)	47,456
Stockholders’ equity:
Common stock	3			—			3
Additional paid-in capital	273,128			—			273,128
Accumulated deficit	(216,897		)	4,428			(212,469		)
Accumulated other comprehensive loss	(4,420		)	—			(4,420		)
Total stockholders’ equity	51,814			4,428			56,242
Total liabilities and stockholders’ equity	$	101,100		$	2,598		$	103,698

Schedule of Changes in Contract Liabilities

The following table presents changes in the Company’s contract liabilities during the three months ended March 31, 2018 (in thousands):



	Balance at						Balance at
	Beginning of Period		Additions		Deductions		End of Period
Three Months Ended March 31, 2018
Contract liabilities:
Deferred revenue	$	14,876	$	—	$	—	$	14,876
Other liabilities (i)	2,231		—		—		2,231
Total contract liabilities	$	17,107	$	—	$	—	$	17,107

Nature of the Business and Basis of Presentation - Liquidity (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017	Mar. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Accumulated deficit	$ 228,357	$ 212,469	$ 216,897
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents, and restricted cash	58,228			$ 26,637
Russian subsidiary \| Unrestricted cash
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents, and restricted cash	$ 1,000		$ 1,200

Summary of Significant Accounting Policies - Segment Information (Details)	3 Months Ended
	Mar. 31, 2018 segment
Accounting Policies [Abstract]
Number of operating segments	1

Summary of Significant Accounting Policies - Cash Equivalents, Short Term Investments and Restricted Cash (Details)	3 Months Ended
	Mar. 31, 2018 USD ($)
Cash and Cash Equivalents [Line Items]
Realized gains or losses on sale of investments	$ 0
Minimum
Cash and Cash Equivalents [Line Items]
Agreements collateralized by U.S. treasury securities (no less than) (as a percent)	102.00%

Summary of Significant Accounting Policies - Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Cash, cash equivalents, and restricted cash	$ 58,228		$ 26,637
Restricted cash	76		81
Restricted cash included in other assets	329		316
Total cash, cash equivalents, and restricted cash shown in the consolidated statement of cash flows	$ 58,633	$ 71,027	$ 27,034	$ 59,050

Summary of Significant Accounting Policies - Concentrations of Credit Risk and Off-Balance Sheet Risk (Details) - Russian subsidiary $ in Millions	Mar. 31, 2018 USD ($)
Concentration Risk [Line Items]
Cash maintained in Russian bank accounts	$ 1.1
U.S. dollars
Concentration Risk [Line Items]
Cash maintained in Russian bank accounts	$ 1.0

Summary of Significant Accounting Policies - Fair Value of Financial Instruments (Details)	Mar. 31, 2018
Accounting Policies [Abstract]
Effective interest rate for debt arrangement	6.77%

Summary of Significant Accounting Policies - Property and Equipment (Details)	3 Months Ended
	Mar. 31, 2018
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Estimated useful life	7 years
Laboratory and office equipment
Property, Plant and Equipment [Line Items]
Estimated useful life	5 years
Computer 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	3 Months Ended
	Mar. 31, 2018 USD ($)
AOCI Attributable to Parent, Net of Tax
Beginning balance	$ 51,814
Other comprehensive loss during the year	22
Ending balance	41,582
Foreign currency translation adjustment
AOCI Attributable to Parent, Net of Tax
Beginning balance	(4,404)
Other comprehensive loss during the year	19
Ending balance	(4,385)
Unrealized gains (losses) on available-for-sale securities
AOCI Attributable to Parent, Net of Tax
Beginning balance	(16)
Other comprehensive loss during the year	3
Ending balance	(13)
Accumulated other comprehensive income (loss)
AOCI Attributable to Parent, Net of Tax
Beginning balance	(4,420)
Ending balance	$ (4,398)

Available-for-Sale Marketable Securities - Summary of Available-for-Sale Marketable Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 24,852	$ 25,956
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(13)	(16)
Estimated Fair Value	24,839	25,940
U.S. government and agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	22,854	12,798
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(13)	(10)
Estimated Fair Value	22,841	12,788
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,998
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Estimated Fair Value	$ 1,998
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		13,158
Gross Unrealized Gains		0
Gross Unrealized Losses		(6)
Estimated Fair Value		$ 13,152

Available-for-Sale Marketable Securities - Schedule of Fair Values and Amortized Cost of Marketable Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value
Less than one year	$ 24,839	$ 25,940
Amortized Cost
Less than one year	$ 24,852	$ 25,956

Available-for-Sale Marketable Securities - Narrative (Details)	3 Months Ended
	Mar. 31, 2018 USD ($) position
Investments, Debt and Equity Securities [Abstract]
Number of investment position in an unrealized loss position, less than twelve months \| position	10
Fair value investments, unrealized loss position, less than twelve months	$ 22,800,000
Unrealized loss through net loss (less than)	100,000
Other-than-temporary impairments	$ 0

Net Loss Per Share - Computation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Numerator:
Net loss	$ (15,888)	$ (15,134)
Denominator:
Weighted‑average common shares outstanding—basic and diluted (in shares)	22,345,523	18,474,227
Net loss per share attributable to common stockholders—basic and diluted (in dollars per share)	$ (0.71)	$ (0.82)

Net Loss Per Share - Schedule of Potential Common Shares Issuable Upon Conversion of Warrants (Details) - shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Potential common shares
Total (in shares)	2,834,467	2,122,966
Stock options to purchase common stock
Potential common shares
Total (in shares)	2,658,035	2,025,664
Stock warrants to purchase common stock
Potential common shares
Total (in shares)	176,432	97,302

Fair Value Measurements - Summary of Assets Measured at Fair Value on a Recurring Basis (Details) - Recurring - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	$ 29,896	$ 40,478
Total short-term investments	24,839	25,940
(Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	24,402	39,478
Total short-term investments	0	13,152
(Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	5,494	1,000
Total short-term investments	24,839	12,788
(Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	0	0
Total short-term investments	0	0
Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total short-term investments		13,152
Corporate bonds \| (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total short-term investments		13,152
Corporate bonds \| (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total short-term investments		0
Corporate bonds \| (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total short-term investments		0
U.S. government and agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total short-term investments	22,841	12,788
U.S. government and agency securities \| (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total short-term investments	0
U.S. government and agency securities \| (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total short-term investments	22,841	12,788
U.S. government and agency securities \| (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total short-term investments	0
Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total short-term investments	1,998
Commercial paper \| (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total short-term investments	0
Commercial paper \| (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total short-term investments	1,998
Commercial paper \| (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total short-term investments	0
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	22,402	39,478
Money market funds \| (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	22,402	39,478
Money market funds \| (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	0	0
Money market funds \| (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	0	0
Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	5,494
Commercial paper \| (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	5,494
Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	2,000
Corporate bonds \| (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	2,000
Corporate bonds \| (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	0
Corporate bonds \| (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	$ 0
U.S. government and agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents		1,000
U.S. government and agency securities \| (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents		0
U.S. government and agency securities \| (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents		1,000
U.S. government and agency securities \| (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents		$ 0

Fair Value Measurements - Narrative (Details)	3 Months Ended		12 Months Ended
Fair Value Measurements - Narrative (Details)	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
U.S. government and agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Average maturity, term	181 days		168 days
Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Average maturity, term	95 days
Corporate Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Average maturity, term		179 days
Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Tri-party repurchase agreements collateralized by government securities (not less than) (as a percent)	102.00%

Property and Equipment - Schedule of Property and Equipment (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 6,590		$ 6,249
Less accumulated depreciation	(4,333)		(4,158)
Property and equipment, net	2,257	$ 2,091	2,091
Laboratory equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	5,191		5,138
Computer equipment and software
Property, Plant and Equipment [Line Items]
Total property and equipment	492		480
Leasehold improvements
Property, Plant and Equipment [Line Items]
Total property and equipment	278		278
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Total property and equipment	253		235
Office equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	118		118
Construction in process
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 258		$ 0

Property and Equipment - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Property, Plant and Equipment [Abstract]
Depreciation expense	$ 169	$ 175

Accrued Expenses (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Payables and Accruals [Abstract]
Payroll and employee related expenses	$ 1,441		$ 1,902
Current portion of deferred rent and lease incentive	83		72
Collaboration and licensing	859		1,020
Accrued patent fees	408		268
Accrued external research and development costs	4,377		3,578
Accrued professional and consulting services	913		859
Accrued grant refund	175		175
Accrued interest	92		89
Other	714		617
Accrued expenses	$ 9,062	$ 8,580	$ 8,580

Commitments and Contingencies - Narrative (Details)	3 Months Ended		12 Months Ended
Commitments and Contingencies - Narrative (Details)	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2017 USD ($)	Oct. 31, 2017 ft²
Operating Leased Assets [Line Items]
Area of additional office space leased \| ft²					5,100
Deferred rent and lease incentive liability	$ 200,000			$ 200,000
Current portion of deferred rent and lease incentive liability (less than)	83,000			72,000
Rent expense	500,000	$ 500,000
Restricted cash and other assets \| Letter of credit
Operating Leased Assets [Line Items]
Letter of credit	300,000.0
Accrued expenses
Operating Leased Assets [Line Items]
Current portion of deferred rent and lease incentive liability (less than)	$ 100,000			$ 100,000
Maximum \| Laboratory and office space
Operating Leased Assets [Line Items]
Tenant improvement allowance			$ 700,000

Commitments and Contingencies - Schedule of Future Minimum Lease Payments (Details) $ in Thousands	Mar. 31, 2018 USD ($)
Year ended December 31,
2018 (remainder)	$ 1,085
2019	1,482
2020	375
Total minimum lease payments	$ 2,942

Debt - Term Loans (Narrative) (Details) - USD ($)		1 Months Ended			3 Months Ended
Debt - Term Loans (Narrative) (Details) - USD ($)	Sep. 12, 2017	Dec. 31, 2016	Jul. 31, 2014	Aug. 31, 2013	Mar. 31, 2018	Sep. 30, 2017	Mar. 31, 2017	Dec. 31, 2017	Jun. 21, 2016	Dec. 31, 2015	Aug. 09, 2013
Line of Credit Facility [Line Items]
Minimum assets trigger					$ 300,000
Minimum amount in excess of indebtedness trigger					300,000
Interest expense related to the Term Loan					$ 350,000		$ 300,000
Conversion of Series D Preferred
Line of Credit Facility [Line Items]
Expiration period of warrants					10 years
Conversion of Series E Preferred
Line of Credit Facility [Line Items]
Warrants issued for purchase of stock (in shares)										37,978
Expiration period of warrants					10 years
Convertible Debt \| Conversion of Series E Preferred
Line of Credit Facility [Line Items]
Exercise price (in dollars per share)										$ 11.32
Term Loans
Line of Credit Facility [Line Items]
Borrowing capacity										$ 12,000,000.0	$ 7,500,000.0
Borrowings on loan and security agreement			$ 4,500,000	$ 3,000,000
Interest rate per annum					8.10%
Number of monthly installments					30 months
Final payment fee (as a percent)					6.00%
Final payment fee					$ 700,000
Initial grant date fair value of warrants				$ 100,000						$ 100,000
Number of warrants exercised to purchase common stock (in shares)		16,493
Net shares of common stock issued (in shares)		4,697
Repayments of debt	$ 10,000,000
Term Loans \| Conversion of Series D Preferred \| Common stock
Line of Credit Facility [Line Items]
Warrants issued for purchase of stock (in shares)									17,888
Term Loans \| Conversion of Series D Preferred \| Maximum
Line of Credit Facility [Line Items]
Warrants issued for purchase of stock (in shares)			40,000	26,668					66,668
Term Loans \| Conversion of Series E Preferred \| Common stock
Line of Credit Facility [Line Items]
Warrants issued for purchase of stock (in shares)										15,094
Term Loans \| Prepaid after to first anniversary but before second
Line of Credit Facility [Line Items]
Prepayment fee (as a percent)					2.00%
Term Loans \| Prepaid after second anniversary
Line of Credit Facility [Line Items]
Prepayment fee (as a percent)					1.00%
Term Loans \| Convertible Debt \| Conversion of Series D Preferred \| Common stock
Line of Credit Facility [Line Items]
Exercise price (in dollars per share)									$ 16.77
2017 Term Loans
Line of Credit Facility [Line Items]
Interest rate per annum					4.00%
Final payment fee (as a percent)					5.00%
Final payment fee					$ 1,100,000
Term loan facility	21,000,000				21,000,000			$ 21,000,000
Loss on extinguishment of debt						$ 700,000
Debt issuance costs, line of credit arrangements (less than)	$ 100,000
Interest expense related to the Term Loan					$ 300,000		$ 300,000
2017 Term Loans \| Prepaid before the first anniversary
Line of Credit Facility [Line Items]
Prepayment fee (as a percent)					3.00%
2017 Term Loans \| Prepaid after to first anniversary but before second
Line of Credit Facility [Line Items]
Prepayment fee (as a percent)					2.00%
2017 Term Loans \| Prepaid after second anniversary
Line of Credit Facility [Line Items]
Prepayment fee (as a percent)					1.00%
Prime rate \| 2017 Term Loans
Line of Credit Facility [Line Items]
Floating rate	0.05%

Debt - Schedule of Future Minimum Payments on the Term Loans (Details) - 2017 Term Loans $ in Thousands	Mar. 31, 2018 USD ($)
Year ended December 31,
2018 (Remainder)	$ 842
2019	4,587
2020	9,128
2021	8,678
2022	1,753
Total minimum debt payments	24,988
Less: Amount representing interest	(2,938)
Less: Debt discount and deferred charges	(923)
Less: Current portion of loans payable	0
Loans payable, net of current portion	$ 21,127

Stockholders' Equity - Narrative (Details)					1 Months Ended	3 Months Ended
Stockholders' Equity - Narrative (Details)	Aug. 10, 2017 USD ($)	Jun. 27, 2017 USD ($) $ / shares shares	Jul. 25, 2016 USD ($) $ / shares shares	Jun. 21, 2016 USD ($) $ / shares shares	Jul. 25, 2016 USD ($) $ / shares	Mar. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 $ / shares shares	Jun. 27, 2016 shares
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)						22,349,840
Common stock, par value (in dollars per share) \| $ / shares						$ 0.0001	$ 0.0001
Common stock issued for convertible preferred stock (in shares)								10,126,118
Common stock, shares authorized (in shares)						200,000,000	200,000,000
Number of votes per share that common stockholders are entitled to						1
Dividends declared or paid on common stock \| $						$ 0
Common stock, shares outstanding (in shares)						22,349,840	22,343,254
At-The-Market Offering
Subsidiary, Sale of Stock [Line Items]
Public securities offering, amount authorized \| $	$ 200,000,000.0
Public stock offering, amount authorized \| $						$ 50,000,000
Sale of stock, number of shares issued in transaction (in shares)						0
2017 PIPE
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)		2,750,000
Common stock, par value (in dollars per share) \| $ / shares		$ 0.0001
Share price (in dollars per share) \| $ / shares		$ 16.00
Proceeds from issuance of common stock \| $		$ 47,100,000
Springer Purchase Agreement
Subsidiary, Sale of Stock [Line Items]
Share price (in dollars per share) \| $ / shares		$ 17.71
Exercise of common warrants
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)		79,130
Exercise price (in dollars per share) \| $ / shares		$ 17.71
Expiration period of warrants		5 years
Share price (in dollars per share) \| $ / shares		$ 0.125
IPO
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)			289,633	5,000,000
Share price (in dollars per share) \| $ / shares				$ 14.00
Proceeds from Initial Public Offering, net of underwriters' discounts and commissions \| $				$ 60,800,000	$ 64,500,000
Over-Allotment Option
Subsidiary, Sale of Stock [Line Items]
Share price (in dollars per share) \| $ / shares			$ 14.00		$ 14.00
Proceeds from Initial Public Offering, net of underwriters' discounts and commissions \| $			$ 3,700,000
Common stock \| Springer Purchase Agreement
Subsidiary, Sale of Stock [Line Items]
Stock issued (in shares)		338,791

Stockholders' Equity - Schedule of Authorized Shares of Common Stock for Future Issuance (Details) - shares	Mar. 31, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Total (in shares)	4,746,018	3,757,059
Shares available for future stock incentive awards
Class of Stock [Line Items]
Total (in shares)	1,911,551	923,440
Outstanding common stock options
Class of Stock [Line Items]
Total (in shares)	2,658,035	2,657,187
Exercise of common warrants
Class of Stock [Line Items]
Total (in shares)	176,432	176,432

Stock Incentive Plans - Additional Information (Narrative) (Details) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) $ / shares shares	Mar. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2008 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available for future grants (in shares)	4,746,018		3,757,059
Shares exercises in period by employees (in shares)	0
Stock‑based compensation expense \| $	$ 1,153	$ 693
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 (in shares)	1,911,551		923,440
Estimated forfeitures rate	10.00%
Weighted average grant date fair value of stock options (in dollars per share) \| $ / shares	$ 7.07	$ 7.77
Aggregate intrinsic value of stock options exercised \| $		$ 600
Unrecognized compensation expense related to unvested employee stock options \| $	$ 8,700		$ 10,200
Weighted average period for recognition	2 years 8 months 12 days	2 years 10 months 24 days
Non-employee stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested stock option awards (in shares)	55,073
Unrecognized compensation expense related to unvested non-employee stock options \| $	$ 400		$ 600
The 2008 Plan and the 2016 Plan \| Shares available for future stock incentive awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares exercises in period by employees (in shares)	0
Aggregate intrinsic value of stock options exercised \| $	$ 5,144
Forfeited (in shares)	51,352
Vested and expected to vest (in shares)	2,255,453
Granted (in shares)	52,200
The 2008 Plan and the 2016 Plan \| Non-employee stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares exercises in period by employees (in shares)	0
Aggregate intrinsic value of stock options exercised \| $	$ 1,444
Forfeited (in shares)	0
Vested and expected to vest (in shares)	245,950
Granted (in shares)	0	0
2008 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for grants (in shares)	0			2,213,412
2008 Plan \| Shares available for future stock incentive awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting term	4 years
Award expiration term	10 years
2008 Plan \| Shares available for future stock incentive awards \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Combined voting power of participants (as a percent)	10.00%
2008 Plan \| Shares available for future stock incentive awards \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Combined voting power of participants (as a percent)	10.00%
2016 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for grants (in shares)	1,210,256
Automatic increase in the number of shares that may be issued (as a percent)	0.04
Number of shares authorized, increase (in shares)	893,730	737,550
Number of shares available for future grants (in shares)	2,841,536
2016 Plan \| Shares available for future stock incentive awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting term	4 years
Award expiration term	10 years
2016 Plan \| Shares available for future stock incentive awards \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Combined voting power of participants (as a percent)	10.00%
2016 Plan \| Shares available for future stock incentive awards \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Combined voting power of participants (as a percent)	10.00%
Tranche One \| 2008 Plan \| Shares available for future stock incentive awards \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price of incentive stock options (not less than)	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 (not less than)	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 (not less than)	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 (not less than)	110.00%
Chief Executive Officer \| The 2008 Plan and the 2016 Plan \| Shares available for future stock incentive awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Forfeited (in shares)			62,546
Vested and expected to vest (in shares)			126,457
Granted (in shares)	20,000

Stock Incentive Plans - Schedule of Weighted Average Assumptions for Employee Stock Options (Details) - Employee stock option grants - $ / shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	2.65%	2.07%
Dividend yield	0.00%	0.00%
Expected term	6 years 7 months 21 days	5 years 11 months 1 day
Expected volatility	84.81%	84.41%
Weighted-average fair value of common stock (in dollars per share)	$ 9.53	$ 10.89

Stock Incentive Plans - Summary of Activity Under the 2008 Plan and the 2016 Plan (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Number of Options
Exercised (in shares)	0
Employee Awards
Aggregate intrinsic value
Vested and expected to vest		$ 600
The 2008 Plan and the 2016 Plan \| Employee Awards
Number of Options
Beginning balance (in shares)	2,411,237
Granted (in shares)	52,200
Exercised (in shares)	0
Forfeited (in shares)	(51,352)
Ending balance (in shares)	2,412,085		2,411,237
Vested (in shares)	1,172,413
Vested and expected to vest (in shares)	2,255,453
Weighted-average exercise price
Beginning balance (in dollars per share)	$ 10.58
Granted (in dollars per share)	9.53
Exercised (in dollars per share)	0.00
Forfeited (in dollars per share)	16.37
Ending balance (in dollars per share)	10.43		$ 10.58
Vested (in dollars per share)	7.12
Vested and expected to vest (in dollars per share)	$ 10.22
Weighted-average remaining contractual term
Outstanding, term	7 years 3 months 29 days	7 years 6 months
Vested, term	5 years 8 months 1 day
Vested and expected to vest, term	7 years 2 months 16 days
Aggregate intrinsic value
Outstanding	$ 5,193		$ 4,729
Vested	4,645
Vested and expected to vest	$ 5,144
The 2008 Plan and the 2016 Plan \| Non‑Employee Awards
Number of Options
Beginning balance (in shares)	245,950
Granted (in shares)	0	0
Exercised (in shares)	0
Forfeited (in shares)	0
Ending balance (in shares)	245,950		245,950
Vested (in shares)	212,308
Vested and expected to vest (in shares)	245,950
Weighted-average exercise price
Beginning balance (in dollars per share)	$ 4.32
Granted (in dollars per share)	0.00
Exercised (in dollars per share)	0.00
Forfeited (in dollars per share)	0.00
Ending balance (in dollars per share)	4.32		$ 4.32
Vested (in dollars per share)	3.89
Vested and expected to vest (in dollars per share)	$ 4.32
Weighted-average remaining contractual term
Outstanding, term	4 years 10 months 10 days	5 years 1 month 6 days
Vested, term	4 years 4 months 13 days
Vested and expected to vest, term	4 years 10 months 10 days
Aggregate intrinsic value
Outstanding	$ 1,444		$ 1,351
Vested	1,338
Vested and expected to vest	$ 1,444

Stock Incentive Plans - Employee Stock Purchase Plan (Narrative) (Details) $ in Thousands		3 Months Ended
	Jun. 20, 2016 offering_period shares	Mar. 31, 2018 USD ($) shares	Mar. 31, 2017 USD ($) shares	Dec. 31, 2017 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)		4,746,018		3,757,059
Stock-based compensation expense \| $		$ 1,153	$ 693
ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of offering periods under the ESPP \| offering_period	2
Term of each offering period	6 months
Eligible compensation withheld for use (up to) (as a percent)	25.00%
Purchase price of shares (not less than) (as a percent)	85.00%
Common stock authorized and reserved for future issuance (in shares)	173,076	558,046
Number of shares of common stock outstanding (as a percent)	0.01
Number of shares authorized, increase (in shares)		223,432	184,387
Shares of common stock issued to employees under the ESPP (in shares)		6,586
Stock-based compensation expense \| $		$ 100	$ 100

Stock Incentive Plans - Schedule of Stock-Based Compensation Expense Related to Stock Options and Restricted Common Stock (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 1,153	$ 693
Research and development
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	511	335
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 642	$ 358

Revenue from Contracts with Customers - Narrative (Details)					1 Months Ended		3 Months Ended		12 Months Ended		40 Months Ended
	Oct. 31, 2017 USD ($) $ / shares shares	Jun. 06, 2017 USD ($) $ / shares shares	Dec. 02, 2016 USD ($) target obligation contract	Nov. 28, 2014 USD ($)	Dec. 31, 2016 USD ($) $ / shares shares	May 31, 2014 USD ($)	Mar. 31, 2018 USD ($) agreement shares	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) $ / shares	Mar. 31, 2018 USD ($) agreement	Jan. 01, 2018 USD ($) agreement
Disaggregation of Revenue [Line Items]
Number of collaboration/grant agreements that required analysis to quantify the impact of adoption \| agreement												3
Number of additional targets \| target			4
Sale of stock, number of shares issued in transaction (in shares) \| shares							22,349,840
Contract liability							$ 17,107,000				$ 17,107,000	$ 17,107,000
Remaining performance obligation							8,600,000				8,600,000
Incremental costs of obtaining a contract							2,600,000				2,600,000
Revenue recognized							0
Deferred revenue related to agreement							14,876,000				14,876,000	14,876,000
Deferred revenue, current portion							959,000		$ 787,000		959,000	787,000
Deferred revenue, net of current portion							13,917,000		$ 15,919,000		13,917,000	14,089,000
Spark Letter Agreement
Disaggregation of Revenue [Line Items]
Sale common stock	$ 2,500,000
Premium of average daily VWAP (as a percent)	115.00%
Contract liability	$ 2,500,000
Aggregate purchase price		$ 5,000,000
Number of shares authorized to be issued pursuant to the Stock Purchase Agreement (in shares) \| shares	205,254	324,362
Fair value of initial purchase of common stock combined with the embedded future stock Acquisition Rights			$ 2,700,000
Revenue recognized							100,000
Aggregate purchase price per share (in dollar per share) \| $ / shares	$ 24.36	$ 15.41
First Acquisition Right
Disaggregation of Revenue [Line Items]
Aggregate purchase price	$ 5,000,000
Skolkovo
Disaggregation of Revenue [Line Items]
Contract liability												200,000
Revenue recognized							$ 1,800,000
Aggregate amount of grant				$ 2,700,000
Percentage of estimated total cost of research plan				48.50%
Remaining percentage of estimated costs to be contributed by the Company				51.50%
Payments received											$ 2,000,000
Period after completion of project when Company may be required to reimburse the funds received				3 years
Second Acquisition Right
Disaggregation of Revenue [Line Items]
Share price (in dollars per share) \| $ / shares	$ 24.36
Premium of average daily VWAP (as a percent)	115.00%
Consecutive calendar days leading up to and ending on the day prior to the Contract Date	30 days
Aggregate price of shares Spark will have the right to purchase	$ 5,000,000.0
Spark License Agreement
Disaggregation of Revenue [Line Items]
Upfront cash payment			10,000,000
Aggregate additional payments due within twelve months			$ 5,000,000
Term loan number of payments \| contract			2
Two payments due within 12 months of contract date			$ 2,500,000.0
Number of additional targets \| target			4
Variable fee the Company is eligible to receive for each additional target option elected			$ 2,000,000
Election period to exercise additional targets			3 years
Additional development and regulatory milestone payments			$ 65,000,000
Commercialization milestone payments			365,000,000
Sales milestone payments			$ 255,000,000
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
Sale of stock, number of shares issued in transaction (in shares) \| shares					197,238
Sale common stock					$ 5,000,000
Share price (in dollars per share) \| $ / shares					$ 25.35					$ 25.35
Premium of average daily 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 (in shares) \| shares									2,758,112
Number of agreements combined and evaluated into a single agreement \| agreement							2				2
Fair value of initial purchase of common stock combined with the embedded future stock Acquisition Rights			$ 2,700,000
Remaining performance obligation			12,300,000				$ 2,300,000				$ 2,300,000
Performance obligation per each option			1,300,000
Incremental costs of obtaining a contract			2,600,000				2,600,000				2,600,000
Revenue recognized							0	$ 100,000
Deferred revenue related to agreement							14,700,000		$ 14,700,000		14,700,000
Deferred revenue, current portion							1,000,000		800,000		1,000,000
Deferred revenue, net of current portion							$ 13,700,000		13,900,000		$ 13,700,000
Spark License Agreement \| MIT
Disaggregation of Revenue [Line Items]
License and option agreement, additional sub-license payment received liability									2,200,000
License and option agreement, payments made relative to calculated premium paid for initial equity investment made under the purchase agreement									$ 400,000
Spark License Agreement \| Consideration for Promises
Disaggregation of Revenue [Line Items]
Upfront cash payment			15,000,000.0
Spark License Agreement \| Development Milestones
Disaggregation of Revenue [Line Items]
Additional development and regulatory milestone payments			35,000,000
Spark License Agreement \| Regulatory Milestones
Disaggregation of Revenue [Line Items]
Additional development and regulatory milestone payments			30,000,000
Spark License Agreement \| Commercial Milestones
Disaggregation of Revenue [Line Items]
Commercialization milestone payments			$ 110,000,000
Spark License Agreement \| License and Supply Obligation
Disaggregation of Revenue [Line Items]
Number of obligations \| obligation			5
Remaining performance obligation			$ 7,100,000
Spark License Agreement \| Discount on Option Obligation
Disaggregation of Revenue [Line Items]
Number of obligations \| obligation			4
Remaining performance obligation			$ 5,200,000
First Grant \| NIH
Disaggregation of Revenue [Line Items]
Aggregate amount of grant						$ 8,100,000
Period after completion of project when Company may be required to reimburse the funds received						3 years
Revenue recognized										$ 7,200,000
Remaining amount not recognized						$ 900,000
Accumulated deficit \| ASU 2014-09 \| Skolkovo
Disaggregation of Revenue [Line Items]
Cumulative effect adjustment												$ 1,800,000

Revenue from Contracts with Customers - Schedule of Impact of New Accounting Pronouncements (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Assets
Cash, cash equivalents, and restricted cash	$ 58,304	$ 70,698	$ 70,698
Short-term deposits and investments	24,839	25,940	25,940
Accounts receivable	52	63	63
Prepaid expenses and other current assets	1,789	2,210	1,979
Total current assets	84,984	98,911	98,680
Property and equipment, net	2,257	2,091	2,091
Restricted cash and other assets	2,696	2,696	329
Total assets	89,937	103,698	101,100
Current liabilities:
Accounts payable	1,789	1,606	1,606
Accrued expenses	9,062	8,580	8,580
Deferred revenue, current portion	959	787	787
Total current liabilities	11,810	10,973	10,973
Deferred rent and lease incentive	129	151	151
Loans payable, net of current portion	21,127	21,042	21,042
Deferred revenue, net of current portion	13,917	14,089	15,919
Other long‑term liabilities	1,372	1,201	1,201
Total liabilities	48,355	47,456	49,286
Stockholders’ equity:
Common stock	3	3	3
Additional paid-in capital	274,334	273,128	273,128
Accumulated deficit	(228,357)	(212,469)	(216,897)
Accumulated other comprehensive loss	(4,398)	(4,420)	(4,420)
Total stockholders’ equity	41,582	56,242	51,814
Total liabilities and stockholders’ equity	89,937	103,698	$ 101,100
Pro forma as if the previous accounting was in effect
Assets
Cash, cash equivalents, and restricted cash	58,304
Short-term deposits and investments	24,839
Accounts receivable	52
Prepaid expenses and other current assets	1,558
Total current assets	84,753
Property and equipment, net	2,257
Restricted cash and other assets	329
Total assets	87,339
Current liabilities:
Accounts payable	1,789
Accrued expenses	9,062
Deferred revenue, current portion	959
Total current liabilities	11,810
Deferred rent and lease incentive	129
Loans payable, net of current portion	21,127
Deferred revenue, net of current portion	15,747
Other long‑term liabilities	1,372
Total liabilities	50,185
Stockholders’ equity:
Common stock	3
Additional paid-in capital	274,334
Accumulated deficit	(232,785)
Accumulated other comprehensive loss	(4,398)
Total stockholders’ equity	37,154
Total liabilities and stockholders’ equity	$ 87,339
Adjustments due to ASU 2014-09 \| ASU 2014-09
Assets
Cash, cash equivalents, and restricted cash		0
Short-term deposits and investments		0
Accounts receivable		0
Prepaid expenses and other current assets		231
Total current assets		231
Property and equipment, net		0
Restricted cash and other assets		2,367
Total assets		2,598
Current liabilities:
Accounts payable		0
Accrued expenses		0
Deferred revenue, current portion		0
Total current liabilities		0
Deferred rent and lease incentive		0
Loans payable, net of current portion		0
Deferred revenue, net of current portion		(1,830)
Other long‑term liabilities		0
Total liabilities		(1,830)
Stockholders’ equity:
Common stock		0
Additional paid-in capital		0
Accumulated deficit		4,428
Accumulated other comprehensive loss		0
Total stockholders’ equity		4,428
Total liabilities and stockholders’ equity		$ 2,598

Revenue from Contracts with Customers - Transaction Price Allocated to Future Performance Obligation (Details) - USD ($) $ in Millions		3 Months Ended
	Dec. 02, 2016	Mar. 31, 2018
Revenue from Contract with Customer [Abstract]
Remaining performance obligation		$ 8.6
Remaining performance obligation, percentage		9.00%
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-04-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, period		12 months
Spark License Agreement
Revenue from Contract with Customer [Abstract]
Remaining performance obligation	$ 12.3	$ 2.3
Spark License Agreement \| License and Supply Obligation
Revenue from Contract with Customer [Abstract]
Remaining performance obligation	$ 7.1
Spark License Agreement \| License and Supply Obligation \| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2016-12-03
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, period	4 years

Revenue from Contracts with Customers - Schedule of Changes in Contract Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Jan. 01, 2018
Contract liabilities:
Deferred revenue, additions	$ 0
Other liabilities, additions	0
Contract liabilities, additions	0
Deferred revenue, deductions	0
Other liabilities, deductions	0
Contract liabilities, deductions	0
Deferred revenue, end of period	14,876
Other liabilities, end of period	2,231	$ 2,231
Contract liabilities, end of period	17,107
Other liabilities, current portion	800
Other liabilities, noncurrent portion	$ 1,400

Related-Party Transactions (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended
	Jun. 27, 2017	Mar. 31, 2018	Mar. 31, 2017
Related Party Transaction [Line Items]
Sale of stock, number of shares issued in transaction (in shares)		22,349,840
Series D Redeemable Convertible Preferred Stock
Related Party Transaction [Line Items]
Expiration period of warrants		10 years
Conversion of Series E Preferred
Related Party Transaction [Line Items]
Expiration period of warrants		10 years
Founders
Related Party Transaction [Line Items]
Expenses for consulting services (less than)		$ 0.1	$ 0.1
Springer Purchase Agreement
Related Party Transaction [Line Items]
Share price (in dollars per share)	$ 17.71
Springer Purchase Agreement \| Common stock
Related Party Transaction [Line Items]
Stock issued (in shares)	338,791
Stock warrants to purchase common stock
Related Party Transaction [Line Items]
Sale of stock, number of shares issued in transaction (in shares)	79,130
Exercise price (in dollars per share)	$ 17.71
Expiration period of warrants	5 years
Share price (in dollars per share)	$ 0.125

Technology License Agreements - MIT (Narrative) (Details) - MIT - USD ($)		3 Months Ended	12 Months Ended
	Nov. 25, 2008	Mar. 31, 2018	Dec. 31, 2017
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Payment amount upon achievement of defined clinical milestones	$ 1,500,000.0
Spark License Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contractual payments defined in the Exclusive Patent License agreement		$ 0	$ 2,200,000
License and option agreement, payments made relative to calculated premium paid for initial equity investment made under the purchase agreement			$ 400,000
Minimum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of sublicense income	10.00%
Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of sublicense income	30.00%

Technology License Agreements - Shenyang Sunshine Pharmaceutical Co., Ltd (Narrative) (Details) - 3SBio License - USD ($) $ in Millions	1 Months Ended
	May 31, 2014	Mar. 31, 2018	Dec. 31, 2016
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Period after approval to commercialize product	48 months
Aggregate amount of upfront and milestone-based payments		$ 1.0
Additional liability for milestone payments expensed in 2016			$ 2.0
Aggregate amount for future payments upon achievement of clinical and regulatory approval milestones for products containing SVP technology	$ 21.0
Aggregate amount for future payments upon achievement of clinical and regulatory approval milestones for products without SVP technology	$ 41.5
Maximum paid on tiered royalties (percent)	10.00%
Period for prior written notice to terminate license	60 days

Technology License Agreements - Massachusetts Eye and Ear Infirmary and The Schepens Eye Research Institute, Inc. (Narrative) (Details) - MEE	1 Months Ended	6 Months Ended
	May 31, 2016 USD ($) product	Jun. 30, 2016 USD ($)	Mar. 31, 2018 USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
License fees due under agreement		$ 100,000
Accrued License Fees			$ 0
Minimum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Minimum number of products for each target sequence \| product	1
Payment under development milestone method	$ 4,200,000
Payment under sales milestone method	50,000,000
Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Payment under development milestone method	37,000,000
Payment under sales milestone method	$ 70,000,000

Income Taxes (Details) - USD ($)	3 Months Ended		12 Months Ended
Income Taxes (Details) - USD ($)	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2014
Income Tax Holiday [Line Items]
Current income tax expense or benefit	$ 0	$ 0
Deferred income tax expense or benefit	0	$ 0
Tax Cuts and Jobs Act, reduction of deferred tax assets	23,400,000
Full valuation allowance	23,400,000
Unrecognized tax benefits	0
Related interest and penalties accrued	0
Accrued interest related to uncertain tax positions	$ 0
Russia \| Russian Federal Tax Service
Income Tax Holiday [Line Items]
Term of tax holiday granted to Russian subsidiary			10 years

Defined Contribution Plan (Details) - USD ($) $ in Millions	3 Months Ended
Defined Contribution Plan (Details) - USD ($) $ in Millions	Mar. 31, 2018	Mar. 31, 2017
Retirement Benefits [Abstract]
Vesting period	4 years
Employer contribution made	$ 0.1	$ 0.1