Voyager Therapeutics (VYGR) 10-K 26 Feb 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Feb. 22, 2019	Jun. 29, 2018
Document and Entity Information
Entity Registrant Name	Voyager Therapeutics, Inc.
Entity Central Index Key	1,640,266
Document Type	10-K
Document Period End Date	Dec. 31, 2018
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Well-known Seasoned Issuer	No
Entity Filer Category	Accelerated Filer
Entity Public Float			$ 442
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		32,616,999
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	FY

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 46,859	$ 31,530
Marketable securities, current	108,947	137,522
Prepaid expenses and other current assets	6,675	2,738
Total current assets	162,481	171,790
Property, Plant and Equipment, Net	12,771	10,283
Deposits and other non-current assets	1,149	1,304
Marketable securities, non-current	628	1,100
Total assets	177,029	184,477
Current liabilities:
Accounts payable	1,038	1,020
Accrued expenses	9,788	11,497
Deferred revenue, current portion	20,847	3,380
Total current liabilities	31,673	15,897
Deferred rent	5,710	5,337
Deferred revenue, net of current portion	92,199	28,180
Other non-current liabilities	1,001	1,012
Total liabilities	130,583	50,426
Commitments and contingencies (see note 7)
Stockholders’ equity:
Preferred stock, $0.001 par value: 5,000,000 shares authorized; no shares issued and outstanding at December 31, 2018 and 2017
Common stock, $0.001 par value: 120,000,000 shares authorized; 32,364,895 and 31,572,044 shares issued and outstanding at December 31, 2018 and 2017, respectively	32	32
Additional paid-in capital	315,598	295,019
Accumulated other comprehensive loss	(133)	(287)
Accumulated deficit	(269,051)	(160,713)
Total stockholders’ equity	46,446	134,051
Total liabilities and stockholders’ equity	$ 177,029	$ 184,477

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Consolidated Balance Sheets
Preferred Stock, par value (in dollars per share)	$ 0.001	$ 0.001
Preferred Stock, Shares Authorized	5,000,000	5,000,000
Preferred Stock, Shares Issued	0	0
Preferred Stock, Shares Outstanding	0	0
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common Stock, Shares Authorized	120,000,000	120,000,000
Common Stock, Shares, Issued	32,364,895	31,572,044
Common Stock, Shares, Outstanding	32,364,895	31,572,044

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Consolidated Statements of Operations and Comprehensive Loss
Collaboration revenue	$ 7,619	$ 10,135	$ 14,220
Operating expenses:
Research and development	64,905	62,260	42,249
General and administrative	33,809	19,738	13,270
Total operating expenses	98,714	81,998	55,519
Operating loss	(91,095)	(71,863)	(41,299)
Other income (expense), net
Interest income, net	3,310	1,227	976
Other (expense) income, net	(683)	(62)	182
Total other income (expense), net	2,627	1,165	1,158
Loss before income taxes	(88,468)	(70,698)	(40,141)
Income tax provision	180		(52)
Net loss	(88,288)	(70,698)	(40,193)
Other comprehensive (loss) income
Net unrealized gain (loss) on available-for-sale-securities, net of tax expense of $128 for the year ended December 31, 2016	34	(235)	199
Total other comprehensive income (loss)	34	(235)	199
Comprehensive loss	$ (88,254)	$ (70,933)	$ (39,994)
Net loss per share, basic and diluted	$ (2.75)	$ (2.64)	$ (1.59)
Weighted-average common shares outstanding, basic and diluted	32,065,781	26,803,711	25,302,414

Consolidated Statements of Op_2

Consolidated Statements of Operations and Comprehensive Loss (Parenthetical) $ in Thousands	12 Months Ended
	Dec. 31, 2016USD ($)
Consolidated Statements of Operations and Comprehensive Loss
Tax expense	$ 128

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit	Total
Stockholders' Equity, Beginning Balance at Dec. 31, 2015	$ 25	$ 219,122	$ (251)	$ (49,822)	$ 169,074
Shares, Outstanding, Beginning Balance at Dec. 31, 2015	24,930,979
Vesting of restricted stock	$ 1	17			18
Vesting of restricted stock (in shares)	601,501
Exercises of vested stock options		514			514
Exercises of vested stock options (in shares)	65,432
Stock based compensation expense		6,310			6,310
Unrealized gain (loss) on available for-sale securities, net of tax			199		199
Net loss				(40,193)	(40,193)
Stockholders' Equity, Ending Balance at Dec. 31, 2016	$ 26	225,963	(52)	(90,015)	135,922
Shares, Outstanding, Ending Balance at Dec. 31, 2016	25,597,912
Vesting of restricted stock	$ 1	12			13
Vesting of restricted stock (in shares)	573,803
Exercises of vested stock options		1,363			1,363
Exercises of vested stock options (in shares)	158,677
Issuance of common stock under ESPP		563			563
Issuance of common stock under ESPP (in shares)	66,652
Issuance of common stock from public offering (net of underwriters discounts and issuance costs of $4,100)	$ 5	57,989			57,994
Issuance of common stock from public offering (net of underwriters discounts and issuance costs of $4,100) (in shares)	5,175,000
Stock based compensation expense		9,129			9,129
Unrealized gain (loss) on available for-sale securities, net of tax			(235)		(235)
Net loss				(70,698)	(70,698)
Stockholders' Equity, Ending Balance at Dec. 31, 2017	$ 32	295,019	(287)	(160,713)	134,051
Shares, Outstanding, Ending Balance at Dec. 31, 2017	31,572,044
Vesting of restricted stock		9			9
Vesting of restricted stock (in shares)	319,891
Exercises of vested stock options		3,891			$ 3,891
Exercises of vested stock options (in shares)	384,186				384,186
Issuance of common stock under ESPP		969			$ 969
Issuance of common stock under ESPP (in shares)	88,774
Stock based compensation expense		15,710			15,710
Unrealized gain (loss) on available for-sale securities, net of tax			34		34
Impact of adopting new accounting standards | ASU 2016-01			120	(120)
Impact of adopting new accounting standards | ASU 2014-09				(19,930)	(19,930)
Impact of adopting new accounting standards					20,050
Net loss				(88,288)	(88,288)
Stockholders' Equity, Ending Balance at Dec. 31, 2018	$ 32	$ 315,598	$ (133)	$ (269,051)	$ 46,446
Shares, Outstanding, Ending Balance at Dec. 31, 2018	32,364,895

Consolidated Statements of Equi

Consolidated Statements of Equity (Parenthetical) $ in Thousands	Dec. 31, 2017USD ($)
Common Stock
Discounts and issuance costs	$ 4,100

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flow from operating activities
Net loss	$ (88,288)	$ (70,698)	$ (40,193)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	15,710	9,238	6,310
Depreciation	2,117	1,595	612
Amortization of premiums and discounts on marketable securities	(2,163)	(24)	696
In-kind research and development expenses	176	113	1,182
Other non-cash items	859	46	709
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(3,937)	1,630	(847)
Other non-current assets	(180)		7
Deferred revenue	61,380	(10,135)	(14,582)
Accounts payable	(282)	470	(62)
Accrued expenses	(1,600)	4,900	2,636
Other non-current liabilities		1,000
Lease incentive benefit	321	515	1,050
Net cash used in operating activities	(15,887)	(61,350)	(42,482)
Cash flow from investing activities
Purchases of property and equipment	(4,305)	(3,985)	(5,029)
Purchases of marketable securities	(333,228)	(147,296)	(112,350)
Proceeds from maturities or sales of marketable securities	364,000	147,600	165,100
Net cash provided by (used in) investing activities	26,467	(3,681)	47,721
Cash flow from financing activities
Proceeds from the issuance of stock net of discount and issuance costs		57,994
Proceeds from the exercise of stock options	3,889	1,363	514
Proceeds from the purchase of common stock under ESPP	860	563
Net cash provided by financing activities	4,749	59,920	514
Net increase (decrease) in cash and cash equivalents	15,329	(5,111)	5,753
Cash, cash equivalents, and restricted cash beginning of period	32,265	37,376	31,623
Cash, cash equivalents, and restricted cash end of period	47,594	$ 32,265	37,376
Supplemental disclosure of cash and non-cash activities
Impact of adopting new accounting standards	20,050
Capital expenditures incurred but not yet paid	$ 300		$ 242

Nature of business

Nature of business	12 Months Ended
	Dec. 31, 2018
Nature of business
Nature of business	VOYAGER THERAPEUTICS INC. NOTES TO CONSOLIDATED FINANCIAL STATEMENTS 1. Nature of business Voyager Therapeutics, Inc. (the “Company”) is a clinical-stage gene therapy company focused on developing life-changing treatments for patients suffering from severe neurological diseases. The Company is focused on neurological diseases where it believes an adeno-associated virus (“AAV”) gene therapy approach that either increases or decreases the production of a specific protein can slow or reduce the symptoms experienced by patients, and therefore have a clinically meaningful impact. The Company has built a gene therapy platform that it believes positions itself to be a leading company at the intersection of AAV gene therapy and severe neurological disease. The Company’s gene therapy platform enables it to engineer, optimize, manufacture and deliver its AAV-based gene therapies that have the potential to provide durable efficacy following a single administration. Additionally, the Company is working to identify novel AAV capsids, which are the outer viral protein shells that enclose the genetic material of the virus payload. The Company’s team of experts in the fields of AAV gene therapy and neuroscience first identifies and selects severe neurological diseases that are well-suited for treatment using AAV gene therapy. The Company then engineers and optimizes AAV vectors for delivery of the virus payload to the targeted tissue or cells. The Company’s manufacturing process employs an established system that it believes will enable production of high quality AAV vectors at commercial-scale. I n addition to the Company’s capsid optimization efforts, it leverages novel delivery paradigms, established routes of administration, and advances in dosing techniques to optimize delivery of its AAV gene therapies to target tissues, regions and cell types that are critical to the disease of interest. The Company believes it can achieve this directly, with targeted infusions to discrete regions of the brain or spinal cord, or systemically, in conjunction with its novel capsids. The Company’s business strategy focuses on discovering, developing, manufacturing and commercializing its gene therapy programs. As part of this strategy, the Company has developed core competencies specific to AAV gene therapy development and manufacturing and is beginning to build its commercial infrastructure. This business strategy also includes business development activities that may include in-licensing activities or partnering certain programs in certain geographies with collaborators, as the Company has demonstrated through its collaboration with Sanofi Genzyme (the “ Sanofi Genzyme Collaboration”), its collaboration with AbbVie Biotechnology Ltd (the “ AbbVie Tau Collaboration”), its collaboration with Neurocrine Biosciences, Inc. (the “Neurocrine Collaboration”), and its collaboration with AbbVie Ireland Unlimited Company (the “AbbVie Alpha-Synuclein Collaboration”) . The Company is devoting substantially all of its efforts to product research and development, market development, and raising capital. The Company is subject to risks common to companies in the biotechnology and gene therapy industry, including but not limited to, the need to obtain sufficient capital to continue to fund its operations, risks of failure of preclinical studies and clinical trials, the need to obtain marketing approval for its product candidates, the need to successfully commercialize and gain market acceptance of its product candidates, dependence on key personnel, protection of proprietary technology, compliance with government regulations, development by competitors of technological innovations, and ability to transition from pilot-scale manufacturing to large-scale production of products. The Company has incurred annual net operating losses in every year since inception. As of December 31, 2018, the Company had an accumulated deficit of $269.1 million. The Company has not generated any product revenue and has financed its operations primarily through public offerings and private placements of its equity securities and funding from its collaborations with Sanofi Genzyme and AbbVie. Additionally, in January 2019, the Company entered into the Neurocrine Collaboration which is expected to commence in the first half of 2019. Under the Neurocrine Collaboration, the Company will receive an upfront payment of $165.0 million, including a $50.0 million investment in 4,179,728 shares of common stock. The effectiveness of this agreement is subject to certain conditions, including the expirations or termination of the applicable waiting period under the Hart-Scott-Rodino Antitrust Improvements Act of 1976, as amended, and other customary closing conditions . In February 2019, the Company announced the AbbVie Alpha-Synuclein Collaboration to develop and commercialize vectorized antibodies directed at pathological species of alpha-synuclein for the potential treatment of Parkinson’s disease and other diseases (synucleinopathies) characterized by the abnormal accumulation of misfolded alpha-synuclein protein. Under the terms of the AbbVie Alpha-Synuclein Collaboration, the Company will receive an upfront payment of $65.0 million. Based upon the current operating plan, the Company expects that its existing cash, cash equivalents, and marketable debt securities, as well as amounts expected from the upfront payment and expected reimbursement of development costs from the Neurocrine Collaboration Agreement and the upfront payment from the AbbVie Alpha-Synuclein Collaboration Agreement entered into in 2019, will enable the Company to fund its operating expenses and capital expenditure requirements into mid-2022 . There can be no assurance that the Company will be able to obtain additional debt or equity financing or generate product revenue or revenue from collaborative partners on terms acceptable to the Company, on a timely basis or at all. The failure of the Company to obtain sufficient funds on acceptable terms when needed could have a material adverse effect on the Company’s business, results of operations, and financial condition.

Summary of significant accounti

Summary of significant accounting policies	12 Months Ended
	Dec. 31, 2018
Summary of significant accounting policies
Summary of significant accounting policies	2. Summary of significant accounting policies The following is a summary of significant accounting policies followed in the preparation of these financial statements. Basis of presentation The accompanying consolidated financial statements include those of the Company and its subsidiary, Voyager Securities Corporation, after elimination of all intercompany accounts and transactions. The accompanying consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). Public offerings On October 29, 2015, in preparation for the Company’s IPO, the Company’s Board of Directors and stockholders approved a 1-for-4.25 reverse split of the Company’s common stock, which became effective on October 29, 2015. All share and per share amounts in the consolidated financial statements and notes thereto have been retroactively adjusted for all periods presented to give effect to this reverse split, including reclassifying an amount equal to the reduction in par value of common stock to additional paid-in capital. On November 16, 2015, the Company completed the sale of 5,750,000 shares of its common stock in its initial public offering (the “IPO”), at a price to the public of $14.00 per share, resulting in net proceeds to the Company of $72.9 million after deducting underwriting discounts, commissions and offering expenses payable by the Company. On November 7, 2017, the Company completed the sale of 5,175,000 shares of its common stock in a public offering at a price to the public of $12.00 per share, resulting in net proceeds to the Company of $58.0 million after deducting underwriting discounts, commissions, and offering expenses payable by the Company. Use of Estimates The preparation of consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. On an ongoing basis, the Company’s management evaluates its estimates, which include, but are not limited to, estimates related to revenue recognition, accrued expenses, stock‑based compensation expense, and income taxes. The Company bases its estimates on historical experience and other market specific or other relevant assumptions that it believes to be reasonable under the circumstances. Actual results may differ from those estimates or assumptions. Fair Value of Financial Instruments ASC Topic 820, Fair Value Measurement (“ASC 820”), establishes a fair value hierarchy for instruments measured at fair value that distinguishes between assumptions based on market data (observable inputs) and the Company’s own assumptions (unobservable inputs). Observable inputs are inputs that market participants would use in pricing the asset or liability based on market data obtained from sources independent of the Company. Unobservable inputs are inputs that reflect the Company’s assumptions about the inputs that market participants would use in pricing the asset or liability, and are developed based on the best information available in the circumstances. ASC 820 identifies fair value as the exchange price, or exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As a basis for considering market participant assumptions in fair value measurements, ASC 820 establishes a three‑tier fair value hierarchy that distinguishes between the following: · Level 1 —Quoted market prices in active markets for identical assets or liabilities. · Level 2 —Inputs other than Level 1 inputs that are either directly or indirectly observable, such as quoted market prices, interest rates, and yield curves. · Level 3 —Unobservable inputs developed using estimates of assumptions developed by the Company, which reflect those that a market participant would use. To the extent that the 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 carrying amounts reflected in the balance sheets for cash and cash equivalents, prepaid expenses and other current assets, accounts payable and accrued expenses approximate their fair values, due to their short‑term nature. Cash and Cash Equivalents The Company considers all highly liquid investments purchased with original maturities of 90 days or less at acquisition to be cash equivalents. Cash and cash equivalents include cash held in banks and amounts held in money market funds. Marketable Securities The Company classifies marketable debt securities with a remaining maturity of greater than three months when purchased as available‑for‑sale. Marketable debt securities with a remaining maturity date greater than one year and marketable equity securities are classified as non‑current where the Company has the intent and ability to hold these securities for at least the next 12 months. During 2016, the Company invested in a supplier and received common stock and warrants to purchase common stock in that entity. The common stock is considered an available-for-sale marketable equity security and is included in non-current marketable securities, and the warrants are included in non-current assets. All available for sale debt securities are carried at fair value with the unrealized gains and losses included in other comprehensive income (loss) as a component of stockholders’ equity until realized. Any premium or discount arising at purchase is amortized and/or accreted to interest income and/or expense. Realized gains and losses are determined using the specific identification method and are included in other income (expense). If any adjustment to fair value reflects a decline in value of the investment, the Company considers all available evidence to evaluate the extent to which the decline is “other than temporary” and, if so, recognizes the loss through a charge to the Company’s statement of operations and comprehensive loss. No other than temporary losses have been recognized. Cash, cash equivalents, and marketable securities as of December 31, 2018 and 2017 consist of the following: Amortized Unrealized Unrealized Fair Cost Gains Losses Value (in thousands) As of December 31, 2018 Money market funds included in cash and cash equivalents $ 46,173 $ — $ — $ 46,173 Marketable securities: U.S. Treasury notes 108,951 1 5 108,947 Equity securities 1,220 — 592 628 Total marketable securities $ 110,171 $ 1 $ 597 $ 109,575 Total money market funds and marketable securities $ 156,344 $ 1 $ 597 $ 155,748 As of December 31, 2017 Money market funds included in cash and cash equivalents $ 30,469 $ — $ — $ 30,469 Marketable securities: U.S. Treasury notes 137,560 — 38 137,522 Equity securities 1,220 — 120 1,100 Total marketable securities $ 138,780 $ — $ 158 $ 138,622 Total money market funds and marketable securities $ 169,249 $ — $ 158 $ 169,091 All of the Company’s marketable debt securities at December 31, 2018 and 2017 have a contractual maturity of one year or less. Restricted Cash At December 31, 2018 and 2017, the Company maintained restricted cash totaling approximately $0.7 million held in the form of money market accounts as collateral for the Company’s facility lease obligation. The balance is included within deposits in other non‑current assets in the accompanying consolidated balance sheets. The following table provides a reconciliation of cash, cash equivalents, and restricted cash within the condensed consolidated balance sheet that sum to the total of the same such amounts shown in the statement of cash flows: As of December 31, 2018 2017 2016 (in thousands) Cash and cash equivalents $ 46,859 $ 31,530 $ 36,641 Restricted cash included in deposits and other noncurrent assets 735 735 735 Total cash, cash equivalents, and restricted cash $ 47,594 $ 32,265 $ 37,376 Property and Equipment Property and equipment consists of laboratory equipment, furniture and office equipment, and leasehold improvements and is stated at cost, less accumulated depreciation. Maintenance and repairs that do not improve or extend the lives of the respective assets are expensed to operations as incurred; while costs of major additions and betterments are capitalized. Depreciation is calculated over the estimated useful lives of the assets using the straight‑line method. Impairment of Long‑Lived Assets The Company evaluates long‑lived assets for potential impairment when events or changes in circumstances indicate the carrying value of the assets may not be recoverable. Recoverability is measured by comparing the book values of the assets to the expected future net undiscounted cash flows that the assets are expected to generate. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the book values of the assets exceed their fair value. The Company has not recognized any impairment losses from inception through December 31, 2018. Revenue Recognition As of December 31, 2018, all of the Company’s revenue is generated from its collaboration agreements with Sanofi Genzyme Corporation, a Sanofi company (“S anofi Genzyme”), and AbbVie Biotechnology Ltd. and its affiliates collectively, (“AbbVie”) . The Company enters into collaboration agreements which are within the scope of ASC 606, Revenue from Contracts with Customers (“ASC 606”), under which the Company licenses rights to certain of the Company’s product candidates and performs research and development services. The terms of these arrangements typically include payment of one or more of the following: non-refundable, upfront fees; reimbursement of research and development costs; development, regulatory, and commercial milestone payments; and royalties on net sales of licensed products. Under ASC 606, an entity recognizes revenue when its 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 the appropriate amount of revenue to be recognized for arrangements determined to be within the scope of ASC 606, the Company performs the following five steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of 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 consideration it is entitled to in exchange for the goods or services it transfers to the customer. The promised goods or services in the Company’s arrangements typically consist of license rights to the Company’s intellectual property and research and development services. The Company provides options to additional items in the contracts, which are accounted for as separate contracts when the customer elects to exercise such options, unless the option provides a material right to the customer. The Company evaluates the customer options for material rights, or options to acquire additional goods or services for free or at a discount. If the customer options are determined to represent a material right, the material right is recognized as a separate performance obligation at the outset of the arrangement. Performance obligations are promised goods or services in a contract to transfer a distinct good or service to the customer and are considered distinct when (i) the customer can benefit from the good or service on its own or together with other readily available resources and (ii) the promised good or service is separately identifiable from other promises in the contract. In assessing whether promised goods or services are distinct, the Company considers factors such as the stage of development of the underlying intellectual property, the capabilities of the customer to develop the intellectual property on its own or whether the required expertise is readily available and whether the goods or services are integral or dependent to other goods or services in the contract. The Company estimates the transaction price based on the amount expected to be received for transferring the promised goods or services in the contract. The consideration may include fixed consideration or variable consideration. At the inception of each arrangement that includes variable consideration, the Company evaluates the amount of potential payments and the likelihood that the payments will be received. The Company utilizes either the most likely amount method or expected amount method to estimate the amount expected to be received based on which method best predicts the amount expected to be received. The amount of variable consideration which is included in the transaction price may be constrained, and is included in the transaction price only to the extent that it is probable that a significant reversal in the amount of the cumulative revenue recognized will not occur in a future period. The Company’s contracts often include development and regulatory milestone payments which are assessed under the most likely amount method and constrained if it is probable that a significant revenue reversal would occur. Milestone payments that are not within the Company’s control or the licensee’s control, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. At the end of each 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 overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect collaboration revenues in the period of adjustment. To date, the Company has not recognized any consideration related to the achievement of development, regulatory, or commercial milestone revenue resulting from any of the Company’s collaboration arrangements. 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 recognizes 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). To date, the Company has not recognized any consideration related to sales-based royalty revenue resulting from any of the Company’s collaboration arrangements. The Company allocates the transaction price based on the estimated stand-alone selling price of each of the performance obligations. 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 utilizes key assumptions to determine the stand-alone selling price for service obligations, which may include other comparable transactions, pricing considered in negotiating the transaction and the estimated costs. Additionally, in determining the standalone selling price for material rights, the Company utilizes comparable transactions, clinical trial success probabilities, and estimates of option exercise likelihood. Variable consideration is allocated specifically to one or more performance obligations in a contract when the terms of the variable consideration relate to the satisfaction of the performance obligation and the resulting amounts allocated are consistent with the amounts the Company would expect to receive for the satisfaction of each performance obligation. The consideration allocated to each performance obligation is recognized as revenue when control is transferred for the related goods or services. For performance obligations which consist of licenses and other promises, the Company utilizes judgment to assess 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. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. Upfront payments and fees are recorded as deferred revenue upon receipt or when due until the Company performs its obligations under these arrangements. Amounts are recorded as accounts receivable when the Company’s right to consideration is unconditional Research and Development Research and development costs are charged to expense as incurred in performing research and development activities. The costs include employee compensation costs, external research, consultant costs, sponsored research, in‑kind services provided under the Sanofi Genzyme agreement, license fees, process development and facilities costs. Facilities costs primarily include the allocation of rent, utilities and depreciation. Research Contract Costs and Accruals The Company has entered into various research and development contracts with research institutions and other companies. These agreements are generally cancelable. The Company records accruals for estimated ongoing research costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates may be made in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs. Patent Costs The Company expenses patent application and related legal costs as incurred and classifies such costs as general and administrative expenses in the accompanying statements of operations. Stock‑Based Compensation Expense The Company accounts for its stock‑based compensation awards in accordance with ASC Topic 718 Compensation—Stock Compensation (“ASC 718”). ASC 718 requires all stock‑based payments to employees and directors, including grants of restricted stock and stock options, to be recognized as expense in the statements of operations based on their grant date fair values. Grants of restricted stock and stock options to other service providers, referred to as non‑employees, are required to be recognized as expense in the statements of operations based on their vesting date fair values. The Company estimates the fair value of options granted using the Black‑Scholes option pricing model. The Company uses the fair value of its common stock to determine the fair value of restricted stock awards. The Black‑Scholes option pricing model requires inputs based on certain subjective assumptions, including (a) the expected stock price volatility, (b) the calculation of expected term of the award, (c) the risk‑free interest rate and (d) expected dividends. Due to a lack of company-specific historical and implied volatility data, the Company bases the estimate of expected volatility on the historical volatility of a group of similar companies that are publicly traded, blended with the most recent period of historic volatility of its common stock. The historical volatility is calculated based on a period of time commensurate with the expected term assumption. The computation of expected volatility is based on the historical volatility of a representative group of companies with similar characteristics to the Company, including stage of product development and life science industry focus. The Company uses the simplified method as prescribed by the SEC Staff Accounting Bulletin No. 107, Share‑Based Payment , to calculate the expected term for stock options granted to employees as it does not have sufficient historical exercise data to provide a reasonable basis upon which to estimate the expected term. For stock options granted to non‑employees, the Company utilizes the contractual term of the arrangement as the basis for the expected term assumption. The risk‑free interest rate is based on a treasury instrument whose term is consistent with the expected term of the stock options. The expected dividend yield is assumed to be zero as the Company has never paid dividends and has no current plans to pay any dividends on its common stock. The Company expenses the fair value of its stock‑based compensation awards to employees on a straight‑line basis over the associated service period, which is generally the period in which the related services are received. Stock‑based compensation awards to non‑employees are adjusted through stock‑based compensation expense at each reporting period end to reflect the current fair value of such awards and are expensed on a straight‑line basis. The Company records the expense for stock‑based compensation awards subject to performance conditions over the remaining service period when management determines that achievement of the performance condition is probable. Management evaluates when the achievement of a performance condition is probable based on the expected satisfaction of the performance conditions as of the reporting date. Income Taxes Income taxes are recorded in accordance with ASC Topic 740, Income Taxes (“ASC 740”), which provides for deferred taxes using an asset and liability approach. Under this method, deferred tax assets and liabilities are determined based on the difference between the financial reporting and the tax reporting basis of assets and liabilities and are measured using the enacted tax rates and laws that are expected to be in effect when the differences are expected to reverse. The Company provides a valuation allowance against net deferred tax assets unless, based upon the weight of available evidence, it is more likely than not that the deferred tax assets will be realized. The Company accounts for uncertain tax positions in accordance with the provisions of ASC 740. When uncertain tax positions exist, the Company recognizes the tax benefit of tax positions to the extent that the benefit will more likely than not be realized. The determination as to whether the tax benefit will more likely than not be realized is based upon the technical merits of the tax position as well as consideration of the available facts and circumstances. As of December 31, 2018, the Company does not have any significant uncertain tax positions. Comprehensive Loss Comprehensive loss is comprised of net loss and other comprehensive income or loss. Other comprehensive income or loss consists of unrealized gains or losses on marketable securities. Net Loss Per Share Basic net loss per share is calculated by dividing the net loss by the weighted‑average number of shares of common stock outstanding during the period, without consideration for potentially dilutive securities. Diluted net loss per share is computed by dividing the net loss by the weighted‑average number of shares of common stock and potentially dilutive securities outstanding for the period determined using the treasury‑stock and if‑converted methods. For purposes of the diluted net loss per share, unvested restricted common stock and outstanding stock options are considered to be potentially dilutive securities, but are excluded from the calculation of diluted net loss per share because their effect would be anti‑dilutive and therefore, basic and diluted net loss per share were the same for all periods presented. The following table sets forth the outstanding potentially dilutive securities that have been excluded in the calculation of diluted net loss per share because to do so would be anti‑dilutive: As of December 31, 2018 2017 2016 Unvested restricted common stock 235,294 557,979 1,167,984 Outstanding stock options 4,225,152 3,143,566 1,871,237 Total 4,460,446 3,701,545 3,039,221 Concentrations of Credit Risk and Off‑Balance Sheet Risk The Company has no financial instruments with off‑balance sheet risk such as foreign exchange contracts, option contracts or other foreign currency hedging arrangements. Financial instruments that potentially subject the Company to a concentration of credit risk are cash and cash equivalents. The Company’s cash is held in accounts at a financial institution that may exceed federally insured limits. The Company has not experienced any credit losses in such accounts and does not believe it is exposed to any significant credit risk on these funds. Concentration of Suppliers The Company is dependent on a third‑party manufacturer to supply certain products for research and development activities in its programs. In particular, the Company relies on a sole manufacturer to supply it with specific vectors related to the Company’s research and development programs. Segment Information Operating segments are defined as components of an enterprise about which separate discrete information is available for evaluation by the chief operating decision maker in deciding how to allocate resources and assess performance. The Company and the Company’s chief operating decision maker, the Company’s Chief Executive Officer, views the Company’s operations and manages its business as a single operating segment, which is the business of developing and commercializing gene therapies. Recently Adopted Accounting Pronouncements In January 2016, the Financial Instruments - Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities (“Update No. 2016-01”). The purpose of Update No. 2016-01 is to improve financial reporting for financial instruments by reducing the number of items recorded to other comprehensive income. The Company adopted Update No. 2016-01 in the first quarter of 2018, using the modified retrospective method. Unrealized gains and losses previously recorded to other comprehensive income (loss) were reclassified to accumulated deficit and all future fair value changes will be recorded to other income (loss). The adoption of the standard on January 1, 2018 did not have a material impact on the Company’s consolidated financial statements. In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (“ASU 2016-15”), which simplifies certain elements of cash flow classification and is intended to reduce diversity in practice in how certain transactions are classified in the statement of cash flows. The update is effective for fiscal years, and interim periods within those years, beginning after December 15, 2017. The Company adopted ASU 2016-15 on January 1, 2018, and such adoption did not have a material impact on the Company's consolidated financial statements. In November 2016, the FASB issued ASU No. 2016-18, Restricted Cash (“ASU 2016-18”). The amendments in ASU 2016-18 require an entity to reconcile and explain the period-over-period change in total cash, cash equivalents and restricted cash within its statements of cash flows and was adopted utilizing a full retrospective approach. The Company adopted the new standard on January 1, 2018. The Company has included the necessary reconciliation within Note 2 “Restricted Cash”. In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (“ASU 2014-09”), which supersedes the revenue recognition requirements in ASC 605, Revenue Recognition (“ASC 605”), and most industry-specific guidance. The new standard requires that an entity recognize revenue to depict the transfer of 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 update also requires additional disclosure about the nature, amount, timing, and uncertainty of revenue and cash flows arising from customer contracts, including significant judgments and changes in judgments and assets recognized from costs incurred to obtain or fulfill a contract. Thereafter, a series of clarifying ASUs, narrow scope improvements and practical expedients were issued. This collective guidance resulted in the new revenue standard, ASC 606. The Company adopted the new standard effective January 1, 2018 using the modified retrospective approach. The Company had one open contract, relating to the Sanofi Genzyme Collaboration, on the adoption date and has assessed it under the new revenue standard. The adoption of ASC 606 resulted in the changes to (i) the allocation of arrangement consideration, including the determination of estimated selling price and the allocation of variable consideration to specific performance obligations and (ii) the application of proportional performance as a measure of progress on service-related deliverables. The Company has accounted for the impact of adopting ASC 606 as a cumulative catch-up under the modified retrospective approach, which is represented as an increase of $20.0 million to deferred revenue with an offset to accumulated deficit, effective January 1, 2018. The following financial statement line items have been shown to reflect comparative balances under ASC 606 and ASC 605 for the year ended December 31, 2018, for both of the Sanofi Genzyme Collaboration and AbbVie Tau Collaboration, collectively. Condensed Consolidated Statements of Operations and Comprehensive Loss Year ended December 31, 2018 Under ASC 606 Under ASC 605 Effect of change (in thousands, except per share data) Collaboration revenue $ 7,619 $ 11,095 $ (3,476) Loss before income taxes (88,468) (84,992) (3,476) Net loss (88,288) (84,812) (3,476) Net loss per share, basic and diluted (2.75) (2.64) (0.11) Condensed Consolidated Balance Sheets As of December 31, 2018 Under ASC 606 Under ASC 605 Effect of change (in thousands) Deferred revenue, current $ 20,847 $ 19,111 $ 1,736 Deferred revenue, non-current 92,199 70,529 21,670 Accumulated deficit (269,051) (245,645) (23,406) Condensed Consolidated Statements of Cash Flows Year ended December 31, 2018 Under ASC 606 Under ASC 605 Effect of change (in thousands) Net loss $ (88,288) $ (84,812) $ (3,476) Adjustments to reconcile net loss to net cash used in operating activities: Deferred revenue 61,380 57,904 3,476 Recent Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, a comprehensive new lease accounting standard, which provides revised guidance on accounting for lease arrangements by both lessors and lessees and requires lessees to recognize a lease liability and a right-of-use asset for most leases. In July 2018, the FASB issued ASU No. 2018-11, Leases (Topic 842), Targeted Improvements , which provides an additional transition method that allows entities to initially apply the new lease requirements at the adoption date, not the earliest period presented, and recognize a cumulative effect adjustment to the opening balance of retained earnings in the period of adoption. The Company expects to elect this transition method at the adoption date of January 1, 2019. The Company also expects to elect a package of practical expedients, under which an entity need not reassess whether any expired or existing contracts are or contain leases, the lease classification for any expired or existing leases, or initial direct costs for any existing leases. The Company is currently in the process of evaluating the impact that the adoption of this guidance will have on its consolidated financial statements and related disclosures. On December 22, 2017, the SEC staff issued Staff Accounting Bulletin No. 118, to address the application of GAAP in situations when a registrant does not have the necessary information available, prepared, or analyzed (including computations) in reasonable detail to complete the accounting for certain income tax effects of the Tax Cuts and Jobs Act (the “Tax Reform Act”). The Company has recognized the provisional tax impacts related to the revaluation of the deferred tax assets and liabilities and included these amounts in its consolidated financial

Fair value measurements

Fair value measurements	12 Months Ended
	Dec. 31, 2018
Fair value measurements
Fair value measurements	3. Fair value measurements Assets and liabilities measured at fair value on a recurring basis as of December 31, 2018 and 2017 are as follows: Quoted Prices Significant in Active Other Significant Markets for Observable Unobservable Identical Assets Inputs Inputs Assets Total (Level 1) (Level 2) (Level 3) (in thousands) December 31, 2018 Money market funds included in cash and cash equivalents $ 46,173 $ 46,173 $ — $ — Marketable securities: U.S. Treasury notes 108,947 108,947 — — Equity securities 628 628 — — Total marketable securities $ 109,575 $ 109,575 $ — $ — Warrants to purchase equity securities 234 — 234 — Total $ 155,982 $ 155,748 $ 234 $ — December 31, 2017 Money market funds included in cash and cash equivalents $ 30,469 $ 30,469 $ — $ — Marketable securities: U.S. Treasury notes 137,522 137,522 — — Equity securities 1,100 1,100 — — Total marketable securities $ 138,622 $ 138,622 $ — $ — Warrants to purchase equity securities 569 — 569 — Total $ 169,660 $ 169,091 $ 569 $ — The Company measures the fair value of money market funds, U.S. Treasuries and equity securities based on quoted prices in active markets for identical securities. The Level 2 equity securities include warrants used to purchase equity securities that are valued using the Black-Scholes model. The Black-Scholes option pricing model requires inputs based on certain subjective assumptions, including (a) the expected stock price volatility, (b) the calculation of expected term of the awards, (c) the risk-free interest rate, and (d) expected dividends. The assumptions utilized to value the warrants to purchase equity securities as of December 31, 2018, 2017, and 2016 are as follows: As of December 31, 2018 2017 2016 Risk-free interest rate 2.5 % 2.0 % 1.8 % Expected dividend yield — % — % — % Expected term (in years) 2.7 3.7 4.7 Expected volatility 112.7 % 103.5 % 97.5 % The expected volatility is based on the historic volatility for the equity securities underlying the warrants and is calculated based on a period of time commensurate with the expected term assumption. The expected term is based on the remaining contractual life of the warrants on each measurement date. The risk-free interest rate is based on a treasury instrument whose term is consistent with the expected term of the warrants. The expected dividend yield is assumed to be zero as the entity that issued the warrants has never paid and has not indicated any intention to pay dividends.

Prepaid expenses and other curr

Prepaid expenses and other current assets	12 Months Ended
	Dec. 31, 2018
Prepaid expense and other current assets
Prepaid expenses and other current assets	4. Prepaid expenses and other current assets Prepaid expenses and other current assets consist of the following: As of December 31, 2018 2017 (in thousands) Prepaid research and development contracts $ 4,497 $ 1,330 Other current assets 1,360 766 Prepaid insurance 617 520 Accrued interest receivable 201 122 Total $ 6,675 $ 2,738

Property and equipment, net

Property and equipment, net	12 Months Ended
	Dec. 31, 2018
Property and equipment, net
Property and equipment, net	5. Property and equipment, net Property and equipment, net consists of the following: As of December 31, 2018 2017 (in thousands) Leasehold improvements $ 7,035 $ 6,421 Laboratory equipment 8,843 5,262 Furniture and office equipment 1,675 1,565 Other 306 25 Construction in progress 19 — Total property and equipment 17,878 13,273 Less: accumulated depreciation (5,107) (2,990) Property and equipment, net $ 12,771 $ 10,283 The Company recorded $2.1 million, $1.6 million, and $0.6 million in depreciation expense during the years ended December 31, 2018, 2017, and 2016, respectively.

Accrued expenses

Accrued expenses	12 Months Ended
	Dec. 31, 2018
Accrued expenses
Accrued expenses	6. Accrued expenses Accrued expenses consist of the following: As of December 31, 2018 2017 (in thousands) Employee compensation costs $ 3,780 $ 3,383 Research and development costs 3,555 5,780 Professional services 1,448 1,762 Accrued goods and services 784 388 Patent costs 120 120 Other 101 64 Total $ 9,788 $ 11,497

Commitments and contingencies

Commitments and contingencies	12 Months Ended
	Dec. 31, 2018
Commitments and contingencies
Commitments and contingencies	7. Commitments and contingencies Operating Leases During April 2014, the Company entered into an agreement to lease the 75 Sidney Street facility under a non‑cancelable operating lease that would have expired December 15, 2019. The lease includes two renewal options, each for five-year terms and at fair market value upon exercise. The lease contains escalating rent clauses which require higher rent payments in future years. The Company expenses rent on a straight‑line basis over the term of the lease, including any rent‑free periods. In December 2015, the Company executed an amendment to the 75 Sidney Street lease to extend its term, and executed an agreement to lease a facility at 64 Sidney Street until December 31, 2024. The facility at 64 Sidney Street includes laboratory and office space, and was ready for occupancy in early 2017. In February 2018, the Company executed a second amendment to the 75 Sidney Street lease to lease additional space to support its continued growth. The additional facility includes laboratory and office space, and was ready for occupancy in mid-2018. In June 2018, the Company executed a third amendment to the 75 Sidney Street lease to lease additional space to further support its continued growth. The additional facility includes laboratory and office space, and was ready for occupancy in late 2018. The third amendment extended the term of the 75 Sidney Street lease until November 30, 2026. Additionally, the Company executed a second amendment to the 64 Sidney Street lease to extend that lease until November 30, 2026. The Company received leasehold improvement incentives from the landlord totaling $1.3 million and $3.5 million for 75 Sidney Street and 64 Sidney Street, respectively. The Company recorded these incentives as a component of deferred rent and is amortizing these incentives as a reduction of rent expense over the life of the lease. The leasehold improvements have been recorded as fixed assets. The Company is entitled to receive approximately $0.3 million of leasehold improvements for the additional space at 75 Sidney Street. Rent expense of approximately $4.0 million, $2.9 million, and $2.0 million was incurred during the years ended December 31, 2018, 2017, and 2016, respectively. Future annual minimum lease payments at December 31, 2018 are as follows: Total Minimum Lease Payments (in thousands) 2019 4,325 2020 4,731 2021 4,863 2022 5,001 2023 5,141 2024+ 16,783 $ 40,844 . Significant Agreements Sanofi Genzyme Collaboration Agreement Summary of Agreement In February 2015, the Company entered into an agreement with Sanofi Genzyme (the “Sanofi Genzyme Collaboration Agreement”) which included a non‑refundable upfront payment of $65.0 million. In addition, contemporaneous with entering into the Sanofi Genzyme Collaboration Agreement, Sanofi Genzyme entered into a Series B Stock Purchase Agreement, under which Sanofi Genzyme purchased 10,000,000 shares of Series B Preferred Stock for $30.0 million. The fair value of the Series B Preferred Stock at the time of issuance was approximately $25.0 million. The $5.0 million premium over the fair value is accounted for as additional consideration under the Sanofi Genzyme Collaboration Agreement. Under the Sanofi Genzyme Collaboration Agreement, the Company granted Sanofi Genzyme an exclusive option to license, develop and commercialize (i) ex‑U.S. rights to the following programs, which are referred to as Split Territory Programs; VY‑AADC (“Parkinson’s Program”), VY‑FXN01 (“Friedreich’s ataxia Program”), a future program to be designated by Sanofi Genzyme (“ Future Program), and VY‑HTT01 (“Huntington’s Program”), with an incremental option to co‑commercialize VY‑HTT01 in the United States and (ii) worldwide rights to VY‑SMN101 (“Spinal Muscular Atrophy Program”). Sanofi Genzyme ’s option for the Split Territory Programs and the Spinal Muscular Atrophy Program is triggered following the completion of the first proof‑of‑principle human clinical study (“POP Study”), on a program by program basis. Prior to any option exercise by Sanofi Genzyme , the Company will collaborate with Sanofi Genzyme in the development of products under each Split Territory Program and the Spinal Muscular Atrophy Program pursuant to a written development plan and under the guidance of an Alliance Joint Steering Committee (“AJSC”), comprised of an equal number of employees from the Company and Sanofi Genzyme . The Company is required to use commercially reasonable efforts to develop products under each Split Territory Program and the Spinal Muscular Atrophy Program through the completion of the applicable POP Study. During the development of these joint programs, the activities are guided by a Development Advisory Committee (“DAC”). The DAC may elect to utilize certain Sanofi Genzyme technology relating to the Parkinson’s Program, the Huntington’s Program or generally with the manufacture of Split Territory Program products. The Company is solely responsible for all costs incurred in connection with the development of the Split Territory Programs and the Spinal Muscular Atrophy Program products prior to the exercise of an option by Sanofi Genzyme with the exception of the following: (i) at the Company’s request and upon mutual agreement, Sanofi Genzyme will provide “in‑kind” services valued at up to $5.0 million and (ii) Sanofi Genzyme shall be responsible for the costs and expenses of activities under the Huntington’s Program development plan to the extent such activities are covered by financial support Sanofi Genzyme is entitled to receive from a patient advocacy group, collectively Sanofi Genzyme “in‑kind” and other funding. Other than the Parkinson’s Program (for which a POP Study has already been completed), if the Company does not initiate a POP Study for a given Split Territory Program by December 31, 2026 (or for the Future Program by the tenth anniversary of the date the Future Program is nominated by Sanofi Genzyme ), and Sanofi Genzyme has not terminated the Sanofi Genzyme Collaboration Agreement with respect to the collaboration program, then Sanofi Genzyme shall be entitled, as its sole and exclusive remedy, to a credit of $10.0 million for each such program against other milestone or royalty payments payable by Sanofi Genzyme under the Sanofi Genzyme Collaboration Agreement. However, if the POP Study is not initiated due to a regulatory delay or a force majeure event, such time period shall be extended for so long as such delay continues. With the exception of the Parkinson’s Program, Sanofi Genzyme is required to pay an option exercise payment of $20.0 million or $30.0 million for each Split Territory Program, as well as the Spinal Muscular Atrophy Program. Upon Sanofi Genzyme ’s exercise of its option to license a given product in a Split Territory Program (“Split Territory Licensed Product”), the Company will have sole responsibility for the development of such Split Territory Licensed Product in the United States and Sanofi Genzyme shall have sole responsibility for development of such Split Territory Licensed Product in the rest of the world. The Company and Sanofi Genzyme will have shared responsibility for execution of ongoing development of such Split Territory Licensed Product that is not specific to either territory, including costs associated therewith. The Company is responsible for all commercialization activities relating to Split Territory Licensed Products in the United States, including all of the associated costs. Sanofi Genzyme is responsible for all commercialization activities relating to the Split Territory Licensed Products in the rest of the world, including all of the associated costs. If Sanofi Genzyme exercised its co‑commercialization rights, Sanofi Genzyme will be the lead party responsible for all commercialization activities related to the Huntington’s Licensed Product in the United States. Upon exercise of the option, Sanofi Genzyme shall have the sole right to develop the Spinal Muscular Atrophy Product worldwide. Sanofi Genzyme shall be responsible for all of the development costs that occur after the option exercise date for the Spinal Muscular Atrophy Program. Sanofi Genzyme is also responsible for commercialization activities relating to the Spinal Muscular Atrophy Product worldwide. Sanofi Genzyme is required to pay the Company for specified regulatory and commercial milestones, if achieved, up to $540.0 million across all programs. The Company is no longer entitled to receive a total of $105.0 million related to regulatory and commercial milestone payments for VY-AADC as a result of Sanofi Genzyme’s decision to not exercise its option for the Parkinson’s Program (the “PD Opt-Out”). The regulatory approval milestones are payable upon either regulatory approval in the United States or regulatory and reimbursement approval in the European Union and range from $40.0 million to $50.0 million per milestone, with an aggregate total of $220.0 million, after accounting for the PD Opt-Out. The commercial milestones are payable upon achievement of specified annual net sales in each program and range from $50.0 million to $100.0 million per milestone, with an aggregate total of $320.0 million, after accounting for the PD Opt-Out . In addition, to the extent any Split Territory Licensed Products or the Spinal Muscular Atrophy Licensed Product are commercialized, the Company is entitled to tiered royalty payments ranging from the mid‑single digits to mid‑teens based on a percentage of net sales by Sanofi Genzyme . Sanofi Genzyme is entitled to receive tiered royalty payments related to sales of the Split Territory Licensed Product ranging from the low‑single digits to mid‑single digits based on a percentage of net sales by the Company depending on whether the Company uses Sanofi Genzyme technology in the Split Territory Licensed Product. If Sanofi Genzyme elects to co‑commercialize VY‑HTT01 in the United States, the Company and Sanofi Genzyme will share in any profits or losses from VY‑HTT01 product sales. The Sanofi Genzyme Collaboration Agreement will continue in effect until the later of (i) the expiration of the last to expire of the option rights and (ii) the expiration of all payment obligations unless sooner terminated by the Company or Sanofi Genzyme . The Company and Sanofi Genzyme have customary termination rights including the right to terminate for an uncured material breach of the agreement committed by the other party and Sanofi Genzyme has the right to terminate for convenience. Accounting Analysis At inception, the Sanofi Genzyme Collaboration Agreement included the following performance obligations: (i) research and development services for each of the Split Territory Programs and the Spinal Muscular Atrophy Program, (ii) participation in the AJSC, (iii) participation in the DAC and (iv) a material right associated with an option to obtain a development and commercial license in the Parkinson’s Program (“PD Material Right”). The Company determined that the option to obtain a development and commercial license in the Parkinson’s Program was a material right under ASC 606 primarily because there were no additional option exercise payments payable by Sanofi Genzyme at the time of option exercise. Therefore, the PD Material Right was considered a performance obligation at the inception of the arrangement. The options in the other Split Territory Programs and the Spinal Muscular Atrophy Program do not provide a material right to the customer that it would receive without entering into the contract principally because the option fees are at least equal to the standalone selling price for the underlying goods. Therefore, the other Split Territory Programs and the Spinal Muscular Atrophy Program options are not performance obligations at inception. The Company has identified $74.6 million of total transaction price consisting of the $65.0 million upfront fee, the $5.0 million premium paid in excess of fair value of the Series B Preferred Stock and $4.6 million of Sanofi Genzyme “in‑kind” funding, which represents the transaction price at adoption. Additional consideration to be paid to the Company upon the exercise of the license options by Sanofi Genzyme or upon reaching certain milestones are excluded from the transaction price as they relate to option fees and milestones that can only be achieved subsequent to the option exercise or are outside of the initial contract term. The Company has allocated the transaction price to the separate performance obligations based on their relative standalone selling price. For all performance obligations, the Company determined the standalone selling price at contract inception based on each obligation’s estimated standalone selling price (“ESP”). The Company determined the ESP for the service related deliverable for the research and development activities based on internal estimates of the costs to perform the services, including expected internal expenses and expenses with third parties for services and supplies, marked up to include a reasonable profit margin and adjusted for the scope of the potential license. Significant inputs used to determine the total expense of the research and development activities include, the length of time required and the number and costs of various studies that will be performed to complete the applicable POP Study. The ESP for the AJSC and DAC have been estimated based on the costs incurred to participate in the committees, marked up to include a reasonable profit margin. The ESP for the PD Material Right was determined based on the estimated value of the license adjusted for the estimated probability that the option would be exercised by Sanofi Genzyme. Based on the relative standalone selling price allocation, the allocation of the transaction price to the separate performance obligations was as follows: Unit of Accounting Amount (in thousands) Research and Development Services for: Huntington’s Program $ 14,228 Parkinson’s Program 6,040 Friedreich’s ataxia Program 14,821 Spinal Muscular Atrophy Program 29,116 Future Program 2,239 Committee Obligations: AJSC 133 DAC 207 PD Material Right 7,855 Total $ 74,639 The Company recognizes the amounts associated with research and development services and committee obligations on a proportional performance basis over the period of service using input-based measurements such as costs incurred to date, to estimate proportion performed, and remeasures its progress towards completion at the end of each reporting period. The amount allocated to the PD Material Right was initially deferred and recognized in full prior to the adoption of ASC 606. In October 2017, Sanofi Genzyme decided not to exercise the PD Material Right. Therefore, in the year ended December 31, 2017, the Company recognized revenue of $7.8 million of consideration which had been allocated to the PD Material Right. In addition, revenue recognized during the years ended December 31, 2017 and 2016 include amounts recognized related to consideration allocated to research and development services for various programs under the Sanofi Genzyme Collaboration Agreement. During 2017 the Company reassessed the estimated period of performance for each of the units of accounting and determined that the estimated period would be extended for two units of accounting. During 2016 the Company deprioritized the development of VY-SMN101 for the treatment of Spinal Muscular Atrophy. As a result, the Company ceased recognizing the revenue allocated to this program. These adjustments were made on a prospective basis and resulted in decreases in revenue recognized by $2.1 million and $9.5 million, respectively, for the year ended December 31, 2017. During the years ended December 31, 2018, 2017, and 2016, the Company recognized $0.7 million, $10.1 million, and $14.2 million, respectively, of revenue associated with its collaboration with Sanofi Genzyme related to research and development services performed during the period and for consideration allocated to the PD Material Right, which was recognized during 2017. As of December 31, 2018, there is $50.9 million of deferred revenue related to the Sanofi Genzyme Collaboration Agreement, which is classified as either current or noncurrent in the accompanying balance sheet based on the period the services are expected to be delivered. Costs incurred relating to the programs that Sanofi Genzyme has the option to license under the Sanofi Genzyme Collaboration Agreement consist of internal and external research and development costs, which primarily include: salaries and benefits, lab supplies and preclinical research studies. All costs are included in research and development expenses in the Company’s statement of operations during the years ended December 31, 2018, 2017, and 2016. AbbVie Tau Collaboration Agreement Summary of Agreement In February 2018, the Company entered into an exclusive collaboration and option agreement (the “AbbVie Tau Collaboration Agreement”) with AbbVie for the research, development and commercialization of AAV and other virus-based gene therapy products for the treatment of diseases of the central nervous system and other neurodegenerative diseases related to defective or excess aggregation of tau protein in the human brain, including Alzheimer’s disease. Under the AbbVie Tau Collaboration Agreement, the Company and AbbVie have agreed to collaborate on the research and development of specified vectorized antibody compounds comprised of an AAV or other viral capsid and a virus vector genome that encodes one or more antibodies that target and bind to a tau protein. The collaboration is comprised of a research period (the “Research Period”), a development period (the “Development Period”), and an exclusive license option (the “License Option”). The AbbVie Tau Collaboration Agreement included a non-refundable upfront payment of $69.0 million for services during the Research Period. During the Research Period, each party has agreed to identify up to five antibodies for inclusion in the collaboration. Subject to certain conditions and exceptions, the parties will select up to three antibodies (each, a “Research Antibody”) as candidates for creation of research compounds (each, a “Research Compound”), with AbbVie having the right to select two of the three Research Antibodies. The Company is required to use diligent efforts to conduct antibody engineering and other research activities to create Research Compounds and to develop product candidates containing or comprised of such Research Compounds (“Product Candidates”). The Company is solely responsible for its costs and expenses during the Research Period. During a specified portion of the Research Period, AbbVie may exercise one or more of its exclusive development options (each, a “Development Option”) to select up to a total of three Research Compounds (the “Selected Research Compounds”) and their corresponding Product Candidates (the “Selected Product Candidates”) to proceed to the Development Period. Upon AbbVie’s exercise of a Development Option, AbbVie will pay the Company $80.0 million for the first Selected Research Compound and $30.0 million each for up to two additional Selected Research Compounds. During the Development Period, the Company is obligated to use diligent efforts to conduct development activities, including Investigational New Drug application-enabling and Phase 1 clinical trial activities, for the Selected Research Compounds and corresponding Selected Product Candidates. The Company will be solely responsible for the costs and expenses during the Development Period. During a specified portion of the Development Period (the “License Option Period”), AbbVie may exercise its License Option to further develop and commercialize all of the Research Compounds (the “Licensed Compounds”), and corresponding product candidates (the “Licensed Products”). Upon AbbVie’s exercise of its License Option, AbbVie will provide a one-time payment of $75.0 million to the Company, and the Company will grant to AbbVie an exclusive, worldwide license, with the right to sublicense, under certain of the Company’s intellectual property rights to develop and commercialize the Licensed Compounds and the Licensed Products for all human diagnostic, prophylactic and therapeutic uses. In addition, after AbbVie’s exercise of the License Option, the Company has certain obligations to complete any remaining research and development activities that have not been completed for any Research Compounds and Product Candidates. The Company’s research and development activities will be conducted pursuant to the plans agreed to by the parties and overseen by a joint governance committee (“JGC”) as detailed in the AbbVie Tau Collaboration Agreement. Any material amendment to the research or development plans must be mutually agreed to by the Company and AbbVie, which may be through the JGC. Under the AbbVie Tau Collaboration Agreement, AbbVie is required to use commercially reasonable efforts to develop and commercialize at least one Licensed Product in each of the United States, Japan, the United Kingdom, Germany, France, Italy, and Spain. After exercise of the License Option, AbbVie is solely responsible for all development and commercialization activities relating to Licensed Compounds and Licensed Products at its sole cost and expense, subject to the agreed-upon research and development plans. The Company may elect to share in AbbVie’s development costs relating to a Licensed Product on an indication-by-indication basis in exchange for a specified increase in royalties (a “Cost-Sharing Option”). If the Company exercises a Cost-Sharing Option, the Company may either reimburse AbbVie for AbbVie’s applicable development costs or, in the case of certain budget overruns, AbbVie may instead deduct applicable development costs, up to a specified cap, from milestone and royalty payments owed by AbbVie to the Company. Under the AbbVie Tau Collaboration, the Company is eligible to receive specified development and first-sale milestone payments for each Licensed Compound of up to an aggregate of $550.0 million in the case of an Alzheimer’s disease indication, up to $230.0 million in the case of the first indication other than Alzheimer’s disease and up to $115.0 million for a subsequent non-Alzheimer’s disease indication. Additionally, the Company is eligible to receive tiered, escalating royalties, in a range from a high-single digit to a mid-to-high teen (or, if the Company has exercised its Cost-Sharing Option, low-twenties) percentage of aggregate net sales of Licensed Products on a Licensed Compound by Licensed Compound basis, subject to potential reductions in certain circumstances. For each Licensed Product, AbbVie also has the right to decrease or eliminate its royalty payments on such Licensed Product in exchange for a one-time payment by AbbVie at a fair market value to be negotiated by the parties or determined pursuant to dispute resolution procedures specified in the AbbVie Tau Collaboration Agreement. Unless earlier terminated, the AbbVie Tau Collaboration Agreement will expire on the earliest to occur of the expiration of (i) the Development Option Period, without AbbVie’s exercise of a Development Option; (ii) the License Option Period, without AbbVie’s exercise of its License Option; and (iii) the last-to-expire royalty term with respect to all Licensed Products in all countries. The Company and AbbVie have customary termination rights including the right to terminate for an uncured material breach of the agreement committed by the other party, and AbbVie has the right to terminate for convenience. Accounting Analysis The Company assessed the promised goods and services under the AbbVie Tau Collaboration Agreement, in accordance with ASC 606, and determined that the AbbVie Tau Collaboration Agreement includes the following performance obligations: (i) research services during the Research Period (through the delivery of the final research report) including the identification of the Research Antibodies, conduct of research activities and provision of information to AbbVie to allow AbbVie to determine whether to exercise up to three Development Options to be rendered (collectively, the “Research Services”), and (ii) a material right associated with the Development Option on the first Research Compound and associated Product Candidates (“First Development Option Material Right”). The first Development Option provides AbbVie with (i) additional development services on a selected Research Compound and (ii) the ability to exercise the License Option. The Company has concluded the option provides a material right as the consideration paid by AbbVie upon exercise of the first Development Option is less than the amount that the Company would otherwise expect to receive outside the context of the contract. The Company has concluded that the First Development Option Material Right is a separate performance obligation under ASC 606 as AbbVie is provided additional services and a License Option for additional consideration that represents a significant discount from amounts that would otherwise be offered outside the context of the contract. The First Development Option Material Right is distinct from the other performance obligations in the arrangement as it is an option in the contract that is not required for AbbVie to obtain the benefit of the other promised goods or services in the arrangement. The First Development Option Material Right does not include the underlying goods or services that are delivered upon exercise of the option, but rather represents the value to the customer of having the right to obtain development services and the right to the License Option at an advantageous price. The Company received a nonrefundable, upfront payment of $69.0 million as consideration under the AbbVie Tau Collaboration Agreement, which represents the transaction price at inception. Additional consideration to be paid to the Company upon the exercise of the Development and License Options by AbbVie or upon reaching certain milestones are excluded from the transaction price as they relate to option fees and milestones that can only be achieved subsequent to the option exercise or are outside of the initial contact term. The Company has allocated the transaction price to the separate performance obligations based on their relative standalone selling price. The Company determined the standalone selling price at contract inception based on each obligation’s ESP. The Company determined the ESP for the research services obligation based on internal estimates of the costs to perform the services, including expected internal expenses and expenses with third parties for services and supplies, inclusive of a reasonable profit margin. Significant inputs used to determine the total expense of the research services include the length of time required, the internal hours expected to be incurred on the services and the number and costs of various studies that will be performed to complete the Research Plan. The ESP for the First Development Option Material Right was determined based on the fees AbbVie would pay to exercise the Development and License Options, the estimated costs to perform the development services, inclusive of a reasonable profit margin, the estimated value of the License Option using comparable transactions, and the probability that the Development and License Options would be exercised by AbbVie. Based on the relative standalone selling price, the allocation of the transaction price to the separate performance obligations was as follows: Performance Obligation Amount (in thousands) Research Services $ 34,482 First Development Option Material Right 34,518 Total $ 69,000 The Company recognizes the amounts associated with Research Services on a proportional performance basis over the period of service using input-based measurements of total cost of research incurred to estimate proportion performed and remeasures its progress towards completion at the end of each reporting period. The amount allocated to the First Development Option Material Right is recorded as deferred revenue and will be recognized either over the period in which goods and services underlying the option are transferred or upon expiry of the option. During the year ended December 31, 2018, the Company recognized $6.9 million of revenue associated with the AbbVie Tau Collaboration related to the Research Services performed during the period. As of December 31, 2018, there is $62.1 million of deferred revenue related to the AbbVie Tau Collaboration Agreement, which is classified as either current or noncurrent in the accompanying balance sheet based on the period the goods or services are expected to be delivered. Costs incurred relating to the AbbVie Tau Collaboration Agreement consist of internal and external research and development costs, which primarily include salaries and benefits, lab supplies, and preclinical research studies. All of these costs are included in research and development expenses in the Company’s statement of operations during the year ended December 31, 2018. MRI Interventions License and Securities Purchase Agreements Summary of Agreement In September 2016, the Company entered into a securities purchase agreement (the “Securities Purchase Agreement”) and a license ® System, which is being used by the Company in ongoing development and clinical trials. Under the Securities Purchase Agreement, the Company paid $2.0 million for shares of MRIC common stock and a warrant to purchase additional shares of MRIC common stock. The Company also entered into the MRIC License Agreement, which provided for certain rights to MRIC technology and for MRIC to transfer the rights and know-how to manufacture the ClearPoint System to enable the Company to utilize an alternative supplier for the ClearPoint System for use in the Company’s development and clinical trials. During 2017, the Company terminated the MRIC License Agreement and all prior and future commitments and obligations under such agreement became null and void. The Company continues to hold the common stock and warrants to purchase additional shares of common stock as an available-for-sale security and non-current asset, respectively. In May 2018, the Company entered into a master services and supply agreement with MRIC (the “MRIC Supply Agreement”) which provides for MRIC to perform certain manufacturing, supply, development and services as requested by the Company, including the supply of the ClearPoint System and cannula devices. As of December 31, 2018, the Company continued to hold the common stock and warrants to purchase additional shares of common stock as an available-for-sale security and non-current asset, respectively. Other Agreements During 2018, 2017, and 2016, the Company entered into various agreements with contract research organizations and institutions to license intellectual property. In consideration for the licensed rights the Company generally made upfront payments, which were recorded as research and development expense as the acquired technologies were considered in‑process research and development. The license agreements obligate the Company to make additional payments that are contingent upon specific clinical trial and regulatory approval milestones being achieved as well as royalties on future product sales. The agreements to license intellectual property include potential milestone payments that are dependent upon the development of products licensed under the agreements and contingent upon the achievement of clinical trial or regulatory approval milestones. As of December 31, 2018 and 2017, there have been no milestones achieved. The Company can generally terminate the license agreements upon 30‑90 days prior written notice. Additionally, certain license agreements require the Company to reimburse the licensor for certain past and ongoing patent related expenses. During the year ended December 31, 2018, 2017, and 2016, the Company incurred $0.6 million, $0.8 million, and $1.8 million of expense, respectively, related to these reimbursable patent costs which are recorded as general and administrative expense During the year ended December 31, 2016, the Company entered into a research and development funding arrangement with a non-profit organization that provides up to $4.0 million in funding to the Company upon the achievement of clinical and development milestones. The agreement provides that the Company repay amou

Preferred stock

Preferred stock	12 Months Ended
	Dec. 31, 2018
Preferred stock
Preferred stock	8. Preferred stock The Company has authorized preferred stock amounting to 5,000,000 shares as of December 31, 2018 and 2017. The authorized preferred stock was classified under stockholders’ equity at December 31, 2018.

Common stock

Common stock	12 Months Ended
	Dec. 31, 2018
Common stock
Common stock	9. Common stock As of December 31, 2018 and 2017, the Company had authorized 120,000,000 shares of common stock, at $0.001 par value per share. General The voting, dividend and liquidation rights of the holders of the common stock are subject to and qualified by the rights, powers and preferences of the holders of preferred stock. The common stock has the following characteristics: Voting The holders of shares of common stock are entitled to one vote for each share of common stock held at all meetings of stockholders and written actions in lieu of meetings. Dividends The holders of shares of common stock are entitled to receive dividends, if and when declared by the Board of Directors. No dividends have been declared or paid by the Company since its inception. Liquidation The holders of shares of common stock are entitled to share ratably in the Company’s remaining assets available for distribution to its stockholders in the event of any voluntary or involuntary liquidation, dissolution or winding up of the Company or upon occurrence of a deemed liquidation event. Shares Reserved For Future Issuance As of December 31, 2018 2017 Shares reserved for vesting of restricted stock awards under the Founder Agreements 235,294 366,914 Shares reserved for vesting of restricted stock awards under the 2014 Option and Stock Plan — 191,065 Shares reserved for exercise of outstanding stock options 4,225,152 3,143,566 Shares reserved for issuances under the 2015 Stock Option Plan 1,973,227 1,501,005 Shares reserved for issuances under the 2015 Employee Stock Purchase Plan 963,386 730,860 7,397,059 5,933,410

Stock based compensation

Stock based compensation	12 Months Ended
	Dec. 31, 2018
Stock based compensation
Stock based compensation	10. Stock‑based compensation 2014 Stock Option and Grant Plan In January 2014, the Company adopted the 2014 Stock Option and Grant Plan (the “2014 Plan”), under which it could grant incentive stock options, non‑qualified stock options, restricted stock awards, unrestricted stock awards, or restricted stock units to purchase up to 823,529 shares of Common Stock to employees, officers, directors and consultants of the Company. In April 2014, the Company amended the Plan to allow for the issuance of up to 1,411,764 shares of Common Stock. In August 2014, April 2015, August 2015 and October 2015 the Company further amended the Plan to allow for the issuance of up to 2,000,000, 2,047,058, 2,669,411 and 2,998,823 shares of Common Stock, respectively. During 2014 the Company issued only restricted stock awards under the Plan and during 2015 the Company only granted stock options. The terms of stock option agreements, including vesting requirements, were determined by the Board of Directors and were subject to the provisions of the 2014 Plan. Restricted stock awards granted by the Company generally vest based on each grantee’s continued service with the Company during a specified period following grant. Stock options granted to employees generally vest over four years, with 25% vesting on the one year anniversary and 75% vesting ratably, on a monthly basis, over the remaining three years. Stock options granted to non‑employee consultants generally vest monthly over a period of one to four years. Founder Awards In January 2014, the Company issued 1,188,233 shares of restricted stock to its Founders at an original issuance price of $0.0425 per share. Of the total restricted shares awarded to the Founders, 835,292 shares generally vest over one to four years, based on each Founder’s continued service to the Company in varying capacity as a Scientific Advisory Board member, consultant, director, officer or employee, as set forth in each grantee’s individual restricted stock purchase agreement. The remaining 352,941 of the shares issued will begin vesting upon the achievement of certain performance objectives as well as continued service to the Company, as set forth in the agreements. These performance conditions are tied to certain milestone events specific to the Company’s corporate goals, including but not limited to preclinical and clinical development milestones related to the Company’s product candidates. Stock‑based compensation expense associated with these performance‑based awards will be recognized when the achievement of the performance condition is considered probable, using management’s best estimates. Management concluded that the achievement of the performance milestone for one of the three performance-based awards had been met during 2016. Accordingly, stock‑based compensation expense in the amount of $0.3 million, $1.4 million, and $1.1 million was recorded in the years ended December 31, 2018, 2017, and 2016, respectively. 2015 Stock Option Plan In October 2015, the Company ’ s board of directors and stockholders approved the 2015 Stock Option and Incentive Plan “(2015 Stock Option Plan”), which became effective upon the completion of the IPO. The 2015 Stock Option Plan provides the Company with the flexibility to use various equity-based incentive and other awards as compensation tools to motivate its workforce. These tools include stock options, stock appreciation rights, restricted stock, restricted stock units, unrestricted stock, performance share awards and cash-based awards. The 2015 Stock Option Plan replaced the 2014 Plan. Any options or awards outstanding under the 2014 Plan remained outstanding and effective. The number of shares initially reserved for issuance under the 2015 Stock Option Plan is the sum of (i) 1,311,812 shares of common stock and (ii) the number of shares under the 2014 Plan that are not needed to fulfill the Company ’ s obligations for awards issued under the 2014 Plan as a result of forfeiture, expiration, cancellation, termination or net issuances of awards thereunder. The number of shares of common stock that may be issued under the 2015 Stock Option Plan is also subject to increase on the first day of each fiscal year by up to 4% of the Company ’ s issued and outstanding shares of common stock on the immediately preceding December 31. Effective January 1, 2016, 2017, 2018 and 2019, an additional 1,069,971, 1,070,635, 1,285,200, and 1,302,830 shares of common stock, respectively, were added to the Company’s 2015 Stock Option Plan pursuant to its “evergreen” provision, for future issuance. During the year ended December 31, 2018, the Company granted options to purchase 2,215,891 shares of common stock to employees and directors. As of December 31, 2018, there were 1,973,227 shares available for future issuance under the 2015 Stock Option Plan. 2015 Employee Stock Purchase Plan In October 2015, the Company’s board of directors and stockholders approved the 2015 Employee Stock Purchase Plan (the “2015 ESPP”). Under the 2015 ESPP, all full-time employees of the Company are eligible to purchase common stock of the Company twice per year, at the end of each six-month payment period. During each payment period, eligible employees who so elect, may authorize payroll deductions in an amount of 1% to 10% (whole percentages only) of the employee’s base pay for each payroll period. At the end of each payment period, the accumulated deductions are used to purchase shares of common stock from the Company at a discount. A total of 262,362 shares of common stock were initially authorized for issuance under this plan. The 2015 ESPP became effective upon the completion of the IPO. Effective January 1, 2016, 2017, 2018, and 2019, a total of 267,492, 267,658, 321,300, and 325,707 shares of common stock, respectively, were added to the 2015 ESPP, pursuant to its evergreen provision, for future issuance . The Company issued 88,774 shares of common stock under the 2015 ESPP in the year ended December 31, 2018. Stock-based Compensation Expense Total compensation cost recognized for all stock-based compensation awards in the statements of operations and comprehensive loss is as follows: Year ended December 31, 2018 2017 2016 (in thousands) Research and development $ 4,717 $ 5,367 $ 4,296 General and administrative 10,993 3,871 2,014 Total stock-compensation expense $ 15,710 $ 9,238 $ 6,310 Restricted Stock A summary of the status of and changes in unvested restricted stock was as follows: Weighted Average Grant Date Fair Value Shares Per Share Unvested restricted common stock as of December 31, 2017 557,979 $ 0.70 Issued — Vested (319,891) $ 0.84 Repurchased (2,794) $ 1.11 Unvested restricted common stock as of December 31, 2018 235,294 $ 0.51 The expense related to awards granted to employees and non‑employees was $0.1 million and $0.4 million, respectively, for the year ended December 31, 2018. The expense related to awards granted to employees and non-employees was $0.5 million and $2.7 million, respectively, for the year ended December 31, 2017. The expense related to awards granted to employees and non-employees was $0.5 million and $2.6 million, respectively, for the year ended December 31, 2016. Stock Options A summary of the status of, and changes in, stock options was as follows: Weighted Remaining Aggregate Average Contractual Intrinsic Exercise Life Value Shares Price (in years) (in thousands) Outstanding at December 31, 2017 3,143,566 $ 11.82 Granted 2,215,891 $ 20.78 Exercised (384,186) $ 10.13 Cancelled or forfeited (750,119) $ 16.13 Outstanding at December 31, 2018 4,225,152 $ 15.91 8.4 $ 578 Exercisable at December 31, 2018 1,531,832 $ 12.55 7.3 $ 507 Vested and expected to vest at December 31, 2018 4,225,152 $ 15.91 8.4 $ 578 Using the Black‑Scholes option pricing model, the weighted average fair value of options granted to employees and directors during the year ended December 31, 2018 was $13.87. The stock-based compensation expense related to stock option awards granted to employees and directors was $14.9 million, $5.5 million, and $3.0 million for the years ended December 31, 2018, 2017, and 2016, respectively. The fair value of each option issued to employees and directors was estimated at the date of grant using the Black‑Scholes option pricing model with the following weighted‑average assumptions: Year ended December 31, 2018 2017 2016 Risk-free interest rate 2.8 % 2.0 % 1.5 % Expected dividend yield — % — % — % Expected term (in years) 6.0 6.0 6.0 Expected volatility 74.4 % 73.7 % 73.1 % There were no new options granted to non‑employees during the year ended December 31, 2018. Unvested options granted to non‑employees are revalued at each measurement period until they vest. The expense related to stock option awards granted to non‑employees was $0.3 million, $0.4 million, and $0.2 million for the years ended December 31, 2018, 2017, and 2016, respectively. As of December 31, 2018, the Company had unrecognized stock‑based compensation expense related to its unvested stock options of $26.1 million which is expected to be recognized over the remaining weighted average vesting period of 2.78 years. The fair value of each option issued to non‑employees was estimated at each vesting and reporting date using the Black‑Scholes option pricing model. The reporting date fair value was determined using the following weighted‑average assumptions: As of December 31, 2018 2017 2016 Risk-free interest rate 2.6 % 2.4 % 2.1 % Expected dividend yield — % — % — % Expected term (in years) 7.6 8.5 9.1 Expected volatility 73.2 % 76.2 % 83.3 %

401(k) Savings Plan

401(k) Savings Plan	12 Months Ended
	Dec. 31, 2018
401(k) Savings Plan
401(k) Savings Plan	11. 401(k) Savings plan The Company has a defined‑contribution savings plan under Section 401(k) of the Internal Revenue Code (the “401(k) Plan”). The 401(k) Plan covers all employees who meet defined minimum age and service requirements, and allows participants to defer a portion of their annual compensation on a pretax basis. The Company expensed approximately $0.8 million, $0.5 million, and $0.3 million related to employer contributions made during the years ended December 31, 2018, 2017, and 2016, respectively.

Income taxes

Income taxes	12 Months Ended
	Dec. 31, 2018
Income taxes
Income taxes	12. Income taxes The Company accounts for income taxes using the asset and liability method. Under the asset and liability method, deferred tax assets and liabilities are recognized for the future tax consequences attributable to temporary differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases and for tax carryforwards, such as net operating losses. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in the provision for income taxes in the period that includes the enactment date. The Company records a valuation allowance to reduce the carrying amount of deferred tax assets if it is more likely than not that such asset will not be realized. The evaluation of uncertain tax positions is based on factors including, but not limited to, changes in the law, the measurement of tax positions taken or expected to be taken in tax returns, the effective settlement of matters subject to audit, new audit activity, and changes in facts or circumstances related to a tax position. The Company evaluates its tax positions on an annual basis. On December 22, 2017, the U.S. federal government enacted comprehensive tax legislation with the U.S. Tax Cuts and Jobs Act (“Tax Act”) that made changes to the U.S. tax code impacting the year ended December 31, 2017 and future years. Effective January 1, 2018, the Tax Act reduced the U.S. federal corporate tax rate from 35% to 21%. For the year ended December 31, 2017, the Tax Act required a one-time transition tax on certain unrepatriated earnings of foreign subsidiaries that is payable over eight years. At December 31, 2018, the Company does not have any foreign subsidiaries and the international aspects of the Tax Act are not applicable. In December 2017, the SEC staff issued Staff Accounting Bulletin No. 118 (“SAB 118”), which provided guidance on accounting for the tax effects of the Tax Act. SAB 118 provided a measurement period that should not extend beyond one year from the date of the Tax Act enactment for companies to complete the accounting under Accounting Standards Codification 740—Income Taxes. In accordance with SAB 118, to the extent that a company’s accounting for certain income tax effects of the Tax Act is incomplete, but the company is able to determine a reasonable estimate, it must record a provisional estimate in its financial statements. The Company’s accounting for the Tax Act was completed in the fourth quarter of 2018 and resulted in no adjustments to the Company’s prior provisional estimates recorded in the period ended December 31, 2017. The benefit for incomes taxes is as follows: Year ended December 31, 2018 2017 (in thousands) Current Federal $ 180 $ — State — — Total current 180 — Deferred Federal — — State — — Total deferred — — Total tax expense $ 180 $ — A reconciliation of the expected income tax (benefit) computed using the federal statutory income tax rate at the Company’s effective tax rate is as follows: Year ended December 31, 2018 2017 2016 Income tax computed at federal statutory tax rate 21.0 % 34.0 % 34.0 % State taxes, net of federal benefit 6.3 % 6.1 % 5.6 % General business credit carryovers 3.1 % 5.0 % 4.2 % Non-deductible expenses (2.1) % (4.1) % (4.0) % Deferred rate change — % (21.8) % — % Change in valuation allowance (28.1) % (19.2) % (40.2) % Total 0.2 % — % (0.4) % The Company has incurred net operating losses (“NOLs”) since June 2013. At December 31, 2018, the Company had federal and state net operating loss carryforwards of $162.9 million and $163.8 million, respectively. During 2018, the company generated federal and state NOLs carryforwards of $73.1 million and $72.1 million, respectively. The federal net operating loss carryforward generated in 2018 is limited to 80% of taxable income and has an indefinite carryforward period. The Company’s federal net operating loss carryforward generated in the period ended December 31, 2017 and prior, as well as the Company’s state NOL carryforwards begin to expire in 2033. As of December 31, 2018, the Company also had federal and state research and development tax credit carryforwards of $9.4 million and $3.0 million, respectively, which expire beginning in 2028. As of December 31, 2018, the Company had state investment credits of $0.4 million, which expire beginning in 2019. Under the provisions of the Internal Revenue Code, certain substantial changes in the Company’s ownership may result in a limitation on the amount of NOL carryforwards and research and development credit carryforwards that may be utilized annually to offset future taxable income and taxes payable. In general, an ownership change, as defined by Section 382, results from transactions that increase the ownership of 5% stockholders or public groups in the stock of a corporation by more than 50 percent in the aggregate over a three-year period. During 2016, the Company completed a study through June 30, 2016, to determine whether any ownership change has occurred since the Company’s formation and has determined that transactions have resulted in three ownership changes, as defined by Section 382. There could be additional ownership changes in the future that could further limit the amount of NOLs and tax credit carryforwards that the Company can utilize. The significant components of the Company’s deferred tax assets and (liabilities) as of December 31, 2018 and 2017 are as follows: Year ended December 31, 2018 2017 (in thousands) Deferred tax assets: Net operating loss carryforwards $ 44,556 $ 24,642 Tax credit carryforwards 12,021 8,832 Deferred rent 1,560 1,458 Deferred revenue 13,926 8,622 Non-deductible accruals and reserves 1,105 817 Intangibles 930 832 Stock compensation 2,802 1,361 Total deferred tax assets 76,900 46,564 Less valuation allowance (75,213) (44,953) Net deferred tax assets 1,687 1,611 Deferred tax liabilities Depreciation and amortization (1,687) (1,611) Net deferred taxes $ — $ — As required by ASC 740, management has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets, which principally comprise NOL carryforwards, research and development credit carryforwards, deferred revenue. Management has determined that it is more likely than not that the Company will not recognize the benefits of its federal and state deferred tax assets, and as a result, a valuation allowance of $75.2 million and $45.0 million has been established at December 31, 2018 and 2017, respectively. The change in valuation allowance was $30.2 million for the year ended December 31, 2018, primarily due to additional operating losses incurred by the Company for the year ended December 31, 2018. The primary reason for the difference between the income tax expense recorded by the Company and the amount of income tax expense at statutory income tax rates was the change in the valuation allowance. At December 31, 2018 and 2017, the Company had no unrecognized tax benefits. The Company has not as 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. Thus, there would be no impact to the balance sheets or statements of operations if an adjustment were required. Interest and penalty charges, if any, related to unrecognized tax benefits would be classified as income tax expense in the accompanying statements of operations. As of December 31, 2018 and 2017, the Company has no accrued interest related to uncertain tax positions. Since the Company is in a loss carryforward position, it is generally subject to examination by the U.S. federal, state, and local income tax authorities for all tax years in which a loss carryforward is available.

Related party transactions

Related party transactions	12 Months Ended
	Dec. 31, 2018
Related party transactions
Related party transactions	13. Related‑party transactions Since inception, the Company received consulting and management services from one of its investors. The total amount of consulting and management services provided by this investor was de minimis during the years ended December 31, 2018 and 2017.

Selected quarterly financial da

Selected quarterly financial data (unaudited)	12 Months Ended
	Dec. 31, 2018
Selected quarterly financial data (unaudited)
Selected quarterly financial data (unaudited)	14. Selected quarterly financial data (unaudited) The following table contains quarterly financial information for 2018 and 2017. The Company believes that the following information reflects all normal recurring adjustments necessary for a fair statement of the information for the periods presented. The operating results for any quarter are not necessarily indicative of results for any future period. 2018 First Second Third Fourth Quarter Quarter Quarter Quarter Total (amounts in thousands, except per share data) Collaboration revenue $ 942 $ 2,575 $ 2,094 $ 2,008 $ 7,619 Total operating expenses 22,035 28,269 23,241 25,169 98,714 Loss from operations (21,093) (25,694) (21,147) (23,161) (91,095) Net loss (19,926) (25,541) (20,289) (22,532) (88,288) Net loss per share $ (0.63) $ (0.80) $ (0.63) $ (0.70) $ (2.75) 2017 First Second Third Fourth Quarter Quarter Quarter Quarter Total (amounts in thousands, except per share data) Collaboration revenue $ 1,464 $ 1,177 $ 1,148 $ 6,346 $ 10,135 Total operating expenses 18,986 19,816 24,503 18,693 81,998 Loss from operations (17,522) (18,639) (23,355) (12,347) (71,863) Net loss (16,648) (18,876) (23,346) (11,828) (70,698) Net loss per share $ (0.65) $ (0.73) $ (0.89) $ (0.40) $ (2.64)

Subsequent events

Subsequent events	12 Months Ended
	Dec. 31, 2018
Subsequent events
Subsequent events	15. Subsequent events Neurocrine Collaboration In January 2019, the Company entered into a collaboration and license agreement with Neurocrine for the research, development and commercialization of four programs including the Company’s Parkinson’s disease program, the Company’s Friedreich’s ataxia program, and two programs to be determined by the Company and Neurocrine at a later date. Under the terms of the agreement, the Company will receive an upfront payment of $165.0 million, inclusive of $50.0 million for the sale of 4,179,728 shares of its common stock, funding of development costs under the Parkinson’s and Friedreich’s ataxi programs, and may receive future development and regulatory milestones and royalties. AbbVie Alpha-Synuclein Collaboration In February 2019, the Company entered into a separate collaboration agreement with AbbVie, the AbbVie Alpha-Synuclein Collaboration Agreement, for the research, development, and commercialization of AAV and other virus based gene therapy products directed against pathological species of alpha-synuclein for the potential treatment of Parkinson’s disease and other synucleinopathies. Under the terms of the AbbVie Alpha-Synuclein Collaboration Agreement, the Company will receive an upfront payment of $65.0 million and may receive option exercise payments, future regulatory and commercial milestone payments and royalties.

Summary of significant accoun_2

Summary of significant accounting policies (Policies)	12 Months Ended
	Dec. 31, 2018
Summary of significant accounting policies
Basis of presentation	Basis of presentation The accompanying consolidated financial statements include those of the Company and its subsidiary, Voyager Securities Corporation, after elimination of all intercompany accounts and transactions. The accompanying consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”).
Public offerings	Public offerings On October 29, 2015, in preparation for the Company’s IPO, the Company’s Board of Directors and stockholders approved a 1-for-4.25 reverse split of the Company’s common stock, which became effective on October 29, 2015. All share and per share amounts in the consolidated financial statements and notes thereto have been retroactively adjusted for all periods presented to give effect to this reverse split, including reclassifying an amount equal to the reduction in par value of common stock to additional paid-in capital. On November 16, 2015, the Company completed the sale of 5,750,000 shares of its common stock in its initial public offering (the “IPO”), at a price to the public of $14.00 per share, resulting in net proceeds to the Company of $72.9 million after deducting underwriting discounts, commissions and offering expenses payable by the Company. On November 7, 2017, the Company completed the sale of 5,175,000 shares of its common stock in a public offering at a price to the public of $12.00 per share, resulting in net proceeds to the Company of $58.0 million after deducting underwriting discounts, commissions, and offering expenses payable by the Company.
Use of Estimates	Use of Estimates The preparation of consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. On an ongoing basis, the Company’s management evaluates its estimates, which include, but are not limited to, estimates related to revenue recognition, accrued expenses, stock‑based compensation expense, and income taxes. The Company bases its estimates on historical experience and other market specific or other relevant assumptions that it believes to be reasonable under the circumstances. Actual results may differ from those estimates or assumptions.
Fair Value of Financial Instruments	Fair Value of Financial Instruments ASC Topic 820, Fair Value Measurement (“ASC 820”), establishes a fair value hierarchy for instruments measured at fair value that distinguishes between assumptions based on market data (observable inputs) and the Company’s own assumptions (unobservable inputs). Observable inputs are inputs that market participants would use in pricing the asset or liability based on market data obtained from sources independent of the Company. Unobservable inputs are inputs that reflect the Company’s assumptions about the inputs that market participants would use in pricing the asset or liability, and are developed based on the best information available in the circumstances. ASC 820 identifies fair value as the exchange price, or exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As a basis for considering market participant assumptions in fair value measurements, ASC 820 establishes a three‑tier fair value hierarchy that distinguishes between the following: · Level 1 —Quoted market prices in active markets for identical assets or liabilities. · Level 2 —Inputs other than Level 1 inputs that are either directly or indirectly observable, such as quoted market prices, interest rates, and yield curves. · Level 3 —Unobservable inputs developed using estimates of assumptions developed by the Company, which reflect those that a market participant would use. To the extent that the 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 carrying amounts reflected in the balance sheets for cash and cash equivalents, prepaid expenses and other current assets, accounts payable and accrued expenses approximate their fair values, due to their short‑term nature.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers all highly liquid investments purchased with original maturities of 90 days or less at acquisition to be cash equivalents. Cash and cash equivalents include cash held in banks and amounts held in money market funds.
Marketable Securities	Marketable Securities The Company classifies marketable debt securities with a remaining maturity of greater than three months when purchased as available‑for‑sale. Marketable debt securities with a remaining maturity date greater than one year and marketable equity securities are classified as non‑current where the Company has the intent and ability to hold these securities for at least the next 12 months. During 2016, the Company invested in a supplier and received common stock and warrants to purchase common stock in that entity. The common stock is considered an available-for-sale marketable equity security and is included in non-current marketable securities, and the warrants are included in non-current assets. All available for sale debt securities are carried at fair value with the unrealized gains and losses included in other comprehensive income (loss) as a component of stockholders’ equity until realized. Any premium or discount arising at purchase is amortized and/or accreted to interest income and/or expense. Realized gains and losses are determined using the specific identification method and are included in other income (expense). If any adjustment to fair value reflects a decline in value of the investment, the Company considers all available evidence to evaluate the extent to which the decline is “other than temporary” and, if so, recognizes the loss through a charge to the Company’s statement of operations and comprehensive loss. No other than temporary losses have been recognized. Cash, cash equivalents, and marketable securities as of December 31, 2018 and 2017 consist of the following: Amortized Unrealized Unrealized Fair Cost Gains Losses Value (in thousands) As of December 31, 2018 Money market funds included in cash and cash equivalents $ 46,173 $ — $ — $ 46,173 Marketable securities: U.S. Treasury notes 108,951 1 5 108,947 Equity securities 1,220 — 592 628 Total marketable securities $ 110,171 $ 1 $ 597 $ 109,575 Total money market funds and marketable securities $ 156,344 $ 1 $ 597 $ 155,748 As of December 31, 2017 Money market funds included in cash and cash equivalents $ 30,469 $ — $ — $ 30,469 Marketable securities: U.S. Treasury notes 137,560 — 38 137,522 Equity securities 1,220 — 120 1,100 Total marketable securities $ 138,780 $ — $ 158 $ 138,622 Total money market funds and marketable securities $ 169,249 $ — $ 158 $ 169,091 All of the Company’s marketable debt securities at December 31, 2018 and 2017 have a contractual maturity of one year or less.
Restricted Cash	Restricted Cash At December 31, 2018 and 2017, the Company maintained restricted cash totaling approximately $0.7 million held in the form of money market accounts as collateral for the Company’s facility lease obligation. The balance is included within deposits in other non‑current assets in the accompanying consolidated balance sheets. The following table provides a reconciliation of cash, cash equivalents, and restricted cash within the condensed consolidated balance sheet that sum to the total of the same such amounts shown in the statement of cash flows: As of December 31, 2018 2017 2016 (in thousands) Cash and cash equivalents $ 46,859 $ 31,530 $ 36,641 Restricted cash included in deposits and other noncurrent assets 735 735 735 Total cash, cash equivalents, and restricted cash $ 47,594 $ 32,265 $ 37,376
Property and Equipment	Property and Equipment Property and equipment consists of laboratory equipment, furniture and office equipment, and leasehold improvements and is stated at cost, less accumulated depreciation. Maintenance and repairs that do not improve or extend the lives of the respective assets are expensed to operations as incurred; while costs of major additions and betterments are capitalized. Depreciation is calculated over the estimated useful lives of the assets using the straight‑line method.
Impairment of Long-Lived Assets	Impairment of Long‑Lived Assets The Company evaluates long‑lived assets for potential impairment when events or changes in circumstances indicate the carrying value of the assets may not be recoverable. Recoverability is measured by comparing the book values of the assets to the expected future net undiscounted cash flows that the assets are expected to generate. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the book values of the assets exceed their fair value. The Company has not recognized any impairment losses from inception through December 31, 2018.
Revenue Recognition	Revenue Recognition As of December 31, 2018, all of the Company’s revenue is generated from its collaboration agreements with Sanofi Genzyme Corporation, a Sanofi company (“S anofi Genzyme”), and AbbVie Biotechnology Ltd. and its affiliates collectively, (“AbbVie”) . The Company enters into collaboration agreements which are within the scope of ASC 606, Revenue from Contracts with Customers (“ASC 606”), under which the Company licenses rights to certain of the Company’s product candidates and performs research and development services. The terms of these arrangements typically include payment of one or more of the following: non-refundable, upfront fees; reimbursement of research and development costs; development, regulatory, and commercial milestone payments; and royalties on net sales of licensed products. Under ASC 606, an entity recognizes revenue when its 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 the appropriate amount of revenue to be recognized for arrangements determined to be within the scope of ASC 606, the Company performs the following five steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of 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 consideration it is entitled to in exchange for the goods or services it transfers to the customer. The promised goods or services in the Company’s arrangements typically consist of license rights to the Company’s intellectual property and research and development services. The Company provides options to additional items in the contracts, which are accounted for as separate contracts when the customer elects to exercise such options, unless the option provides a material right to the customer. The Company evaluates the customer options for material rights, or options to acquire additional goods or services for free or at a discount. If the customer options are determined to represent a material right, the material right is recognized as a separate performance obligation at the outset of the arrangement. Performance obligations are promised goods or services in a contract to transfer a distinct good or service to the customer and are considered distinct when (i) the customer can benefit from the good or service on its own or together with other readily available resources and (ii) the promised good or service is separately identifiable from other promises in the contract. In assessing whether promised goods or services are distinct, the Company considers factors such as the stage of development of the underlying intellectual property, the capabilities of the customer to develop the intellectual property on its own or whether the required expertise is readily available and whether the goods or services are integral or dependent to other goods or services in the contract. The Company estimates the transaction price based on the amount expected to be received for transferring the promised goods or services in the contract. The consideration may include fixed consideration or variable consideration. At the inception of each arrangement that includes variable consideration, the Company evaluates the amount of potential payments and the likelihood that the payments will be received. The Company utilizes either the most likely amount method or expected amount method to estimate the amount expected to be received based on which method best predicts the amount expected to be received. The amount of variable consideration which is included in the transaction price may be constrained, and is included in the transaction price only to the extent that it is probable that a significant reversal in the amount of the cumulative revenue recognized will not occur in a future period. The Company’s contracts often include development and regulatory milestone payments which are assessed under the most likely amount method and constrained if it is probable that a significant revenue reversal would occur. Milestone payments that are not within the Company’s control or the licensee’s control, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. At the end of each 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 overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect collaboration revenues in the period of adjustment. To date, the Company has not recognized any consideration related to the achievement of development, regulatory, or commercial milestone revenue resulting from any of the Company’s collaboration arrangements. 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 recognizes 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). To date, the Company has not recognized any consideration related to sales-based royalty revenue resulting from any of the Company’s collaboration arrangements. The Company allocates the transaction price based on the estimated stand-alone selling price of each of the performance obligations. 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 utilizes key assumptions to determine the stand-alone selling price for service obligations, which may include other comparable transactions, pricing considered in negotiating the transaction and the estimated costs. Additionally, in determining the standalone selling price for material rights, the Company utilizes comparable transactions, clinical trial success probabilities, and estimates of option exercise likelihood. Variable consideration is allocated specifically to one or more performance obligations in a contract when the terms of the variable consideration relate to the satisfaction of the performance obligation and the resulting amounts allocated are consistent with the amounts the Company would expect to receive for the satisfaction of each performance obligation. The consideration allocated to each performance obligation is recognized as revenue when control is transferred for the related goods or services. For performance obligations which consist of licenses and other promises, the Company utilizes judgment to assess 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. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. Upfront payments and fees are recorded as deferred revenue upon receipt or when due until the Company performs its obligations under these arrangements. Amounts are recorded as accounts receivable when the Company’s right to consideration is unconditional
Research and Development	Research and Development Research and development costs are charged to expense as incurred in performing research and development activities. The costs include employee compensation costs, external research, consultant costs, sponsored research, in‑kind services provided under the Sanofi Genzyme agreement, license fees, process development and facilities costs. Facilities costs primarily include the allocation of rent, utilities and depreciation.
Research Contract Costs and Accruals	Research Contract Costs and Accruals The Company has entered into various research and development contracts with research institutions and other companies. These agreements are generally cancelable. The Company records accruals for estimated ongoing research costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates may be made in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs.
Patent Costs	Patent Costs The Company expenses patent application and related legal costs as incurred and classifies such costs as general and administrative expenses in the accompanying statements of operations.
Stock-Based Compensation Expense	Stock‑Based Compensation Expense The Company accounts for its stock‑based compensation awards in accordance with ASC Topic 718 Compensation—Stock Compensation (“ASC 718”). ASC 718 requires all stock‑based payments to employees and directors, including grants of restricted stock and stock options, to be recognized as expense in the statements of operations based on their grant date fair values. Grants of restricted stock and stock options to other service providers, referred to as non‑employees, are required to be recognized as expense in the statements of operations based on their vesting date fair values. The Company estimates the fair value of options granted using the Black‑Scholes option pricing model. The Company uses the fair value of its common stock to determine the fair value of restricted stock awards. The Black‑Scholes option pricing model requires inputs based on certain subjective assumptions, including (a) the expected stock price volatility, (b) the calculation of expected term of the award, (c) the risk‑free interest rate and (d) expected dividends. Due to a lack of company-specific historical and implied volatility data, the Company bases the estimate of expected volatility on the historical volatility of a group of similar companies that are publicly traded, blended with the most recent period of historic volatility of its common stock. The historical volatility is calculated based on a period of time commensurate with the expected term assumption. The computation of expected volatility is based on the historical volatility of a representative group of companies with similar characteristics to the Company, including stage of product development and life science industry focus. The Company uses the simplified method as prescribed by the SEC Staff Accounting Bulletin No. 107, Share‑Based Payment , to calculate the expected term for stock options granted to employees as it does not have sufficient historical exercise data to provide a reasonable basis upon which to estimate the expected term. For stock options granted to non‑employees, the Company utilizes the contractual term of the arrangement as the basis for the expected term assumption. The risk‑free interest rate is based on a treasury instrument whose term is consistent with the expected term of the stock options. The expected dividend yield is assumed to be zero as the Company has never paid dividends and has no current plans to pay any dividends on its common stock. The Company expenses the fair value of its stock‑based compensation awards to employees on a straight‑line basis over the associated service period, which is generally the period in which the related services are received. Stock‑based compensation awards to non‑employees are adjusted through stock‑based compensation expense at each reporting period end to reflect the current fair value of such awards and are expensed on a straight‑line basis. The Company records the expense for stock‑based compensation awards subject to performance conditions over the remaining service period when management determines that achievement of the performance condition is probable. Management evaluates when the achievement of a performance condition is probable based on the expected satisfaction of the performance conditions as of the reporting date.
Income Taxes	Income Taxes Income taxes are recorded in accordance with ASC Topic 740, Income Taxes (“ASC 740”), which provides for deferred taxes using an asset and liability approach. Under this method, deferred tax assets and liabilities are determined based on the difference between the financial reporting and the tax reporting basis of assets and liabilities and are measured using the enacted tax rates and laws that are expected to be in effect when the differences are expected to reverse. The Company provides a valuation allowance against net deferred tax assets unless, based upon the weight of available evidence, it is more likely than not that the deferred tax assets will be realized. The Company accounts for uncertain tax positions in accordance with the provisions of ASC 740. When uncertain tax positions exist, the Company recognizes the tax benefit of tax positions to the extent that the benefit will more likely than not be realized. The determination as to whether the tax benefit will more likely than not be realized is based upon the technical merits of the tax position as well as consideration of the available facts and circumstances. As of December 31, 2018, the Company does not have any significant uncertain tax positions.
Comprehensive Loss	Comprehensive Loss Comprehensive loss is comprised of net loss and other comprehensive income or loss. Other comprehensive income or loss consists of unrealized gains or losses on marketable securities.
Net Loss Per Share	Net Loss Per Share Basic net loss per share is calculated by dividing the net loss by the weighted‑average number of shares of common stock outstanding during the period, without consideration for potentially dilutive securities. Diluted net loss per share is computed by dividing the net loss by the weighted‑average number of shares of common stock and potentially dilutive securities outstanding for the period determined using the treasury‑stock and if‑converted methods. For purposes of the diluted net loss per share, unvested restricted common stock and outstanding stock options are considered to be potentially dilutive securities, but are excluded from the calculation of diluted net loss per share because their effect would be anti‑dilutive and therefore, basic and diluted net loss per share were the same for all periods presented. The following table sets forth the outstanding potentially dilutive securities that have been excluded in the calculation of diluted net loss per share because to do so would be anti‑dilutive: As of December 31, 2018 2017 2016 Unvested restricted common stock 235,294 557,979 1,167,984 Outstanding stock options 4,225,152 3,143,566 1,871,237 Total 4,460,446 3,701,545 3,039,221
Concentration of Credit Risk and Off-Balance Sheet Risk	Concentrations of Credit Risk and Off‑Balance Sheet Risk The Company has no financial instruments with off‑balance sheet risk such as foreign exchange contracts, option contracts or other foreign currency hedging arrangements. Financial instruments that potentially subject the Company to a concentration of credit risk are cash and cash equivalents. The Company’s cash is held in accounts at a financial institution that may exceed federally insured limits. The Company has not experienced any credit losses in such accounts and does not believe it is exposed to any significant credit risk on these funds.
Concentration of Suppliers	Concentration of Suppliers The Company is dependent on a third‑party manufacturer to supply certain products for research and development activities in its programs. In particular, the Company relies on a sole manufacturer to supply it with specific vectors related to the Company’s research and development programs.
Segment Information	Segment Information Operating segments are defined as components of an enterprise about which separate discrete information is available for evaluation by the chief operating decision maker in deciding how to allocate resources and assess performance. The Company and the Company’s chief operating decision maker, the Company’s Chief Executive Officer, views the Company’s operations and manages its business as a single operating segment, which is the business of developing and commercializing gene therapies.
Recent Accounting Pronouncements	Recently Adopted Accounting Pronouncements In January 2016, the Financial Instruments - Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities (“Update No. 2016-01”). The purpose of Update No. 2016-01 is to improve financial reporting for financial instruments by reducing the number of items recorded to other comprehensive income. The Company adopted Update No. 2016-01 in the first quarter of 2018, using the modified retrospective method. Unrealized gains and losses previously recorded to other comprehensive income (loss) were reclassified to accumulated deficit and all future fair value changes will be recorded to other income (loss). The adoption of the standard on January 1, 2018 did not have a material impact on the Company’s consolidated financial statements. In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (“ASU 2016-15”), which simplifies certain elements of cash flow classification and is intended to reduce diversity in practice in how certain transactions are classified in the statement of cash flows. The update is effective for fiscal years, and interim periods within those years, beginning after December 15, 2017. The Company adopted ASU 2016-15 on January 1, 2018, and such adoption did not have a material impact on the Company's consolidated financial statements. In November 2016, the FASB issued ASU No. 2016-18, Restricted Cash (“ASU 2016-18”). The amendments in ASU 2016-18 require an entity to reconcile and explain the period-over-period change in total cash, cash equivalents and restricted cash within its statements of cash flows and was adopted utilizing a full retrospective approach. The Company adopted the new standard on January 1, 2018. The Company has included the necessary reconciliation within Note 2 “Restricted Cash”. In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (“ASU 2014-09”), which supersedes the revenue recognition requirements in ASC 605, Revenue Recognition (“ASC 605”), and most industry-specific guidance. The new standard requires that an entity recognize revenue to depict the transfer of 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 update also requires additional disclosure about the nature, amount, timing, and uncertainty of revenue and cash flows arising from customer contracts, including significant judgments and changes in judgments and assets recognized from costs incurred to obtain or fulfill a contract. Thereafter, a series of clarifying ASUs, narrow scope improvements and practical expedients were issued. This collective guidance resulted in the new revenue standard, ASC 606. The Company adopted the new standard effective January 1, 2018 using the modified retrospective approach. The Company had one open contract, relating to the Sanofi Genzyme Collaboration, on the adoption date and has assessed it under the new revenue standard. The adoption of ASC 606 resulted in the changes to (i) the allocation of arrangement consideration, including the determination of estimated selling price and the allocation of variable consideration to specific performance obligations and (ii) the application of proportional performance as a measure of progress on service-related deliverables. The Company has accounted for the impact of adopting ASC 606 as a cumulative catch-up under the modified retrospective approach, which is represented as an increase of $20.0 million to deferred revenue with an offset to accumulated deficit, effective January 1, 2018. The following financial statement line items have been shown to reflect comparative balances under ASC 606 and ASC 605 for the year ended December 31, 2018, for both of the Sanofi Genzyme Collaboration and AbbVie Tau Collaboration, collectively. Condensed Consolidated Statements of Operations and Comprehensive Loss Year ended December 31, 2018 Under ASC 606 Under ASC 605 Effect of change (in thousands, except per share data) Collaboration revenue $ 7,619 $ 11,095 $ (3,476) Loss before income taxes (88,468) (84,992) (3,476) Net loss (88,288) (84,812) (3,476) Net loss per share, basic and diluted (2.75) (2.64) (0.11) Condensed Consolidated Balance Sheets As of December 31, 2018 Under ASC 606 Under ASC 605 Effect of change (in thousands) Deferred revenue, current $ 20,847 $ 19,111 $ 1,736 Deferred revenue, non-current 92,199 70,529 21,670 Accumulated deficit (269,051) (245,645) (23,406) Condensed Consolidated Statements of Cash Flows Year ended December 31, 2018 Under ASC 606 Under ASC 605 Effect of change (in thousands) Net loss $ (88,288) $ (84,812) $ (3,476) Adjustments to reconcile net loss to net cash used in operating activities: Deferred revenue 61,380 57,904 3,476 Recent Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, a comprehensive new lease accounting standard, which provides revised guidance on accounting for lease arrangements by both lessors and lessees and requires lessees to recognize a lease liability and a right-of-use asset for most leases. In July 2018, the FASB issued ASU No. 2018-11, Leases (Topic 842), Targeted Improvements , which provides an additional transition method that allows entities to initially apply the new lease requirements at the adoption date, not the earliest period presented, and recognize a cumulative effect adjustment to the opening balance of retained earnings in the period of adoption. The Company expects to elect this transition method at the adoption date of January 1, 2019. The Company also expects to elect a package of practical expedients, under which an entity need not reassess whether any expired or existing contracts are or contain leases, the lease classification for any expired or existing leases, or initial direct costs for any existing leases. The Company is currently in the process of evaluating the impact that the adoption of this guidance will have on its consolidated financial statements and related disclosures. On December 22, 2017, the SEC staff issued Staff Accounting Bulletin No. 118, to address the application of GAAP in situations when a registrant does not have the necessary information available, prepared, or analyzed (including computations) in reasonable detail to complete the accounting for certain income tax effects of the Tax Cuts and Jobs Act (the “Tax Reform Act”). The Company has recognized the provisional tax impacts related to the revaluation of the deferred tax assets and liabilities and included these amounts in its consolidated financial statements for the year ended December 31, 2017. The ultimate impact may differ from these provisional amounts due to, among other things, additional analysis, changes in interpretations and assumptions the Company has made, additional regulatory guidance that may be issued, and actions the Company may take as a result of the Tax Reform Act. The accounting was completed when the Company’s 2017 U.S. corporate income tax return was filed in 2018, and no material differences arose as compared to provisional amounts initially reflected in the consolidated financial statements for the year ended December 31, 2017. In June 2018, the FASB issued ASU No. 2018-07, Compensation-Stock Compensation: Improvements to Nonemployee Share-Based Payment Accounting (“ASC 718”). The new standard largely aligns the accounting for share-based payment awards issued to employees and nonemployees by expanding the scope of ASC 718 to apply to nonemployee share-based transactions, as long as the transaction is not effectively a form of financing. The new guidance will be effective for the Company on January 1, 2019. The Company is currently evaluating the potential impact that this guidance may have on its consolidated financial statements. In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Requirements for Fair Value Measurement (“ASU 2018-13”). The new standard added, modified or removed disclosure requirements under Topic 820 for clarity and consistency. ASU 2018-13 is effective for all entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. The Company is currently evaluating the potential impact that this guidance may have on its consolidated financial statements.

Summary of significant accoun_3

Summary of significant accounting policies (Tables)	12 Months Ended
	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cash, cash equivalents and marketable securities	Cash, cash equivalents, and marketable securities as of December 31, 2018 and 2017 consist of the following: Amortized Unrealized Unrealized Fair Cost Gains Losses Value (in thousands) As of December 31, 2018 Money market funds included in cash and cash equivalents $ 46,173 $ — $ — $ 46,173 Marketable securities: U.S. Treasury notes 108,951 1 5 108,947 Equity securities 1,220 — 592 628 Total marketable securities $ 110,171 $ 1 $ 597 $ 109,575 Total money market funds and marketable securities $ 156,344 $ 1 $ 597 $ 155,748 As of December 31, 2017 Money market funds included in cash and cash equivalents $ 30,469 $ — $ — $ 30,469 Marketable securities: U.S. Treasury notes 137,560 — 38 137,522 Equity securities 1,220 — 120 1,100 Total marketable securities $ 138,780 $ — $ 158 $ 138,622 Total money market funds and marketable securities $ 169,249 $ — $ 158 $ 169,091
Reconciliation of cash, cash equivalents, and restricted cash	As of December 31, 2018 2017 2016 (in thousands) Cash and cash equivalents $ 46,859 $ 31,530 $ 36,641 Restricted cash included in deposits and other noncurrent assets 735 735 735 Total cash, cash equivalents, and restricted cash $ 47,594 $ 32,265 $ 37,376
Outstanding potentially dilutive securities excluded in the calculation of diluted net loss per share	The following table sets forth the outstanding potentially dilutive securities that have been excluded in the calculation of diluted net loss per share because to do so would be anti‑dilutive: As of December 31, 2018 2017 2016 Unvested restricted common stock 235,294 557,979 1,167,984 Outstanding stock options 4,225,152 3,143,566 1,871,237 Total 4,460,446 3,701,545 3,039,221
ASU 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Schedule of impact of adoption of financial statement line items shows comparative balances under ASC 606 and ASC 605	Condensed Consolidated Statements of Operations and Comprehensive Loss Year ended December 31, 2018 Under ASC 606 Under ASC 605 Effect of change (in thousands, except per share data) Collaboration revenue $ 7,619 $ 11,095 $ (3,476) Loss before income taxes (88,468) (84,992) (3,476) Net loss (88,288) (84,812) (3,476) Net loss per share, basic and diluted (2.75) (2.64) (0.11) Condensed Consolidated Balance Sheets As of December 31, 2018 Under ASC 606 Under ASC 605 Effect of change (in thousands) Deferred revenue, current $ 20,847 $ 19,111 $ 1,736 Deferred revenue, non-current 92,199 70,529 21,670 Accumulated deficit (269,051) (245,645) (23,406) Condensed Consolidated Statements of Cash Flows Year ended December 31, 2018 Under ASC 606 Under ASC 605 Effect of change (in thousands) Net loss $ (88,288) $ (84,812) $ (3,476) Adjustments to reconcile net loss to net cash used in operating activities: Deferred revenue 61,380 57,904 3,476

Fair value measurements (Tables

Fair value measurements (Tables)	12 Months Ended
	Dec. 31, 2018
Fair value measurements
Assets and liabilities measured at fair value on a recurring basis	Assets and liabilities measured at fair value on a recurring basis as of December 31, 2018 and 2017 are as follows: Quoted Prices Significant in Active Other Significant Markets for Observable Unobservable Identical Assets Inputs Inputs Assets Total (Level 1) (Level 2) (Level 3) (in thousands) December 31, 2018 Money market funds included in cash and cash equivalents $ 46,173 $ 46,173 $ — $ — Marketable securities: U.S. Treasury notes 108,947 108,947 — — Equity securities 628 628 — — Total marketable securities $ 109,575 $ 109,575 $ — $ — Warrants to purchase equity securities 234 — 234 — Total $ 155,982 $ 155,748 $ 234 $ — December 31, 2017 Money market funds included in cash and cash equivalents $ 30,469 $ 30,469 $ — $ — Marketable securities: U.S. Treasury notes 137,522 137,522 — — Equity securities 1,100 1,100 — — Total marketable securities $ 138,622 $ 138,622 $ — $ — Warrants to purchase equity securities 569 — 569 — Total $ 169,660 $ 169,091 $ 569 $ —
Schedule of fair value assumptions	As of December 31, 2018 2017 2016 Risk-free interest rate 2.5 % 2.0 % 1.8 % Expected dividend yield — % — % — % Expected term (in years) 2.7 3.7 4.7 Expected volatility 112.7 % 103.5 % 97.5 %

Prepaid expenses and other cu_2

Prepaid expenses and other current assets (Tables)	12 Months Ended
	Dec. 31, 2018
Prepaid expense and other current assets
Schedule of prepaid expense and other current assets	Prepaid expenses and other current assets consist of the following: As of December 31, 2018 2017 (in thousands) Prepaid research and development contracts $ 4,497 $ 1,330 Other current assets 1,360 766 Prepaid insurance 617 520 Accrued interest receivable 201 122 Total $ 6,675 $ 2,738

Property and equipment, net (Ta

Property and equipment, net (Tables)	12 Months Ended
	Dec. 31, 2018
Property and equipment, net
Schedule of property and equipment, net	Property and equipment, net consists of the following: As of December 31, 2018 2017 (in thousands) Leasehold improvements $ 7,035 $ 6,421 Laboratory equipment 8,843 5,262 Furniture and office equipment 1,675 1,565 Other 306 25 Construction in progress 19 — Total property and equipment 17,878 13,273 Less: accumulated depreciation (5,107) (2,990) Property and equipment, net $ 12,771 $ 10,283

Accrued expenses (Tables)

Accrued expenses (Tables)	12 Months Ended
	Dec. 31, 2018
Accrued expenses
Schedule of accrued expenses	Accrued expenses consist of the following: As of December 31, 2018 2017 (in thousands) Employee compensation costs $ 3,780 $ 3,383 Research and development costs 3,555 5,780 Professional services 1,448 1,762 Accrued goods and services 784 388 Patent costs 120 120 Other 101 64 Total $ 9,788 $ 11,497

Commitments and contingencies (

Commitments and contingencies (Tables)	12 Months Ended
	Dec. 31, 2018
Commitments and contingencies
Future annual minimum lease payments	Future annual minimum lease payments at December 31, 2018 are as follows: Total Minimum Lease Payments (in thousands) 2019 4,325 2020 4,731 2021 4,863 2022 5,001 2023 5,141 2024+ 16,783 $ 40,844
Allocable arrangement consideration allocation	Unit of Accounting Amount (in thousands) Research and Development Services for: Huntington’s Program $ 14,228 Parkinson’s Program 6,040 Friedreich’s ataxia Program 14,821 Spinal Muscular Atrophy Program 29,116 Future Program 2,239 Committee Obligations: AJSC 133 DAC 207 PD Material Right 7,855 Total $ 74,639
Schedule of allocation of transaction price	Performance Obligation Amount (in thousands) Research Services $ 34,482 First Development Option Material Right 34,518 Total $ 69,000

Common stock (Tables)

Common stock (Tables)	12 Months Ended
	Dec. 31, 2018
Common stock
Schedule of shares reserved for future issuance	Shares Reserved For Future Issuance As of December 31, 2018 2017 Shares reserved for vesting of restricted stock awards under the Founder Agreements 235,294 366,914 Shares reserved for vesting of restricted stock awards under the 2014 Option and Stock Plan — 191,065 Shares reserved for exercise of outstanding stock options 4,225,152 3,143,566 Shares reserved for issuances under the 2015 Stock Option Plan 1,973,227 1,501,005 Shares reserved for issuances under the 2015 Employee Stock Purchase Plan 963,386 730,860 7,397,059 5,933,410

Stock based compensation (Table

Stock based compensation (Tables)	12 Months Ended
	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation cost recognized for all stock-based compensation awards	Year ended December 31, 2018 2017 2016 (in thousands) Research and development $ 4,717 $ 5,367 $ 4,296 General and administrative 10,993 3,871 2,014 Total stock-compensation expense $ 15,710 $ 9,238 $ 6,310
Summary of status and changes in unvested restricted stock	Weighted Average Grant Date Fair Value Shares Per Share Unvested restricted common stock as of December 31, 2017 557,979 $ 0.70 Issued — Vested (319,891) $ 0.84 Repurchased (2,794) $ 1.11 Unvested restricted common stock as of December 31, 2018 235,294 $ 0.51
Summary of status and changes in stock options	Weighted Remaining Aggregate Average Contractual Intrinsic Exercise Life Value Shares Price (in years) (in thousands) Outstanding at December 31, 2017 3,143,566 $ 11.82 Granted 2,215,891 $ 20.78 Exercised (384,186) $ 10.13 Cancelled or forfeited (750,119) $ 16.13 Outstanding at December 31, 2018 4,225,152 $ 15.91 8.4 $ 578 Exercisable at December 31, 2018 1,531,832 $ 12.55 7.3 $ 507 Vested and expected to vest at December 31, 2018 4,225,152 $ 15.91 8.4 $ 578
Employees and directors stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Assumptions	The fair value of each option issued to employees and directors was estimated at the date of grant using the Black‑Scholes option pricing model with the following weighted‑average assumptions: Year ended December 31, 2018 2017 2016 Risk-free interest rate 2.8 % 2.0 % 1.5 % Expected dividend yield — % — % — % Expected term (in years) 6.0 6.0 6.0 Expected volatility 74.4 % 73.7 % 73.1 %
Non-employees stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Assumptions	As of December 31, 2018 2017 2016 Risk-free interest rate 2.6 % 2.4 % 2.1 % Expected dividend yield — % — % — % Expected term (in years) 7.6 8.5 9.1 Expected volatility 73.2 % 76.2 % 83.3 %

Income taxes (Tables)

Income taxes (Tables)	12 Months Ended
	Dec. 31, 2018
Income taxes
Schedule of components of income expense (benefit)	Year ended December 31, 2018 2017 (in thousands) Current Federal $ 180 $ — State — — Total current 180 — Deferred Federal — — State — — Total deferred — — Total tax expense $ 180 $ —
Schedule of reconciliation of the expected income tax (benefit) computed using the federal statutory income tax rate to the effective income tax rate	A reconciliation of the expected income tax (benefit) computed using the federal statutory income tax rate at the Company’s effective tax rate is as follows: Year ended December 31, 2018 2017 2016 Income tax computed at federal statutory tax rate 21.0 % 34.0 % 34.0 % State taxes, net of federal benefit 6.3 % 6.1 % 5.6 % General business credit carryovers 3.1 % 5.0 % 4.2 % Non-deductible expenses (2.1) % (4.1) % (4.0) % Deferred rate change — % (21.8) % — % Change in valuation allowance (28.1) % (19.2) % (40.2) % Total 0.2 % — % (0.4) %
Schedule of principal components of the Company’s deferred tax assets and liabilities	Year ended December 31, 2018 2017 (in thousands) Deferred tax assets: Net operating loss carryforwards $ 44,556 $ 24,642 Tax credit carryforwards 12,021 8,832 Deferred rent 1,560 1,458 Deferred revenue 13,926 8,622 Non-deductible accruals and reserves 1,105 817 Intangibles 930 832 Stock compensation 2,802 1,361 Total deferred tax assets 76,900 46,564 Less valuation allowance (75,213) (44,953) Net deferred tax assets 1,687 1,611 Deferred tax liabilities Depreciation and amortization (1,687) (1,611) Net deferred taxes $ — $ —

Selected quarterly financial _2

Selected quarterly financial data (unaudited) (Tables)	12 Months Ended
	Dec. 31, 2018
Selected quarterly financial data (unaudited)
Schedule of Quarterly Financial Information	2018 First Second Third Fourth Quarter Quarter Quarter Quarter Total (amounts in thousands, except per share data) Collaboration revenue $ 942 $ 2,575 $ 2,094 $ 2,008 $ 7,619 Total operating expenses 22,035 28,269 23,241 25,169 98,714 Loss from operations (21,093) (25,694) (21,147) (23,161) (91,095) Net loss (19,926) (25,541) (20,289) (22,532) (88,288) Net loss per share $ (0.63) $ (0.80) $ (0.63) $ (0.70) $ (2.75) 2017 First Second Third Fourth Quarter Quarter Quarter Quarter Total (amounts in thousands, except per share data) Collaboration revenue $ 1,464 $ 1,177 $ 1,148 $ 6,346 $ 10,135 Total operating expenses 18,986 19,816 24,503 18,693 81,998 Loss from operations (17,522) (18,639) (23,355) (12,347) (71,863) Net loss (16,648) (18,876) (23,346) (11,828) (70,698) Net loss per share $ (0.65) $ (0.73) $ (0.89) $ (0.40) $ (2.64)

Nature of business (Details)

Nature of business (Details) - USD ($) $ in Thousands	1 Months Ended
	Feb. 28, 2019	Jan. 31, 2019	Feb. 28, 2018	Dec. 31, 2018	Dec. 31, 2017
Accumulated deficit				$ (269,051)	$ (160,713)
Neurocrine | Forecast | Collaborative Arrangement
Non-refundable upfront payment received		$ 165,000
Proceeds raised		$ 50,000
Shares from collaboration		4,179,728
AbbVie | Collaborative Arrangement
Non-refundable upfront payment received			$ 69,000
AbbVie | Forecast | Collaborative Arrangement
Non-refundable upfront payment received	$ 65,000

Summary of significant accoun_4

Summary of significant accounting policies - Basis of Presentation - (Details) $ / shares in Units, $ in Millions	Nov. 07, 2017USD ($)$ / sharesshares	Nov. 16, 2015USD ($)$ / sharesshares	Oct. 29, 2015	Dec. 31, 2018shares	Dec. 31, 2017shares
Common stock sales | shares	5,175,000			32,364,895	31,572,044
Price per share | $ / shares	$ 12
Net proceeds | $	$ 58
Reverse split ratio			0.235
IPO
Common stock sales | shares		5,750,000
Price per share | $ / shares		$ 14
Net proceeds | $		$ 72.9

Summary of significant accoun_5

Summary of significant accounting policies - Marketable Securities - (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Amortized Cost
Cash and cash equivalents	$ 46,859,000	$ 31,530,000	$ 36,641,000
Total marketable securities, amortized cost	110,171,000	138,780,000
Total money market funds and marketable securities	156,344,000	169,249,000
Fair Value
Total marketable securities	109,575,000	138,622,000
Total money market funds and marketable securities	155,748,000	169,091,000
Unrealized Gains and Losses
Total marketable securities, unrealized gains	1,000
Total money market funds and marketable securities	1,000
Total marketable securities, unrealized losses	597,000	158,000
Total money market funds and marketable securities	597,000	$ 158,000
Other than temporary losses	$ 0
Maximum
Unrealized Gains and Losses
Debt securities, contractual maturity period	1 year	1 year
Money market funds
Amortized Cost
Cash and cash equivalents	$ 46,173,000	$ 30,469,000
Fair Value
Cash and cash equivalents	46,173,000	30,469,000
U.S. Treasury notes
Amortized Cost
Marketable securities, amortized cost	108,951,000	137,560,000
Fair Value
Marketable debt securities, fair value	108,947,000	137,522,000
Unrealized Gains and Losses
Marketable securities, unrealized gains	1,000
Marketable securities, unrealized losses	5,000	38,000
Equity securities
Amortized Cost
Marketable equity securities, amortized cost	1,220,000	1,220,000
Fair Value
Marketable equity securities, fair value	628,000	1,100,000
Unrealized Gains and Losses
Equity securitiess, unrealized losses	$ 592,000	$ 120,000

Summary of significant accoun_6

Summary of significant accounting policies - Restricted Cash - (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Summary of significant accounting policies
Cash and cash equivalents	$ 46,859	$ 31,530	$ 36,641
Restricted cash included in deposits and other noncurrent assets	735	735	735
Total cash, cash equivalents, and restricted cash	$ 47,594	$ 32,265	$ 37,376	$ 31,623

Summary of significant accoun_7

Summary of significant accounting policies - Stock Based Compensation Expense - (Details)	12 Months Ended
	Dec. 31, 2018
Summary of significant accounting policies
Expected dividend yield	0.00%

Summary of significant accoun_8

Summary of significant accounting policies - Net loss per share - (Details) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities	4,460,446	3,701,545	3,039,221
Unvested restricted stock awards
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities	235,294	557,979	1,167,984
Stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities	4,225,152	3,143,566	1,871,237

Summary of significant accoun_9

Summary of significant accounting policies - ASU (Details) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018USD ($)$ / shares	Sep. 30, 2018USD ($)$ / shares	Jun. 30, 2018USD ($)$ / shares	Mar. 31, 2018USD ($)$ / shares	Dec. 31, 2017USD ($)$ / shares	Sep. 30, 2017USD ($)$ / shares	Jun. 30, 2017USD ($)$ / shares	Mar. 31, 2017USD ($)$ / shares	Dec. 31, 2018USD ($)item$ / shares	Dec. 31, 2017USD ($)$ / shares	Dec. 31, 2016USD ($)$ / shares
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of open contracts | item									1
Increase to deferred revenue with an offset to accumulated deficit									$ 20,050
Consolidated Statements of Operations and Comprehensive Loss
Collaboration revenue	$ 2,008	$ 2,094	$ 2,575	$ 942	$ 6,346	$ 1,148	$ 1,177	$ 1,464	7,619	$ 10,135	$ 14,220
Loss before income taxes									(88,468)	(70,698)	(40,141)
Net loss	$ (22,532)	$ (20,289)	$ (25,541)	$ (19,926)	$ (11,828)	$ (23,346)	$ (18,876)	$ (16,648)	$ (88,288)	$ (70,698)	$ (40,193)
Net loss per share, basic and diluted | $ / shares	$ (0.70)	$ (0.63)	$ (0.80)	$ (0.63)	$ (0.40)	$ (0.89)	$ (0.73)	$ (0.65)	$ (2.75)	$ (2.64)	$ (1.59)
Consolidated Balance Sheets
Deferred revenue, current	$ 20,847				$ 3,380				$ 20,847	$ 3,380
Deferred revenue, non-current	92,199				28,180				92,199	28,180
Accumulated deficit	(269,051)				$ (160,713)				(269,051)	(160,713)
Consolidated Statement of Cash Flows
Net loss									(88,288)	(70,698)	$ (40,193)
Adjustments to reconcile net loss to net cash provided by operating activities:
Deferred revenue									61,380	$ (10,135)	$ (14,582)
ASU 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase to deferred revenue with an offset to accumulated deficit									(19,930)
ASU 2014-09 | Under ASC 605
Consolidated Statements of Operations and Comprehensive Loss
Collaboration revenue									11,095
Loss before income taxes									(84,992)
Net loss									$ (84,812)
Net loss per share, basic and diluted | $ / shares									$ (2.64)
Consolidated Balance Sheets
Deferred revenue, current	19,111								$ 19,111
Deferred revenue, non-current	70,529								70,529
Accumulated deficit	(245,645)								(245,645)
Consolidated Statement of Cash Flows
Net loss									(84,812)
Adjustments to reconcile net loss to net cash provided by operating activities:
Deferred revenue									57,904
ASU 2014-09 | Effect of change
Consolidated Statements of Operations and Comprehensive Loss
Collaboration revenue									(3,476)
Loss before income taxes									(3,476)
Net loss									$ (3,476)
Net loss per share, basic and diluted | $ / shares									$ (0.11)
Consolidated Balance Sheets
Deferred revenue, current	1,736								$ 1,736
Deferred revenue, non-current	21,670								21,670
Accumulated deficit	$ (23,406)								(23,406)
Consolidated Statement of Cash Flows
Net loss									(3,476)
Adjustments to reconcile net loss to net cash provided by operating activities:
Deferred revenue									$ 3,476

Fair value measurements - Asset

Fair value measurements - Assets measured on a recurring basis - (Details) $ in Thousands	Dec. 31, 2018USD ($)item	Dec. 31, 2017USD ($)item	Dec. 31, 2016USD ($)item
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	$ 46,859	$ 31,530	$ 36,641
Measurement Input, Risk-Free Interest Rate
Fair value assumptions
Warrants, measurement input | item	0.025	0.020	0.018
Measurement Input, Expected Term
Fair value assumptions
Warrants, measurement input | item	2.700	3.700	4.700
Measurement Input, Expected Volatility
Fair value assumptions
Warrants, measurement input | item	1.127	1.035	0.975
Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Marketable securities	$ 109,575	$ 138,622
Total assets measured at fair value	155,982	169,660
Level 1 | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Marketable securities	109,575	138,622
Total assets measured at fair value	155,748	169,091
Level 2 | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Total assets measured at fair value	234	569
U.S. Treasury notes | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Marketable securities	108,947	137,522
U.S. Treasury notes | Level 1 | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Marketable securities	108,947	137,522
Equity securities | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Marketable securities	628	1,100
Equity securities | Level 1 | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Marketable securities	628	1,100
Warrants to purchase equity securities | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Warrants to purchase equity securities	234	569
Warrants to purchase equity securities | Level 2 | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Warrants to purchase equity securities	234	569
Money market funds
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	46,173	30,469
Money market funds | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	46,173	30,469
Money market funds | Level 1 | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	$ 46,173	$ 30,469

Prepaid expenses and other cu_3

Prepaid expenses and other current assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Prepaid expense and other current assets
Prepaid research and development contracts	$ 4,497	$ 1,330
Other current assets	1,360	766
Prepaid insurance	617	520
Accrued interest receivable	201	122
Total	$ 6,675	$ 2,738

Property and equipment, net (De

Property and equipment, net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 17,878	$ 13,273
Less: accumulated depreciation	(5,107)	(2,990)
Property and equipment, net	12,771	10,283
Depreciation expense	2,117	1,595	$ 612
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	7,035	6,421
Laboratory equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	8,843	5,262
Furniture and office equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	1,675	1,565
Other
Property, Plant and Equipment [Line Items]
Property and equipment, gross	306	$ 25
Construction in progress
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 19

Accrued expenses (Details)

Accrued expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accrued expenses
Employee compensation costs	$ 3,780	$ 3,383
Research and development costs	3,555	5,780
Professional services	1,448	1,762
Accrued goods and services	784	388
Patent costs	120	120
Other	101	64
Total	$ 9,788	$ 11,497

Commitments and contingencies -

Commitments and contingencies - Operating Leases - (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Mar. 31, 2014item	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Rent expense		$ 4,000	$ 2,900	$ 2,000
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2,019		4,325
2,020		4,731
2,021		4,863
2,022		5,001
2,023		5,141
2024 +		16,783
Total minimum lease payments		40,844
75 Sidney Street
Number of renewal options | item	2
Leasehold improvement receivable		300
Term of each renewal option	5 years
Leasehold incentive received from landlord		1,300
64 Sidney Street
Leasehold incentive received from landlord		$ 3,500

Commitments and contingencies_2

Commitments and contingencies - Significant Agreements - (Details) - Sanofi Genzyme - USD ($) $ in Millions	1 Months Ended
	Oct. 31, 2017	Feb. 28, 2015
Regulatory approval milestone
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Aggregate maximum milestone payments to be received from collaborative partner	$ 220
Regulatory approval milestone | Minimum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Per Milestone, maximum milestone payments to be received from collaborative partner	40
Regulatory approval milestone | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Per Milestone, maximum milestone payments to be received from collaborative partner	50
Commercial Milestone
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Aggregate maximum milestone payments to be received from collaborative partner	320
Commercial Milestone | Minimum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Per Milestone, maximum milestone payments to be received from collaborative partner	50
Commercial Milestone | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Per Milestone, maximum milestone payments to be received from collaborative partner	$ 100
Collaborative Arrangement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Non-refundable upfront payment received		$ 65
Premium over fair value on temporary equity issued		5
Amount of reduction in milestone or royalty payments to be made by collaborative partner for each program specified		10
Per Milestone, maximum milestone payments to be received from collaborative partner		540
Parkinson's Program | Regulatory and commercial milestone
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments no longer receivable		105
Spinal Muscular Atrophy Program | Collaborative Arrangement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Maximum potential in-kind services to be provided by collaborative partner		5
Option exercise payment required from collaborative partner for each program specified		30
Split Territory Program | Collaborative Arrangement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Option exercise payment required from collaborative partner for each program specified		20
Redeemable Convertible Preferred Stock Series B | Collaborative Arrangement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Non-refundable upfront payment received		$ 30
Temporary equity issued (in shares)		10,000,000
Fair value of temporary equity issued		$ 25

Commitments and contingencies_3

Commitments and contingencies - Accounting Analysis - (Details) $ in Thousands	1 Months Ended		12 Months Ended
	Feb. 28, 2018USD ($)	Feb. 28, 2015USD ($)	Dec. 31, 2018USD ($)item	Dec. 31, 2017USD ($)item	Dec. 31, 2016USD ($)
Allocable arrangement consideration allocation
Committee Obligations: Alliance Joint Steering Committee ("AJSC")		$ 133
Committee Obligations: Development Advisory Committee ("DAC")		207
PD Material Right		7,855
Total allocable arrangement consideration allocations.		74,639
Huntington's Program
Allocable arrangement consideration allocation
Research and development services		14,228
Parkinson's Program
Allocable arrangement consideration allocation
Research and development services		6,040
Friedreich's Ataxia Program
Allocable arrangement consideration allocation
Research and development services		14,821
Spinal Muscular Atrophy Program
Allocable arrangement consideration allocation
Research and development services		29,116
Future Program
Allocable arrangement consideration allocation
Research and development services		2,239
Sanofi Genzyme
Income Statement Related Disclosures [Abstract]
Revenue recognized				$ 7,800
Number of accounting units | item				2
Sanofi Genzyme | Spinal Muscular Atrophy Program
Income Statement Related Disclosures [Abstract]
Revenue adjustment due to development deprioritization				$ 9,500
Sanofi Genzyme | PD Material Right Program
Income Statement Related Disclosures [Abstract]
Revenue adjustment due to development deprioritization				2,100
Sanofi Genzyme | Collaborative Arrangement
Allocable arrangement consideration
Collaborative Arrangement Nonredeemable Up Front Fee Received		65,000
Additional consideration, premium over fair value on temporary equity issued		5,000
In-kind services and other funding to be provided by collaborative partner		4,600
Total allocable arrangement consideration		74,600
Allocable arrangement consideration allocation
Additional option exercise payments | item			0
Income Statement Related Disclosures [Abstract]
Revenue recognized			$ 700	$ 10,100	$ 14,200
Deferred revenue			50,900
AbbVie | Collaborative Arrangement
Allocable arrangement consideration
Collaborative Arrangement Nonredeemable Up Front Fee Received	$ 69,000
Allocable arrangement consideration allocation
Research and development services			34,482
First Development Option Material Right			34,518
Total allocable arrangement consideration allocations.			69,000
Income Statement Related Disclosures [Abstract]
Revenue recognized	$ 6,900
Deferred revenue			$ 62,100
Redeemable Convertible Preferred Stock Series B | Sanofi Genzyme | Collaborative Arrangement
Allocable arrangement consideration
Collaborative Arrangement Nonredeemable Up Front Fee Received		$ 30,000

Commitments and contingencies_4

Commitments and contingencies - AbbVie Collaboration Agreement - (Details) $ in Thousands	1 Months Ended		12 Months Ended
	Feb. 28, 2018USD ($)item	Feb. 28, 2015USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
AbbVie | Minimum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of development options | item	1
Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue recognized				$ 7,800
Collaborative Arrangement | AbbVie
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Non-refundable upfront payment received	$ 69,000
First Research Compound payment	80,000
Two additional research compounds payment to be received for each	$ 30,000
Number of additional research compounds | item	2
One-time payment upon exercise of license option	$ 75,000
Milestone payments to be received for licensed compound in case Alzheimer disease indication	550,000
Milestone payments to be received for licensed compound in case first indication other than Alzheimer disease	230,000
Milestone payments to be received for licensed compound for subsequent non-Alzheimer disease indication	$ 115,000
Number of research antibodies | item	1
The number of Licensed Products | item	1
Deferred revenue			$ 62,100
Revenue recognized	$ 6,900
Collaborative Arrangement | AbbVie | Minimum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Antibodies to include in Research Period | item	2
Collaborative Arrangement | AbbVie | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of antibodies encoded by virus | item	5
Antibodies to include in Research Period | item	3
Collaborative Arrangement | Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Non-refundable upfront payment received		$ 65,000
Deferred revenue			50,900
Revenue recognized			$ 700	$ 10,100	$ 14,200

Commitments and contingencies_5

Commitments and contingencies - Other Agreements - (Details)	12 Months Ended
	Dec. 31, 2018USD ($)item	Dec. 31, 2017USD ($)item	Dec. 31, 2016USD ($)multiple	Sep. 30, 2016USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Patent costs incurred	$ 600,000
Loss Contingency [Abstract]
Contingency reserves for litigation liabilities	0
General and administrative
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Patent costs incurred	$ 600,000	$ 800,000	$ 1,800,000
Various contract research organizations and institutions
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of milestones achieved | item	0	0
Various contract research organizations and institutions | Minimum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of days written notice required by reporting entity to terminate agreement	30 days
Various contract research organizations and institutions | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of days written notice required by reporting entity to terminate agreement	90 days
Non-profit organization agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Maximum funding			$ 4,000,000
Funding multiple | multiple			2.60
Milestone payment liability		$ 1,000,000
MRIC
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amount of investment				$ 2,000,000

Preferred stock (Details)

Preferred stock (Details) - shares	Dec. 31, 2018	Dec. 31, 2017
Preferred stock
Preferred stock, authorized	5,000,000	5,000,000

Common stock (Details)

Common stock (Details)	12 Months Ended
	Dec. 31, 2018Vote$ / sharesshares	Dec. 31, 2017$ / sharesshares	Oct. 31, 2015shares
Class of Stock [Line Items]
Common Stock, Shares Authorized	120,000,000	120,000,000
Common stock, par value (in dollars per share) | $ / shares	$ 0.001	$ 0.001
Number of votes entitled for each share of common stock | Vote	1
Dividends declared | $ / shares	$ 0
Dividends paid | $ / shares	$ 0
Shares available for future issuance (in shares)	7,397,059	5,933,410
2015 Stock Option Plan
Class of Stock [Line Items]
Shares available for future issuance (in shares)	1,973,227		1,311,812
Vesting of restricted stock awards | Founder Awards
Class of Stock [Line Items]
Shares available for future issuance (in shares)	235,294	366,914
Vesting of restricted stock awards | 2014 Stock Option and Stock Plan
Class of Stock [Line Items]
Shares available for future issuance (in shares)		191,065
Exercise of stock options
Class of Stock [Line Items]
Shares available for future issuance (in shares)	4,225,152	3,143,566
Issuance under share based compensation plans
Class of Stock [Line Items]
Shares available for future issuance (in shares)	963,386	730,860
Issuance under share based compensation plans | 2015 Stock Option Plan
Class of Stock [Line Items]
Shares available for future issuance (in shares)	1,973,227	1,501,005

Stock based compensation (Detai

Stock based compensation (Details) $ / shares in Units, $ in Thousands

Jan. 01, 2019shares

Jan. 01, 2018shares

Jan. 01, 2017shares

Jan. 01, 2016shares

Oct. 31, 2015shares

Jan. 31, 2014$ / sharesshares

Dec. 31, 2018USD ($)$ / sharesshares

Dec. 31, 2017USD ($)$ / sharesshares

Dec. 31, 2016USD ($)item

Aug. 31, 2015shares

Apr. 30, 2015shares

Aug. 31, 2014shares

Apr. 30, 2014shares

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Shares available for future issuance (in shares)

7,397,059

5,933,410

Stock-based compensation | $

$ 15,710

$ 9,238

$ 6,310

Unrecognized stock-based compensation expense | $

$ 26,100

Stock options granted (in shares)

2,215,891

Exercise price | $ / shares

$ 15.91

$ 11.82

2014 Stock Option and Grant Plan

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Number of shares authorized for stock-based compensation plan (in shares)

2,998,823

823,529

2,669,411

2,047,058

2,000,000

1,411,764

2015 Employee Stock Purchase Plan

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

New stock options issued

88,774

Number of shares authorized for stock-based compensation plan (in shares)

321,300

267,658

267,492

262,362

Period to purchase shares

6 months

2015 Employee Stock Purchase Plan | Forecast

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Number of shares authorized for stock-based compensation plan (in shares)

325,707

2015 Employee Stock Purchase Plan | Minimum

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Percentage of payroll deduction

1

2015 Employee Stock Purchase Plan | Maximum

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Percentage of payroll deduction

10

2015 Stock Option Plan

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Shares available for future issuance (in shares)

1,311,812

1,973,227

Percentage limit of issued and outstanding shares up to which additional shares may be issued

4.00%

Additional shares added to stock option plan

1,285,200

1,070,635

1,069,971

2015 Stock Option Plan | Forecast

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Additional shares added to stock option plan

1,302,830

Performance based milestone awards

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Probable performance milestones | item

1

Performance milestones | item

3

Founders restricted stock

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Stock-based compensation | $

$ 300

$ 1,400

$ 1,100

Restricted stock granted (in shares)

1,188,233

Restricted stock granted, original issued price (in dollars per share) | $ / shares

$ 0.0425

Founders restricted stock | Vesting period, tranche one

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Restricted stock granted (in shares)

835,292

Founders restricted stock | Vesting period, tranche two

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Restricted stock granted (in shares)

352,941

Founders restricted stock | Minimum | Vesting period, tranche one

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Vesting period

1 year

Founders restricted stock | Maximum | Vesting period, tranche one

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Vesting period

4 years

Employees and directors stock options

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Stock-based compensation | $

$ 14,900

5,500

3,000

Employees and directors stock options | 2014 Stock Option and Grant Plan

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Vesting period

4 years

Employees and directors stock options | 2014 Stock Option and Grant Plan | Vesting period, tranche one

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Vesting period

1 year

Vesting percentage

25.00%

Employees and directors stock options | 2014 Stock Option and Grant Plan | Vesting period, tranche two

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Vesting period

3 years

Vesting percentage

75.00%

Employees and directors stock options | 2015 Stock Option Plan

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Stock options granted (in shares)

2,215,891

Non-employees stock options

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Stock-based compensation | $

$ 300

400

200

Stock options granted (in shares)

0

Non-employees stock options | 2014 Stock Option and Grant Plan | Minimum

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Vesting period

1 year

Non-employees stock options | 2014 Stock Option and Grant Plan | Maximum

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Vesting period

4 years

Research and development

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Stock-based compensation | $

$ 4,717

5,367

4,296

General and administrative

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Stock-based compensation | $

$ 10,993

$ 3,871

$ 2,014

Stock based compensation - Rest

Stock based compensation - Restricted Stock - (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Restricted stock disclosures
Stock-based compensation	$ 15,710	$ 9,238	$ 6,310
Unrecognized stock-based compensation expense	$ 26,100
Remaining weighted-average remaining vesting period	2 years 9 months 11 days
Unvested restricted stock awards
Shares
Balance, beginning (in shares)	557,979
Vested (in shares)	(319,891)
Repurchased (in shares)	(2,794)
Balance, ending (in shares)	235,294	557,979
Weighted Average Grant Date Fair Value Per Share
Balance, beginning (in dollars per share)	$ 0.70
Vested (in dollars per share)	0.84
Repurchased (in dollars per share)	1.11
Balance, ending (in dollars per share)	$ 0.51	$ 0.70
Employees restricted stock
Restricted stock disclosures
Stock-based compensation	$ 100	$ 500	500
Non-employee consultants restricted stock
Restricted stock disclosures
Stock-based compensation	$ 400	$ 2,700	$ 2,600

Stock based compensation - Stoc

Stock based compensation - Stock Options - (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding, beginning balance (in shares)	3,143,566
Granted (in shares)	2,215,891
Exercised (in shares)	(384,186)
Cancelled or forfeited (in shares)	(750,119)
Outstanding, ending balance (in shares)	4,225,152	3,143,566
Exercisable (in shares)	1,531,832
Vested and expected to vest (in shares)	4,225,152
Weighted Average Exercise Price
Outstanding (in dollars per share)	$ 11.82
Granted (in dollars per share)	20.78
Exercised (in dollars per share)	10.13
Cancelled or forfeited (in dollars per share)	16.13
Outstanding (in dollars per share)	15.91	$ 11.82
Exercisable (in dollars per share)	12.55
Vested and expected to vest (in dollars per share)	$ 15.91
Remaining Contractual Life
Outstanding	8 years 4 months 24 days
Exercisable	7 years 3 months 18 days
Vested and expected to vest	8 years 4 months 24 days
Aggregate Intrinsic Value
Outstanding	$ 578
Exercisable	507
Vested and expected to vest	578
Options disclosures
Stock-based compensation	$ 15,710	$ 9,238	$ 6,310
Employees and directors stock options
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 13.87
Stock-based compensation	$ 14,900	5,500	3,000
Non-employees stock options
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Granted (in shares)	0
Options disclosures
Stock-based compensation	$ 300	$ 400	$ 200

Stock based compensation - Blac

Stock based compensation - Black-Scholes options pricing model - (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair value assumptions
Expected dividend yield	0.00%
Options disclosures
Stock-based compensation	$ 15,710	$ 9,238	$ 6,310
Stock options granted (in shares)	2,215,891
Unrecognized stock-based compensation expense	$ 26,100
Remaining weighted-average remaining vesting period	2 years 9 months 11 days
Employees and directors stock options
Fair value assumptions
Risk-free interest rate	2.80%	2.00%	1.50%
Expected term	6 years	6 years	6 years
Expected volatility	74.40%	73.70%	73.10%
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 13.87
Stock-based compensation	$ 14,900	$ 5,500	$ 3,000
Non-employees stock options
Fair value assumptions
Risk-free interest rate	2.60%	2.40%	2.10%
Expected term	7 years 7 months 6 days	8 years 6 months	9 years 1 month 6 days
Expected volatility	73.20%	76.20%	83.30%
Options disclosures
Stock-based compensation	$ 300	$ 400	$ 200
Stock options granted (in shares)	0

401(k) Savings Plan (Details)

401(k) Savings Plan (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
401(k) Savings Plan
Employer contribution expense	$ 0.8	$ 0.5	$ 0.3

Income taxes (Details)

Income taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2016
Current
Federal	$ 180
Total current	180
Total tax expense	$ 180	$ (52)

Income taxes - Net operating lo

Income taxes - Net operating loss carryforwards (Details) $ in Thousands	Dec. 31, 2017USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016item
Operating Loss Carryforwards [Line Items]
Valuation Allowance	$ 44,953	$ 75,213	$ 44,953
Change in the valuation allowance		$ 30,200
Number of ownership changes | item				3
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Income tax computed at federal statutory tax rate	35.00%	21.00%	34.00%	34.00%
State taxes, net of federal benefit		6.30%	6.10%	5.60%
General business credit carryovers		3.10%	5.00%	4.20%
Non‑deductible expenses		(2.10%)	(4.10%)	(4.00%)
Deferred rate change			(21.80%)
Change in valuation allowance		(28.10%)	(19.20%)	(40.20%)
Total		0.20%		(0.40%)
Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards		$ 162,900
Maximum NOL carryforwards (as a percent)		80.00%
State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards		$ 163,800
Investment Tax Credit expiring in 2018		400
Tax Year 2018 | Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards		73,100
Tax Year 2018 | State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards		72,100
Research and development tax credits carryforwards | Federal
Operating Loss Carryforwards [Line Items]
Tax credits carryforwards		9,400
Research and development tax credits carryforwards | State
Operating Loss Carryforwards [Line Items]
Tax credits carryforwards		$ 3,000

Income taxes - Deferred tax ass

Income taxes - Deferred tax assets (liabilities) and unrecognized tax benefits (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Income taxes
Unrecognized tax benefits	$ 0	$ 0
Amounts in uncertain tax position	0
Accrued interest	0	0
Deferred tax assets:
Net operating loss carryforwards	44,556,000	24,642,000
Tax credit carryforwards	12,021,000	8,832,000
Deferred rent	1,560,000	1,458,000
Deferred revenue	13,926,000	8,622,000
Non-deductible accruals and reserves	1,105,000	817,000
Intangibles	930,000	832,000
Stock compensation	2,802,000	1,361,000
Total deferred tax assets	76,900,000	46,564,000
Less valuation allowance	(75,213,000)	(44,953,000)
Net deferred tax assets	1,687,000	1,611,000
Deferred tax liabilities
Depreciation and amortization	(1,687,000)	(1,611,000)
Net deferred taxes	$ 0	$ 0

Selected quarterly financial _3

Selected quarterly financial data (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Selected quarterly financial data (unaudited)
Collaboration revenue	$ 2,008	$ 2,094	$ 2,575	$ 942	$ 6,346	$ 1,148	$ 1,177	$ 1,464	$ 7,619	$ 10,135	$ 14,220
Total operating expenses	25,169	23,241	28,269	22,035	18,693	24,503	19,816	18,986	98,714	81,998	55,519
Loss from operations	(23,161)	(21,147)	(25,694)	(21,093)	(12,347)	(23,355)	(18,639)	(17,522)	(91,095)	(71,863)	(41,299)
Net loss	$ (22,532)	$ (20,289)	$ (25,541)	$ (19,926)	$ (11,828)	$ (23,346)	$ (18,876)	$ (16,648)	$ (88,288)	$ (70,698)	$ (40,193)
Net loss per share	$ (0.70)	$ (0.63)	$ (0.80)	$ (0.63)	$ (0.40)	$ (0.89)	$ (0.73)	$ (0.65)	$ (2.75)	$ (2.64)	$ (1.59)

Subsequent events - (Details)

Subsequent events - (Details) - Collaborative Arrangement $ in Millions	1 Months Ended
	Feb. 28, 2019USD ($)	Jan. 31, 2019USD ($)Programshares	Feb. 28, 2018USD ($)
AbbVie
Subsequent events
Non-refundable upfront payment received			$ 69
AbbVie | Subsequent Event
Subsequent events
Non-refundable upfront payment received	$ 65
Neurocrine | Subsequent Event
Subsequent events
Non-refundable upfront payment received		$ 165
Proceeds raised		$ 50
Shares from collaboration | shares		4,179,728
Number of collaboration programs | Program		4
Programs determined by the company | Program		2