Voyager Therapeutics (VYGR) 10-K 25 Feb 21 2020 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Feb. 19, 2021	Jun. 30, 2020
Document and Entity Information
Document Type	10-K
Document Annual Report	true
Document Transition Report	false
Document Period End Date	Dec. 31, 2020
Entity File Number	001-37625
Entity Registrant Name	Voyager Therapeutics, Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	46-3003182
Entity Address, Address Line One	75 Sidney Street
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02139
City Area Code	857
Local Phone Number	259-5340
Title of 12(b) Security	Common Stock, $0.001 par value
Trading Symbol	VYGR
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Central Index Key	0001640266
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Entity Public Float			$ 291.7
Entity Common Stock, Shares Outstanding		37,653,805

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 104,440	$ 86,042
Marketable securities, current	76,698	195,491
Related party collaboration receivable	8,012	18,496
Prepaid expenses and other current assets	8,619	4,630
Total current assets	197,769	304,659
Property and equipment, net	25,435	17,986
Deposits and other non-current assets	2,316	1,723
Marketable securities, non-current		1,920
Operating lease, right-of-use assets	36,064	28,472
Total assets	261,584	354,760
Current liabilities:
Accounts payable	634	4,070
Accrued expenses	14,205	21,516
Other current liabilities	4,198	3,193
Deferred revenue, current	7,729	47,233
Total current liabilities	26,766	76,012
Deferred revenue, non-current	36,088	147,260
Other non-current liabilities	44,410	31,976
Total liabilities	107,264	255,248
Commitments and contingencies (see note 9)
Stockholders' equity:
Preferred stock, $0.001 par value: 5,000,000 shares authorized; no shares issued and outstanding at December 31, 2020 and 2019
Common stock, $0.001 par value: 120,000,000 shares authorized; 37,368,027 and 36,865,116 shares issued and outstanding at December 31, 2020 and 2019, respectively	37	37
Additional paid-in capital	430,324	412,227
Accumulated other comprehensive loss	(134)	(104)
Accumulated deficit	(275,907)	(312,648)
Total stockholders' equity	154,320	99,512
Total liabilities and stockholders' equity	$ 261,584	$ 354,760

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2020	Dec. 31, 2019
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	37,368,027	36,865,116
Common Stock, Shares, Outstanding	37,368,027	36,865,116

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Income (Loss) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Consolidated Statements of Operations and Comprehensive Income (Loss)
Collaboration revenue	$ 171,128	$ 104,391	$ 7,619
Operating expenses:
Research and development	108,753	119,735	64,905
General and administrative	34,991	36,335	33,809
Total operating expenses	143,744	156,070	98,714
Operating income (loss)	27,384	(51,679)	(91,095)
Other income (expense), net:
Interest income	1,659	6,457	3,310
Other income (expense), net	7,698	1,625	(683)
Total other income	9,357	8,082	2,627
Income (loss) before income taxes	36,741	(43,597)	(88,468)
Income tax benefit			180
Net income (loss)	36,741	(43,597)	(88,288)
Other comprehensive (loss) income
Net unrealized (loss) gain on available-for-sale-securities	(30)	29	34
Total other comprehensive (loss) income	(30)	29	34
Comprehensive income (loss)	$ 36,711	$ (43,568)	$ (88,254)
Net income (loss) per share, basic	$ 0.99	$ (1.21)	$ (2.75)
Net income (loss) per share, diluted	$ 0.98	$ (1.21)	$ (2.75)
Weighted-average common shares outstanding, basic	37,132,447	35,898,266	32,065,781
Weighted-average common shares outstanding, diluted	37,348,514	35,898,266	32,065,781

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)Cumulative Effect, Period of Adoption, Adjustment	Accumulated Other Comprehensive Income (Loss)	Accumulated DeficitCumulative Effect, Period of Adoption, Adjustment	Accumulated Deficit	Cumulative Effect, Period of Adoption, Adjustment	Total
Stockholders' Equity, Beginning Balance (ASU 2016-01) at Dec. 31, 2017			$ 120		$ (120)
Stockholders' Equity, Beginning Balance (ASU 2014-09) at Dec. 31, 2017					$ (19,930)		$ (19,930)
Stockholders' Equity, Beginning Balance at Dec. 31, 2017	$ 32	$ 295,019		$ (287)		$ (160,713)		$ 134,051
Shares, Outstanding, Beginning Balance at Dec. 31, 2017	31,572,044
Exercises of vested stock options		3,891						3,891
Exercises of vested stock options (in shares)	384,186
Vesting of restricted stock units		9						9
Vesting of restricted stock units (in shares)	319,891
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
Net (loss) income						(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
Exercises of vested stock options	$ 1	2,713						2,714
Exercises of vested stock options (in shares)	250,276
Issuance of common stock in connection with Neurocrine Collaboration Agreement	$ 4	77,613						77,617
Issuance of common stock in connection with Neurocrine Collaboration Agreement (in shares)	4,179,728
Issuance of common stock under ESPP		663						663
Issuance of common stock under ESPP (in shares)	70,217
Stock based compensation expense		15,640						15,640
Unrealized gain (loss) on available for-sale securities, net of tax				29				29
Net (loss) income						(43,597)		(43,597)
Stockholders' Equity, Ending Balance at Dec. 31, 2019	$ 37	412,227		(104)		(312,648)		99,512
Shares, Outstanding, Ending Balance at Dec. 31, 2019	36,865,116
Exercises of vested stock options		2,319						$ 2,319
Exercises of vested stock options (in shares)	228,436							228,436
Vesting of restricted stock units (in shares)	170,367
Issuance of common stock under ESPP		1,279						$ 1,279
Issuance of common stock under ESPP (in shares)	104,108
Stock based compensation expense		14,499						14,499
Unrealized gain (loss) on available for-sale securities, net of tax				(30)				(30)
Net (loss) income						36,741		36,741
Stockholders' Equity, Ending Balance at Dec. 31, 2020	$ 37	$ 430,324		$ (134)		$ (275,907)		$ 154,320
Shares, Outstanding, Ending Balance at Dec. 31, 2020	37,368,027

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flow from operating activities
Net income (loss)	$ 36,741	$ (43,597)	$ (88,288)
Adjustments to reconcile net income (loss) to net cash (used in) provided by operating activities:
Stock-based compensation expense	14,934	15,640	15,710
Depreciation	3,817	2,765	2,117
Amortization of premiums and discounts on marketable securities	27	(3,584)	(2,163)
In-kind research and development expenses		616	176
Deferred rent			52
Change in fair value for common stock and warrants to purchase equity securities	(7,698)	(1,612)	807
Changes in operating assets and liabilities:
Related party collaboration receivable	10,484	(18,496)
Prepaid expenses and other current assets	(551)	1,675	(3,937)
Operating lease, right-of-use assets	(7,592)	2,951
Other non-current assets	275	(343)	(180)
Accounts payable	(3,436)	2,598	(282)
Accrued expenses	(6,480)	11,728	(1,600)
Operating lease liabilities	13,439	(2,506)
Lease incentive benefit			321
Deferred revenue	(150,676)	80,831	61,380
Net cash (used in) provided by operating activities	(96,716)	48,666	(15,887)
Cash flow from investing activities
Purchases of property and equipment	(12,097)	(7,718)	(4,305)
Proceeds from sale of equipment		172
Purchases of marketable securities	(70,403)	(494,231)	(333,228)
Proceeds from maturities of marketable securities	195,495	411,300	364,000
Net cash provided by (used in) investing activities	112,995	(90,477)	26,467
Cash flow from financing activities
Proceeds from the issuance of common stock in connection with the Neurocrine Collaboration Agreement, net		77,617
Proceeds from the exercise of stock options	2,319	2,714	3,889
Proceeds from the purchase of common stock under ESPP	844	663	860
Net cash provided by financing activities	3,163	80,994	4,749
Net increase in cash and cash equivalents	19,442	39,183	15,329
Cash, cash equivalents, and restricted cash beginning of period	86,777	47,594	32,265
Cash, cash equivalents, and restricted cash end of period	106,219	86,777	47,594
Supplemental disclosure of cash and non-cash activities
Impact of adopting new accounting standards			(20,050)
Operating lease right-of-use assets obtained in exchange for operating lease liabilities	10,818	30,964
Capital expenditures incurred but not yet paid	$ 831	$ 434	$ 300

Nature of business

Nature of business	12 Months Ended
	Dec. 31, 2020
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. In 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, the 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. 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 ongoing collaboration with Neurocrine Biosciences, Inc. (the “Neurocrine Collaboration Agreement”) The Company has incurred annual net operating losses in every year prior to 2020. As of December 31, 2020, the Company had an accumulated deficit of $275.9 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 prior collaborations with Sanofi Genzyme Corporation (“Sanofi Genzyme”), AbbVie Biotechnology Ltd and AbbVie Ireland Unlimited Company (collectively, “AbbVie”), and its ongoing collaboration with Neurocrine Biosciences, Inc. (“Neurocrine”). Based upon its current operating plan, the Company expects that its existing cash, cash equivalents, and marketable debt securities, as well as ongoing reimbursement amounts expected from development costs related to the Neurocrine Collaboration Agreement, quirements 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, 2020
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”). 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 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 current marketable securities, and the warrants are included in current assets since they expire in September of 2021. 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 uses a forward-looking approach based on expected losses to estimate credit losses on certain types of financial instruments, including trade receivables and available-for-sale debt securities. No other than temporary losses have been recognized. Cash, cash equivalents, and marketable securities as of December 31, 2020 and 2019 consist of the following: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Amortized ​ Unrealized ​ Unrealized ​ Fair ​ Cost Gains Losses Value ​ ​ (in thousands) As of December 31, 2020 ​ ​ ​ ​ ​ Money market funds included in cash and cash equivalents ​ $ 103,992 ​ $ — ​ $ — ​ $ 103,992 ​ Marketable securities: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ U.S. Treasury notes ​ 70,348 ​ ​ — ​ ​ 6 ​ 70,342 ​ Equity securities ​ ​ 1,220 ​ ​ 5,136 ​ ​ — ​ ​ 6,356 ​ Total marketable securities ​ $ 71,568 ​ $ 5,136 ​ $ 6 ​ $ 76,698 ​ Total money market funds and marketable securities ​ $ 175,560 ​ $ 5,136 ​ $ 6 ​ $ 180,690 ​ As of December 31, 2019 ​ ​ ​ ​ ​ Money market funds included in cash and cash equivalents ​ $ 78,303 ​ $ — ​ $ — ​ $ 78,303 ​ Marketable securities: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ U.S. Treasury notes ​ 195,467 ​ ​ 52 ​ ​ 28 ​ 195,491 ​ Equity securities ​ ​ 1,220 ​ ​ 700 ​ ​ — ​ ​ 1,920 ​ Total marketable securities ​ $ 196,687 ​ $ 752 ​ $ 28 ​ $ 197,411 ​ Total money market funds and marketable securities ​ $ 274,990 ​ $ 752 ​ $ 28 ​ $ 275,714 ​ ​ All of the Company’s marketable debt securities at December 31, 2020 and 2019 have a contractual maturity of one year or less. Restricted Cash As of December 31, 2020 and 2019, the Company maintained restricted cash totaling approximately $1.8 million and $0.7 million, respectively, held in the form of money market accounts as collateral for the Company’s facility lease obligations. 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 consolidated balance sheets that sum to the total of the same such amounts shown in the statements of cash flows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of December 31, ​ ​ ​ 2020 2019 2018 ​ ​ ​ (in thousands) ​ Cash and cash equivalents ​ $ 104,440 ​ $ 86,042 ​ $ 46,859 ​ Restricted cash included in deposits and other noncurrent assets ​ ​ 1,779 ​ ​ 735 ​ ​ 735 ​ Total cash, cash equivalents, and restricted cash ​ $ 106,219 ​ $ 86,777 ​ $ 47,594 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 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, 2020. Revenue Recognition As of December 31, 2020, all of the Company’s revenue has been generated from its collaboration agreements with Sanofi Genzyme, AbbVie, and Neurocrine The Company enters into collaboration agreements which are within the scope of ASC 606, Revenue from Contracts with Customers 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 contract liabilities within deferred revenue on the consolidated balance sheets 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, license fees, process development and facilities costs. Facilities costs primarily include the allocation of rent, utilities and depreciation. Leases Under Accounting Standards Codification (ASC) 842 Leases Right-of-use assets represent the Company’s right to use an underlying asset for the lease term and lease liabilities represent the Company’s obligation to make lease payments arising from the lease. Operating lease right-of-use assets and liabilities are recognized at the lease commencement date based on the present value of lease payments over the lease term. As the Company’s leases typically do not provide an implicit rate, the Company uses an estimate of its incremental borrowing rate based on the information available at the lease commencement date in determining the present value of lease payments. Operating lease right-of-use assets also include the effect of any lease prepaid or deferred lease payments and are reduced by lease incentives. The lease terms may include options to extend or terminate the lease when it is reasonably certain that the Company will exercise that option. Lease expense is recognized on a straight-line basis over the lease term. The Company has lease agreements with lease and non-lease components, which are generally accounted for separately. Non-lease components as it pertains to the Company's leased premises generally refer to common area maintenance charges related to the premises. Prior to January 1, 2019, rent expense and lease incentives from operating leases were recognized on a straight-line basis over the lease term. The difference between rent expenses recognized and rental payments was recorded as deferred rent in the accompanying consolidated balance sheets. 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 The Black-Scholes option pricing model requires inputs based on certain subjective assumptions, including (i) the expected stock price volatility, (ii) the calculation of expected term of the award, (iii) the risk-free interest rate and (iv) 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 historical 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 The Company expenses the fair value of its stock-based compensation awards on a straight-line basis over the associated service period, which is generally the period in which the related services are received, adjusted for actual forfeitures of unvested awards as they occur. 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 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, 2020, the Company does not have any significant uncertain tax positions. Comprehensive Income (Loss) Comprehensive income (loss) is comprised of net income (loss) and other comprehensive income or loss. Other comprehensive income or loss consists of unrealized gains or losses on marketable securities. Net Income (Loss) Per Share Basic net income (loss) per share is calculated by dividing the net income (loss) by the weighted-average number of shares of common stock outstanding during the period, without consideration for potentially dilutive securities. Diluted net income (loss) per share is computed by dividing the net income (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 income (loss) per share, unvested restricted common stock and outstanding stock options are considered to be potentially dilutive securities. Unvested restricted common stock and outstanding stock options were excluded from the calculation of diluted net loss per share in the years ended December 31, 2019 and 2018, because their effect would be anti-dilutive and therefore, basic and diluted net loss per share were the same for the years ended December 31, 2019 and 2018. The following table sets forth the outstanding potentially dilutive securities that have been excluded in the calculation of diluted net income (loss) per share because to do so would be anti-dilutive: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of December 31, ​ ​ ​ 2020 ​ 2019 ​ 2018 ​ Unvested restricted common stock awards ​ 156,863 ​ 176,471 ​ 235,294 ​ Unvested restricted common stock units ​ ​ 527,625 ​ ​ 455,404 ​ ​ — ​ Outstanding stock options ​ 5,379,856 ​ 5,317,326 ​ 4,225,152 ​ Total ​ 6,064,344 ​ 5,949,201 ​ 4,460,446 ​ ​ Basic net income (loss) and diluted weighted-average shares outstanding are as follows for the year ended December 31, 2020, 2019, and 2018. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ ​ 2020 ​ 2019 ​ 2018 ​ ​ (in thousands, except share data) Numerator: ​ ​ ​ ​ ​ ​ ​ ​ ​ Net income (loss) ​ $ 36,741 ​ $ (43,597) ​ $ (88,288) Denominator for basic net income (loss) per share: ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted average shares outstanding-basic ​ ​ 37,132,447 ​ ​ 35,898,266 ​ ​ 32,065,781 Denominator for diluted net income (loss) per share: ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted average shares outstanding ​ ​ 37,132,447 ​ ​ 35,898,266 ​ ​ 32,065,781 Common stock options and restricted stock units ​ ​ 216,068 ​ ​ — ​ ​ — Weighted average shares outstanding-diluted ​ ​ 37,348,514 ​ ​ 35,898,266 ​ ​ 32,065,781 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 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 financial institutions 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 third-party manufacturers 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 2016, the Financial Accounting Standards Board, or FASB, issued ASU 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial In August 2018, the FASB issued ASU No. 2018-13, Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement Recent Accounting Pronouncements In August 2020, the FASB issued ASU No. 2020-06, Debt – Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging—Contracts in Entity’s Own Equity (Subtopic 815-40) In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes

Fair value measurements

Fair value measurements	12 Months Ended
	Dec. 31, 2020
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, 2020 and 2019 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, 2020 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Money market funds included in cash and cash equivalents ​ $ 103,992 ​ $ 103,992 ​ $ — ​ $ — ​ Marketable securities: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ U.S. Treasury notes ​ ​ 70,342 ​ ​ 70,342 ​ ​ — ​ ​ — ​ Equity securities ​ ​ 6,356 ​ ​ 6,356 ​ ​ — ​ ​ — ​ Total marketable securities ​ $ 76,698 ​ $ 76,698 ​ $ — ​ $ — ​ Warrants to purchase equity securities ​ ​ 3,816 ​ ​ — ​ ​ 3,816 ​ ​ — ​ Total ​ $ 184,506 ​ $ 180,690 ​ $ 3,816 ​ $ — ​ December 31, 2019 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Money market funds included in cash and cash equivalents $ 78,303 ​ $ 78,303 ​ $ — ​ $ — ​ Marketable securities: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ U.S. Treasury notes ​ ​ 195,491 ​ ​ 195,491 ​ ​ — ​ ​ — ​ Equity securities ​ ​ 1,920 ​ ​ 1,920 ​ ​ — ​ ​ — ​ Total marketable securities ​ $ 197,411 ​ $ 197,411 ​ $ — ​ $ — ​ Warrants to purchase equity securities ​ ​ 554 ​ ​ — ​ ​ 554 ​ ​ — ​ Total ​ $ 276,268 ​ $ 275,714 ​ $ 554 ​ $ — ​ ​ 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 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 (i) the expected stock price volatility, (ii) the calculation of expected term of the awards, (iii) the risk-free interest rate, and (iv) expected dividends. The assumptions utilized to value the warrants to purchase equity securities as of December 31, 2020 and 2019 are as follows. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of December 31, ​ ​ 2020 ​ 2019 ​ Risk-free interest rate ​ 0.1 % ​ 1.6 % ​ Expected dividend yield ​ — % ​ — % ​ Expected term (in years) ​ 0.7 ​ ​ 1.7 ​ ​ Expected volatility ​ 89.2 % ​ 71.6 % ​ ​ 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, 2020
Prepaid expense and other current assets
Prepaid expenses and other current assets	4. Prepaid expenses and other current assets ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of December 31, ​ 2020 2019 ​ ​ (in thousands) ​ Warrants to purchase equity securities ​ $ 3,816 ​ $ 554 ​ Prepaid research and development contracts ​ 1,674 ​ ​ 1,999 ​ Other current assets ​ ​ 1,575 ​ ​ 1,398 ​ Prepaid insurance ​ 1,459 ​ 203 ​ Accrued interest receivable ​ ​ 95 ​ ​ 476 ​ Total ​ $ 8,619 ​ $ 4,630 ​ ​

Property and equipment, net

Property and equipment, net	12 Months Ended
	Dec. 31, 2020
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, ​ ​ 2020 2019 ​ ​ (in thousands) ​ Laboratory equipment ​ $ 18,093 ​ $ 13,748 ​ Leasehold improvements ​ ​ 15,469 ​ ​ 7,129 ​ Furniture and office equipment ​ ​ 2,410 ​ ​ 1,888 ​ Construction in progress ​ 866 ​ 2,188 ​ Other ​ ​ 167 ​ 690 ​ Total property and equipment ​ 37,005 ​ 25,643 ​ Less: accumulated depreciation ​ (11,570) ​ (7,657) ​ Property and equipment, net ​ $ 25,435 ​ $ 17,986 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ The Company recorded $3.8 million, $2.8 million, and $2.1 million in depreciation expense during the years ended December 31, 2020, 2019, and 2018, respectively.

Accrued expenses

Accrued expenses	12 Months Ended
	Dec. 31, 2020
Accrued expenses
Accrued expenses	6. Accrued expenses Accrued expenses consist of the following: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of December 31, ​ ​ 2020 2019 ​ ​ (in thousands) ​ Research and development costs ​ $ 6,624 ​ $ 13,248 ​ Employee compensation costs ​ ​ 5,857 ​ ​ 5,733 ​ Professional services ​ 1,153 ​ 897 ​ Accrued goods and services ​ 496 ​ 1,386 ​ Other ​ ​ 75 ​ ​ 252 ​ Total ​ $ 14,205 ​ $ 21,516 ​ ​

Lease obligation

Lease obligation	12 Months Ended
	Dec. 31, 2020
Lease obligation
Lease obligation	7. Lease obligation Operating Leases As of December 31, 2020, the Company has leases for office and lab space at 75 and 64 Sidney Street in Cambridge, Massachusetts through November 30, 2026. In March 2020, the Company entered into an agreement to lease additional laboratory and office space at 75 Hayden Avenue in Lexington, Massachusetts through January 31, 2031. The Company gained control of and occupied the space in November of 2020. The Company received leasehold improvement incentives from the landlord totaling $5.3 million for the 75 and 64 Sidney Street leases. The Company also received $5.6 million of leasehold improvement incentives for the 75 Hayden Avenue lease. The leasehold improvements have been capitalized as leasehold improvements and the Company recorded the incentives as a component of its right-of-use assets and is amortizing them as a reduction of lease expense over the life of the lease. The Company’s lease agreements require the Company to maintain a cash deposit or irrevocable letter of credit of $1.8 million payable to the landlord as security for the performance of its obligations under the leases. These amounts are recorded as restricted cash and included in deposits and other non-current assets in the accompanying consolidated balance sheets. The following table summarizes the Company’s significant contractual obligations under operating leases as of payment due date by period at December 31, 2020: ​ ​ ​ ​ ​ ​ Total Minimum ​ Lease Payments ​ ​ (in thousands) ​ 2021 ​ $ 7,822 ​ 2022 ​ 8,469 ​ 2023 ​ 8,723 ​ 2024 ​ ​ 8,985 ​ 2025 ​ ​ 9,644 ​ Thereafter ​ ​ 19,945 ​ Total future minimum lease payments ​ $ 63,588 ​ Less: imputed interest ​ ​ (15,981) ​ Total lease liabilities ​ $ 47,607 ​ Reported as: ​ ​ ​ ​ Other current liabilities ​ $ 4,198 ​ Other non-current liabilities ​ ​ 43,409 ​ Total lease liabilities ​ $ 47,607 ​ ​ Total lease cost for operating leases of approximately $6.2 million and $5.7 million was incurred during the years ended December 31, 2020 and 2019, respectively. As of December 31, 2020, the weighted average remaining lease term was 6.8 years and the weighted average incremental borrowing rate used to determine the operating lease liabilities was 8.0%. Total rent expense for the year ended December 31, 2018 recorded under ASC 840 Leases

Other liabilities

Other liabilities	12 Months Ended
	Dec. 31, 2020
Other liabilities
Other liabilities	8. Other liabilities As of December 31, 2020 and 2019, other current and non-current liabilities consisted of the following: ​ ​ ​ ​ ​ ​ ​ ​ ​ As of December 31, ​ ​ 2020 2019 ​ ​ (in thousands) ​ Other current liabilities ​ ​ ​ ​ ​ ​ Lease liabilities ​ 4,198 ​ ​ 3,193 ​ Total other current liabilities $ 4,198 ​ $ 3,193 ​ ​ ​ ​ ​ ​ ​ ​ Other non-current liabilities ​ ​ ​ ​ ​ ​ Lease liabilities $ 43,409 ​ $ 30,975 ​ Other ​ 1,001 ​ ​ 1,001 ​ Total other non-current liabilities $ 44,410 ​ $ 31,976 ​ ​

Commitments and contingencies

Commitments and contingencies	12 Months Ended
	Dec. 31, 2020
Commitments and contingencies
Commitments and contingencies	9. Commitments and contingencies Significant Agreements Neurocrine Collaboration Agreement Summary of Agreement In March 2019, the Company entered into the Neurocrine Collaboration Agreement for the research, development and commercialization of certain of its AAV gene therapy products. Under the Neurocrine Collaboration Agreement, the Company agreed to collaborate on the conduct of four collaboration programs (the “Neurocrine Programs”) which include: (i) the VY-AADC Program, (ii) the FA Program (collectively, the “Legacy Programs”); and (iii) two programs to be determined by the Company and Neurocrine at a later date (the “Discovery Programs”) In June 2019, in conjunction with the termination of the Sanofi Genzyme Collaboration Agreement, the Company gained ex-U.S. rights to the FA Program . The Company’s ex-U.S. rights to the FA Program were subsequently transferred to Neurocrine under the terms of the Neurocrine Collaboration Agreement. To facilitate the transfer of the ex-U.S. rights to the FA Program to Neurocrine, the Company and Neurocrine executed an amendment to the Neurocrine Collaboration Agreement (the “June 2019 Modification”), and Neurocrine paid $5.0 million to the Company . There were no other changes in pricing or scope of the obligations required to be performed under the Neurocrine Collaboration Agreement Under the terms of the Neurocrine Collaboration Agreement, the Company has agreed to collaborate with Neurocrine on, and to grant, exclusive, royalty-bearing, non-transferable, sublicensable licenses to certain of its intellectual property rights, for all human and veterinary diagnostic, prophylactic, and therapeutic uses, for the research, development, and commercialization of gene therapy products (the “Collaboration Products”) on a worldwide basis under (i) the VY-AADC Program; (ii) the FA Program; and (iii) each Discovery Program. Pursuant to development plans agreed by the parties, which are overseen by a joint steering committee (“JSC”), the Company has operational responsibility, subject to certain exceptions, for the conduct of each Neurocrine Program prior to the occurrence of a specified event for such Neurocrine Program (a “Transition Event”), as described below, and is required to use commercially reasonable efforts to develop the corresponding Collaboration Products. Neurocrine has agreed to be responsible for all costs incurred by the Company in conducting these activities for each Neurocrine Program, in accordance with an agreed budget for each Neurocrine Program. If the Company breaches its development responsibilities or in certain circumstances upon a change in control, Neurocrine has the right but not the obligation to assume the activities under such Neurocrine Program. Upon the occurrence of a Transition Event for each Neurocrine Program, Neurocrine has agreed to assume responsibility for development, manufacturing and commercialization activities for such Neurocrine Program from the Company and to pay milestones and royalties on future net sales as described further below. The Transition Events are (i) with respect to the VY-AADC Program, the Company’s receipt of topline data for the RESTORE-1 Phase 2 clinical trial for VY-AAD C (NBIb-1817) C (NBIb-1817) Under the Neurocrine Collaboration Agreement, subject to exceptions specified therein, the Company and Neurocrine agreed that profits and losses under the Company’s Co-Co Option would be allocated (i) 50% to Neurocrine and 50% to the Company for a Collaboration Product from the VY-AADC Program and (ii) 60% to Neurocrine and 40% to the Company for a Collaboration Product from the FA Program; provided, however, that Neurocrine would have the right to elect, within a specified period following the acceptance for filing of a biologics license application from the FDA, to pay a $35.0 million rate-shifting fee to the Company to change the allocation for the VY-AADC Program to 55% to Neurocrine and 45% to the Company. The parties agreed that each Co-Co Agreement would provide the Company the right to terminate for any reason upon prior written notice to Neurocrine and Neurocrine the right to terminate in certain circumstances upon change of control. The Company’s research and development activities under the Neurocrine Collaboration Agreement are conducted pursuant to plans agreed to by the parties, on a program-by-program basis, and are overseen by the JSC, as detailed in the Neurocrine Collaboration Agreement. The parties have committed to agree on a list of up to eight target genes (the “Targets”) from which Neurocrine has the right to nominate Targets for the two Discovery Programs. The Targets nominated for the Discovery Programs must be approved by a consensus of the JSC or the executive officers. The Neurocrine Collaboration Agreement provides for an upfront non-refundable payment of $115.0 million, as well as for aggregate development and regulatory milestone payments from Neurocrine to the Company for Collaboration Products under (i) the VY-AADC Program of up to $170.0 million; (ii) the FA Program of up to $195.0 million, and (iii) each of the two Discovery Programs of up to $130.0 million per Discovery Program. The Company may be entitled to receive aggregate commercial milestone payments for each Collaboration Product of up to $275.0 million, subject to an aggregate cap on commercial milestone payments across all Neurocrine Programs of $1.1 billion. Furthermore, in connection with the Neurocrine Collaboration Agreement, Neurocrine purchased 4,179,728 shares of the Company’s common stock at a price of $11.9625 per share, for an aggregate purchase price of $50.0 million. Neurocrine also agreed to pay the Company royalties, based on future net sales of the Collaboration Products. Such royalty percentages, for net sales in and outside the United States, as applicable, range (i) for the VY-AADC Program, from the mid-teens to low thirties and the low-teens to low twenties, respectively; (ii) for the FA Program, from the low-teens to high-teens and high-single digits to mid-teens, respectively; and (iii) for each Discovery Program, from the high-single digits to mid-teens and mid-single digits to low-teens, respectively. On a country-by-country and program-by-program basis, royalty payments would commence on the first commercial sale of a Collaboration Product and terminate on the later of (a) the expiration of the last patent covering the Collaboration Product or its method of use in such country, (b) ten years from the first commercial sale of the Collaboration Product in such country and (c) the expiration of regulatory exclusivity in such country, or the Royalty Term. Royalty payments may be reduced by up to 50% in specified circumstances, including expiration of patents rights related to a Collaboration Product, approval of biosimilar products in a given country or required payment of licensing fees to third parties related to the development and commercialization of any Collaboration Product. Additionally, the licenses granted to Neurocrine shall automatically convert to fully paid-up, non-royalty bearing, perpetual, irrevocable, exclusive licenses on a country-by-country and product-by-product basis upon the expiration of the Royalty Term applicable to such Collaboration Product in such country. Under the terms of the Neurocrine Collaboration Agreement and subject to specified exceptions therein, each party owns the entire right, title and interest in and to all intellectual property rights made solely by its employees or agents in the course of the collaboration. The parties jointly own all rights, title and interest in and to all intellectual property rights made or invented jointly by employees or agents of both parties. During the term of the Neurocrine Collaboration Agreement, neither party nor any of its respective affiliates is permitted to directly or indirectly exploit any AAV-based gene therapy products directed to a Target to which a Collaboration Product is directed, subject to specified exceptions including the parties’ conduct of basic research activities. Unless earlier terminated, the Neurocrine Collaboration Agreement expires on the later of (i) the expiration of the last to expire royalty term with respect to a Collaboration Product in all countries in the relevant territory or (ii) the expiration or termination of all Co-Co Agreements. Neurocrine may terminate the Neurocrine Collaboration Agreement in its entirety or on a program-by-program or country-by-country basis by providing at least (a) 180-day Upon termination in certain cases, Neurocrine has agreed to grant to the Company licenses to certain Neurocrine intellectual property, subject to a negotiation between the parties to establish royalty rates for use of such intellectual property. In the event of a breach by the Company with respect to a Neurocrine Program, if such termination were to occur after a Transition Event, then (i) if a Co-Co Agreement is in effect with respect to such program, Neurocrine can terminate the Co-Co Agreement for such program and the Company would no longer have co-development and co-commercialization rights with respect to the Collaboration Product and (ii) subject to any license agreements, Neurocrine would no longer have any obligations with respect to any Collaboration Products resulting from such program. Termination of VY-AADC Program In February 2021, Neurocrine notified the Company that it had elected to terminate the Neurocrine Collaboration solely with regards to the VY-AADC Program, effective August 2, 2021 (the “ Neurocrine VY-AADC Program Termination As a result of the termination, as of the Neurocrine VY-AADC Program Termination Effective Date, the license granted by the Company to Neurocrine thereunder regarding the VY-AADC Program shall expire and the Company shall regain worldwide intellectual property rights regarding the VY-AADC Program, in each case in accordance with the terms of the Neurocrine Collaboration Agreement. The Company intends to support Neurocrine, the study sponsor and IND holder, on ongoing matters related to the completion of imaging and clinical assessments requested by the DSMB and the provision of other information requested by the FDA for the RESTORE-1 Phase 2 clinical trial. Accounting Analysis At inception, the Neurocrine Collaboration Agreement included the following performance obligations: (i) research and development services for each Legacy Program combined with a development and commercialization license for each such program and (ii) research and development services for each Discovery Program combined with a development and commercialization license for each program. The research services and license on a program by program basis are not distinct as Neurocrine cannot benefit from such license on its own or from other resources commonly available in the industry, without the corresponding research services due to the unique and specialized expertise of the Company that is not readily available in the marketplace. The Company identified $92.4 million of fixed transaction price consisting of the $115.0 million upfront fee and $5.0 million payment from the June 2019 Modification, offset by a discount of $27.6 million related to the $50.0 million equity investment of 4,179,728 shares when measured at fair value on the date of issuance. The Company is also entitled to reimbursement of costs incurred by the Company prior to the Transition Events associated with each Neurocrine Program. These amounts are determinable based on program plans and budgets, and the Company has a contractual right to the payment of cost incurred under the agreed upon program plans. The Company utilized the most likely amount approach and estimated the expected cost reimbursement to be $431.1 million at inception. The Company concluded that these amounts do not require a constraint and are included in the transaction price at inception. The Company considers this estimate at each reporting date and updates the estimate based on information available. During the fourth quarter of 2020, the Company further revised the estimate of the expected reimbursement to $316.2 million based on current expectations. Additional consideration to be paid to the Company upon reaching certain milestones are excluded from the transaction price at inception due to the uncertainty of achieving the development and regulatory milestones. The Company allocated the fixed transaction price to the separate performance obligations based on the relative standalone selling price of each performance obligation or in the case of certain variable consideration to one or more performance obligations. The estimated standalone selling prices for performance obligations, that include a license and research services, were developed using the estimated selling price of the license, using comparable and market data, and an estimate of the overall effort to perform the research services along with a reasonable profit for research services. The Company has concluded that the variable consideration related to the cost reimbursement of each program will be allocated to each respective program as the cost reimbursement relates specifically to the respective program services being performed under the Neurocrine Collaboration Agreement. The reimbursement of research services is considered to be at a market rate and the allocation of the fixed consideration to all of the performance obligations depicts the estimated amounts in which it would expect to receive for these obligations, absent the variable consideration related to the research reimbursement. The total variable consideration allocated to each program related to the expected cost reimbursement was as follows at December 31, 2020: ​ ​ ​ ​ ​ ​ Performance Obligation ​ Amount ​ ​ ​ (in thousands) ​ Variable Consideration ​ ​ ​ VY-AADC Program ​ $ 86,480 ​ FA Program ​ ​ 87,991 ​ Discovery Program 1 ​ ​ 72,247 ​ Discovery Program 2 ​ ​ 69,515 ​ Total ​ $ 316,233 ​ ​ Based on the relative standalone selling price allocation, the allocation of the transaction price, exclusive of the variable consideration allocated to the individual performance obligations, to the separate performance obligations was as follows: ​ ​ ​ ​ ​ ​ Performance Obligation ​ Amount ​ ​ ​ (in thousands) ​ Fixed Consideration ​ ​ ​ VY-AADC Program ​ $ 80,373 ​ FA Program ​ ​ 6,005 ​ Discovery Program 1 ​ ​ 3,002 ​ Discovery Program 2 ​ ​ 3,002 ​ Total ​ $ 92,382 ​ ​ The Company recognizes the transaction price associated with each performance obligation 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. During the years ended December 31, 2020 and 2019, the Company recognized $56.7 million and $60.0 million of revenue, respectively, associated with its collaboration with Neurocrine related to research and development services performed during the period and the corresponding cost reimbursement receivable. As of December 31, 2020, there was $43.8 million of deferred revenue related to the Neurocrine Collaboration Agreement, which is classified as either current or non-current in the accompanying consolidated balance sheet based on the period the services are expected to be delivered. Additionally, as of December 31, 2020, there was $8.0 million of collaboration receivables related to reimbursable costs expected to be received from Neurocrine for research and development services performed. The following table presents changes in the balances of the Company’s related party collaboration receivables and contract liabilities during the year ended December 31, 2020: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ ​ ​ Balance at ​ ​ ​ December 31, 2019 ​ Additions ​ Deductions ​ December 31, 2020 ​ ​ ​ ​ ( in thousands) ​ Related party collaboration receivable ​ $ 18,496 ​ $ 35,952 ​ $ (46,436) ​ $ 8,012 ​ Contract liabilities: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Deferred revenue ​ $ 70,040 ​ $ ​ ​ $ (26,351) ​ $ 43,689 ​ ​ The change in the receivables balance for the year ended December 31, 2020 is primarily driven by amounts owed to the Company for research and development services provided, offset by amounts collected from Neurocrine during the period. Costs incurred relating to the Collaboration Programs consist of internal and external research and development costs, which primarily include: salaries and benefits, lab supplies, preclinical research studies, clinical studies, consulting services, and commercial development. These costs are included in research and development expenses in the Company’s consolidated statements of operations during the year ended December 31, 2020. The Company incurred approximately $0.8 million of costs to obtain the Neurocrine Collaboration Agreement which were payable only upon the close of the deal and therefore considered incremental costs of obtaining a contract with a customer and capitalized. The costs are recorded in prepaid expenses and other non-current assets and are being amortized over the period in which the research services will be provided. Sanofi Genzyme Collaboration Agreement Summary of Agreement In February 2015, the Company entered into 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. 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 (NBIb-1817) for Parkinson’s disease (the “VY-AADC Program”), VY-FXN01 for Friedreich’s ataxia (the “FA Program”), a future program to be designated by Sanofi Genzyme (the “Future Program), and VY-HTT01 for Huntington’s disease (the “Huntington’s Program”), with an incremental option to co-commercialize VY-HTT01 in the United States and (ii) worldwide rights to VY-SMN101 (the “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. The Company was solely responsible for all costs incurred in connection with the development of the Split Territory Programs and the Spinal Muscular Atrophy Program 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 would provide “in-kind” services valued at up to $5.0 million and (ii) Sanofi Genzyme would be responsible for the costs and expenses of activities under the Huntington’s Program development plan to the extent such activities were covered by financial support Sanofi Genzyme is entitled to receive from a patient advocacy group. Termination of Agreement On June 14, 2019 (the “Termination Date”), the Company and Sanofi Genzyme executed a termination agreement to terminate the Sanofi Genzyme Collaboration Agreement (the “Sanofi Genzyme Termination Agreement”). Under the terms of the Sanofi Genzyme Termination Agreement, Sanofi Genzyme relinquished its rights to the exclusive license options to the Huntington’s Program, the FA Program and the Future Program. The Company was relieved of its obligations to perform the research and development services under those programs through completion of the respective POP Studies. As a result, the Company gained worldwide rights to the Huntington’s Program and ex-U.S. rights to the FA Program. The ex-U.S. rights to the FA Program were, in turn, transferred from the Company to Neurocrine pursuant to the collaboration and option agreement with Neurocrine. Additionally, the Company and Sanofi Genzyme entered into an Amended and Restated Option and License Agreement related to AAV capsids (the “Amended Capsid Agreement”). Under the Amended Capsid Agreement, Sanofi Genzyme obtained exclusive option rights to exclusively license up to two select novel AAV capsids owned or controlled by the Company for exclusive use for up to two non-central nervous system (“non-CNS”) indications. Sanofi Genzyme granted the Company exclusive, irrevocable, perpetual, royalty-free, fully-paid sublicensable (through multiple tiers), non-transferable, worldwide licenses in Sanofi Genzyme’s interests in the collaboration technology generated under or used in the Huntington’s Program and the FA Program with respect to those programs pursuant to the Sanofi Genzyme Collaboration Agreement. In addition, Sanofi Genzyme has granted the Company non-exclusive, irrevocable, perpetual, royalty-free, fully-paid, sublicensable (through multiple tiers), non-transferable, worldwide licenses to the Sanofi Genzyme technology that was contributed to the Sanofi Genzyme Collaboration Agreement and was used in the development or manufacture of product candidates prior to the termination date. Under the Sanofi Genzyme Termination Agreement, the Company made a $10.0 million upfront payment to Sanofi Genzyme and paid a $10.0 million milestone payment to Sanofi Genzyme within fifteen days of the filing of an investigational new drug (“IND”) application for a product candidate incorporating certain intellectual property rights developed under or substantially related to the Huntington’s Program (a “Post-Termination HD Product”). The Company has agreed to pay Sanofi Genzyme (i) 50% of any income received from sublicensing arrangements related to Post- Termination HD Products in excess of specified thresholds and entered into prior to (a) the filing of an IND application for a Post-Termination HD Product or (b) the dosing of the first patient in a clinical trial for a Post-Termination HD Product in the United States or certain European countries, respectively and (ii) a low-double digit percentage of any income received from sublicensing arrangements outside the United States related to products incorporating intellectual property rights developed under, or substantially related to, the FA Program (collectively, “Post-Termination FA Products”), that are in excess of a specified threshold and entered into prior to the dosing of the first patient in a clinical trial for a Post-Termination FA Product in the United States or certain European countries, in each case, subject to certain limitations. The Company also agreed to pay low-single-digit royalties on net sales of Post-Termination HD Products. Under the Sanofi Genzyme Collaboration Agreement, the Company had rights to certain in-kind services. As of the effective date of the Sanofi Genzyme Termination Agreement, the Company waived its right to approximately $0.4 million in unused in-kind services, relinquished its rights to the Spinal Muscular Atrophy Program, and no longer has the right to receive any option payments, regulatory or commercial milestone payments or royalties from Sanofi Genzyme under the Sanofi Genzyme Collaboration Agreement. The Company granted Sanofi Genzyme an exclusive royalty-free, fully-paid, sublicensable (through multiple tiers), non-transferable, worldwide license under the Company’s interest in the collaboration technology generated under or used in the Spinal Muscular Atrophy Program pursuant to the Sanofi Genzyme Collaboration Agreement to manufacture, develop, and commercialize any Spinal Muscular Atrophy product. Under the Amended Capsid Agreement, the Company has granted Sanofi Genzyme an exclusive option to evaluate up to four capsids for no consideration. During the capsid evaluation period, the Company has granted Sanofi Genzyme a non-exclusive license to the capsid intellectual property to conduct evaluation studies. In addition, Sanofi Genzyme is able to evaluate up to two additional capsids for a low six-figure payment per additional capsid. The Company is not obligated to perform any additional research on the capsids. Sanofi Genzyme shall have the right to obtain an exclusive license for up to two capsids, each in a specified non-CNS indication. At its discretion, Sanofi Genzyme may exercise both its options for the same capsid for different specified non-CNS indications. Upon its exercise of each option, Sanofi Genzyme has agreed to pay the Company a $1.0 million option exercise fee. Under the Amended Capsid Agreement, the Company is also entitled to receive potential development and regulatory milestone payments upon the achievement of certain milestone events for products containing licensed capsids (“Sanofi Licensed Products”) of up to an aggregate of $15.0 million per Sanofi Licensed Product. In addition, for each specified indication, Sanofi Genzyme has agreed to pay to the Company a one-time sales milestone payment of $20.0 million, if aggregate worldwide net sales for all Sanofi Licensed Products for such specified indication surpass a specified amount, and low-to-mid single-digit tiered royalty payments on worldwide net sales of Sanofi Licensed Products, on a Sanofi Licensed Product-by-Sanofi Licensed Product basis. Accounting Analysis The Sanofi Genzyme Termination Agreement modified both the pricing and scope of the Sanofi Genzyme Collaboration Agreement. As the modification does not add distinct goods or services to the Sanofi Genzyme Collaboration Agreement, the agreement is considered a modification of the original contract. The Sanofi Genzyme Termination Agreement included the following performance obligations: (i) worldwide license to collaboration technology and Sanofi Genzyme technology for the development, manufacturing and commercialization of the Huntington’s Program and (ii) worldwide license to collaboration technology and Sanofi Genzyme technology for the development, manufacturing and commercialization for the FA Program. Such performance obligations were satisfied upon the Termination Date as control had transferred upon execution of the Sanofi Genzyme Termination Agreement. Therefore, the remainder of the transaction price under the Sanofi Genzyme Collaboration Agreement, which had not yet been recognized, was recognized as revenue upon the Termination Date. The Company recognized $28.7 million of revenue upon the Termination Date. This amount consists of $48.7 million of deferred revenue related to the original agreement as of the Termination Date, offset by (x) $10.0 million related to the fee paid by the Company to Sanofi Genzyme on the Termination Date, and (y) $10.0 million related to the milestone payment paid to Sanofi Genzyme upon the Company’s filing in September 2020 of an IND application for VY-HTT01 for the treatment of Huntington’s disease. Following the milestone payment related to the filing of the IND for VY-HTT01 in September 2020, the Company recorded the payment as a reversal of deferred revenue. The $20.0 million payable by the Company to Sanofi Genzyme in aggregate was treated as consideration payable to a customer and therefore accounted for as a reduction of the transaction price. During the years ended December 31, 2019 and 2018, the Company recognized $32.6 million and $0.9 million of revenue, respectively, related to obligations performed under the Sanofi Genzyme Collaboration. 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 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 was 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 to the Company for services during the Research Period. During the Research Period, each party agreed to identify up to five antibodies for inclusion in the collaboration. Subject to certain conditions and exceptions, the parties agreed to 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 was 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 was solely responsible for its costs and expenses during the Research Period. During a specified portion of the Research Period, AbbVie had the right to 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 agreed to 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 was obligated to use diligent efforts to conduct development activities, including IND application-enabling and Phase 1 clinical trial activities, for the Selected Research Compounds and corresponding Selected Product Candidates. The Company was solely responsible for the costs and expenses during the Development Period. During a specified portion of the Development Period (the “License Option Period”), AbbVie had the right to 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 agreed to provide a one-time payment of $75.0 million to the Company, and the Company agreed to 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 had certain obligations to complete any remaining research and development activities that had not been completed for any Research Compounds and Product Candidates. The Company’s research and development activities were to 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 were required to be mutually agreed to by the Company and AbbVie, which could be through the JGC. Under the AbbVie Tau Collaboration Agreement, AbbVie was 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 was 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 had the option to 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 exercised a Cost-Sharing Option, the Company would either reimburse AbbVie for AbbVie’s applicable development costs or, in the case of certain budget overruns, AbbVi

Preferred stock

Preferred stock	12 Months Ended
	Dec. 31, 2020
Preferred stock
Preferred stock	10. Preferred stock The Company has authorized preferred stock amounting to 5,000,000 shares as of December 31, 2020 and 2019.

Common stock

Common stock	12 Months Ended
	Dec. 31, 2020
Common stock.
Common stock	11. Common stock As of December 31, 2020 and 2019, 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, ​ ​ 2020 2019 Shares reserved for vesting of restricted stock awards under the Founder Agreements 156,863 ​ 176,471 ​ Shares reserved for exercise of outstanding stock options 5,485,078 ​ 5,317,326 Shares reserved for vesting of outstanding restricted stock units 638,471 ​ 455,404 Shares reserved for issuances under the 2015 Stock Option Plan ​ 2,827,185 ​ 1,875,078 ​ Shares reserved for issuances under the 2015 Employee Stock Purchase Plan ​ 1,484,923 ​ 1,218,876 ​ ​ 10,592,520 ​ 9,043,155 ​

Stock-based compensation

Stock-based compensation	12 Months Ended
	Dec. 31, 2020
Stock-based compensation
Stock-based compensation	12. 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. 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 Founder Awards In January 2014, the Company issued 1,188,233 shares of restricted stock to its founders (each, a “Founder”) 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 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 is 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. Stock-based compensation expense in the amount of $0.3 million was recorded in the year ended December 31, 2018 related to this award. In December 2019, the Company modified one of the remaining performance awards, repurchasing 58,823 shares of common stock previously issued to one of the Company’s founders. Additionally, the Company modified the award to vest solely based on time rather than on performance. The Company revalued the award at the modification date and is recognizing expense on a straight-line basis over the three 2015 Stock Option Plan In October 2015, the Company s board of directors and stockholders 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 and every anniversary thereafter an additional 4% of outstanding common stock was added to the Company’s 2015 Stock Option Plan pursuant to its “evergreen” provision, for future issuance. This has accumulated to a total of 7,707,888 shares through January 1, 2021. During the year ended December 31, 2020, the Company granted options to purchase 1,388,075 shares of common stock to employees and directors under the 2015 Stock Option Plan. As of December 31, 2020, there were 2,827,185 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 and every anniversary thereafter an additional 1% of outstanding common stock was added to the 2015 ESPP, pursuant to its evergreen provision, for future issuance . This has accumulated to a total of 1,926,969 shares through January 1, 2021. The Company issued 104,108 and 70,217 shares of common stock under the 2015 ESPP in the years ended December 31, 2020 and 2019. As of December 31, 2020, there were 1,484,923 shares available for future purchase under the 2015 ESPP. Inducement Awards In the year ended December 31, 2020, the Company issued non-statutory stock options to purchase an aggregate of 172,500 shares of the Company’s common stock and restricted stock unit awards for an aggregate of 29,000 shares of the Company’s common stock, respectively, to three individuals in each case outside of the Company’s 2015 Stock Option Plan as an inducement material to such individual’s acceptance of an offer of employment with the Company in accordance with Nasdaq Listing Rule 5635(c)(4). In the year ended December 31, 2019, the Company issued non-statutory stock options to purchase an aggregate of 338,750 shares of the Company’s common stock and restricted stock unit awards for an aggregate of 58,125 shares of the Company’s common stock, respectively, to two executives in each case outside of the Company’s 2015 Stock Option Plan as an inducement material to such executive’s acceptance of an offer of employment with the Company in accordance with Nasdaq Listing Rule 5635(c)(4). In the year ended December 31, 2018, the Company issued a non-statutory stock option to purchase an aggregate of 650,000 shares of the Company’s common stock to one executive, outside of the Company’s 2015 Stock Option Plan as an inducement material to the executive’s acceptance of an offer of employment with the Company in accordance with Nasdaq Stock Market Listing Rule 5635(c)(4). The stock options will vest over a four-year period, with 25% of the shares underlying the option award vesting on the first anniversary of the award and the remaining 75% of the shares underlying the award vesting monthly thereafter over the subsequent 36-month Stock-based Compensation Expense Total compensation cost recognized for all stock-based compensation awards in the statements of operations and comprehensive income (loss) is as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year ended December 31, ​ 2020 2019 2018 ​ ​ (in thousands) Research and development ​ $ 6,357 ​ $ 7,383 ​ $ 4,717 ​ General and administrative ​ 8,577 ​ 8,257 ​ 10,993 ​ Total stock-based compensation expense ​ $ 14,934 ​ $ 15,640 ​ $ 15,710 ​ ​ Stock-based compensation expense by type of award included within the consolidated statements of operations and ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year ended December 31, ​ ​ ​ 2020 2019 2018 ​ ​ ​ (in thousands) ​ Stock options ​ $ 11,387 ​ $ 13,380 ​ $ 14,956 ​ Restricted stock awards and units ​ ​ 3,110 ​ ​ 1,935 ​ ​ 482 ​ Employee stock purchase plan awards ​ 437 ​ 325 ​ 272 ​ Total stock-based compensation expense ​ $ 14,934 ​ $ 15,640 ​ $ 15,710 ​ ​ In June 2019, the Company entered into a consulting agreement (the “Sah Agreement”) with Dr. Dinah Sah, Ph.D., the Company’s former Chief Scientific Officer, pursuant to which Dr. Sah has agreed to provide consulting and advisory services, including but not limited to scientific guidance in connection with certain of the Company’s collaborations and research and development programs for a three-year period which commenced on June 28, 2019. In accordance with its terms, the Sah Agreement triggered an equity modification resulting in the recognition in 2019 of $2.2 million of stock-based compensation expense related to the non-substantive service period of the Sah Agreement. In August 2018, the Company entered into a consulting agreement (the “Paul Agreement”) with Dr. Steven M. Paul, M.D., the Company’s former President and Chief Executive Officer, pursuant to which Dr. Paul has agreed to provide consulting and advisory services, including but not limited to scientific guidance in connection with certain of the Company’s collaborations and research and development programs for a three-year period which commenced on August 2, 2018. In accordance with its terms, the Paul Agreement triggered an equity modification resulting in the recognition in 2018 of $5.4 million of stock-based compensation expense related to the non-substantive service period of the Paul Agreement. Restricted Stock Units A summary of the status of and changes in unvested restricted stock unit activity under the Company’s equity award plans for the year ended December 31, 2020 was as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted ​ ​ ​ ​ ​ Average ​ ​ ​ ​ ​ Grant Date ​ ​ ​ ​ ​ Fair Value ​ ​ ​ Units Per Unit ​ Unvested restricted stock units as of December 31, 2019 ​ 455,404 ​ $ 12.16 ​ Awarded ​ 494,604 ​ $ 12.67 ​ Vested ​ (150,759) ​ $ 12.12 ​ Forfeited ​ (160,778) ​ $ 11.47 ​ Unvested restricted stock units as of December 31, 2020 ​ 638,471 ​ $ 12.74 ​ ​ Stock-based compensation of restricted stock units is based on the fair value of the Company’s common stock on the date of grant and recognized over the vesting period. The restricted stock units granted in the year ended December 31, 2020 vest in equal amounts, annually over three years. The stock-based compensation expense related to awards granted was $2.8 million, $1.9 million, and $0.1 million for the years ended December 31, 2020, 2019, and 2018 respectively. As of December 31, 2020, the Company had unrecognized stock-based compensation expense related to its unvested restricted stock units of $5.6 million which is expected to be recognized over the remaining weighted average vesting period of 1.8 years 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, 2019 5,317,326 ​ $ 15.98 ​ ​ ​ ​ ​ Granted ​ 1,560,575 ​ $ 11.87 ​ ​ ​ ​ ​ ​ Exercised ​ (228,436) ​ $ 10.15 ​ ​ ​ ​ ​ ​ Cancelled or forfeited ​ (1,164,387) ​ $ 17.33 ​ ​ ​ ​ ​ ​ Outstanding at December 31, 2020 ​ 5,485,078 ​ $ 14.77 ​ 7.5 ​ $ 1,313 ​ Exercisable at December 31, 2020 3,027,116 ​ $ 15.24 6.5 ​ $ — ​ ​ 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, 2020 was $7.64. The stock-based compensation expense related to stock option awards granted to employees and directors was $11.2 million, $13.3 million, and $14.6 million for the years ended December 31, 2020, 2019, and 2018, respectively. The fair value of each option was estimated at the date of grant using the Black-Scholes option pricing model with the following weighted-average assumptions: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year ended December 31, ​ ​ 2020 2019 2018 ​ Risk-free interest rate 1.0 % 2.2 % ​ 2.8 % ​ Expected dividend yield — % — % ​ — % ​ Expected term (in years) 6.0 ​ 6.0 ​ ​ 6.0 ​ ​ Expected volatility 73.7 % 74.7 % ​ 74.4 % ​ ​ As of December 31, 2020, the Company had unrecognized stock-based compensation expense related to its unvested stock options of $20.0 million which is expected to be recognized over the remaining weighted average vesting period of 2.5 years.

401(k) Savings Plan

401(k) Savings Plan	12 Months Ended
	Dec. 31, 2020
401(k) Savings Plan
401(k) Savings Plan	13. 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 $1.1 million, $1.0 million, and $0.8 million related to employer contributions made during the years ended December 31, 2020, 2019, and 2018, respectively.

Income taxes

Income taxes	12 Months Ended
	Dec. 31, 2020
Income taxes
Income taxes	14. Income taxes The Company r ecognized deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the Company’s financial statements or tax returns. Under this method, deferred tax assets and liabilities are determined based on differences between the financial statement carrying amounts and the tax bases of the assets and liabilities using the enacted tax rates in effect in the years in which the differences are expected to reverse. A valuation allowance against deferred tax assets is recorded if, based on the weight of the available evidence, it is more likely than not that some or all the deferred tax assets will not be realized. The Company accounts for uncertain tax positions using a more-likely-than-not threshold for recognizing and resolving uncertain tax positions. 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. The benefit for incomes taxes is as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year ended December 31, ​ 2020 ​ 2019 ​ 2018 ​ (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 for the years ended December 31, 2020, 2019, and 2018 is as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year ended December 31, ​ ​ 2020 ​ 2019 ​ 2018 ​ Income tax computed at federal statutory tax rate ​ 21.0 % ​ 21.0 % ​ 21.0 % ​ Non-deductible expenses ​ 5.0 % ​ (2.2) % ​ (2.1) % ​ Other ​ 1.9 % ​ — % ​ — % ​ State taxes, net of federal benefit ​ (2.3) % ​ 8.0 % ​ 6.3 % ​ Change in valuation allowance ​ (5.0) % ​ (36.2) % ​ (28.1) % ​ General business credit carryovers ​ (20.6) % ​ 9.4 % ​ 3.1 % ​ Total ​ — % ​ — % ​ 0.2 % ​ ​ The Company incurred net operating losses (“NOLs”) through December 31, 2019. As of December 31, 2020, the Company had federal and state net operating loss carryforwards of $153.4 million and $140.9 million, respectively, with the pre-2018 NOLs beginning to expire in 2033, and the post 2019 portion limited to 80% of taxable income and carried forward indefinitely. During 2020, the Company did not generate federal and state NOL carryforwards. As of December 31, 2020, the Company had federal and state research and development tax credit carryforwards of $21.1 million and $8.3 million, respectively, which expire beginning in 2030. As of December 31, 2020, the Company had state investment credits of $0.9 million, which expire beginning in 2021. 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 had occurred since the Company’s formation and determined that its transactions had resulted in three ownership changes, as defined by Section 382. Additionally, in the first quarter of the 2020 calendar year, and in the first quarter of the 2021 calendar year, the Company completed additional studies that did not identify additional ownership changes. 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, 2020 and 2019 are as follows: ​ ​ ​ ​ ​ ​ ​ ​ As of December 31, ​ ​ 2020 ​ 2019 ​ (in thousands) ​ Deferred tax assets: ​ ​ ​ ​ ​ ​ Net operating loss carryforwards $ 41,130 ​ $ 49,713 ​ Tax credit carryforwards 28,173 ​ 17,297 ​ Deferred revenue ​ 12,176 ​ ​ 16,805 ​ Lease liability ​ 13,006 ​ ​ 9,335 ​ Stock compensation ​ 4,368 ​ ​ 5,157 ​ Non-deductible accruals and reserves 1,288 ​ 1,502 ​ Intangibles 721 ​ 776 ​ Total deferred tax assets 100,862 ​ 100,585 ​ Less valuation allowance (84,866) ​ (90,920) ​ Net deferred tax assets 15,996 ​ 9,665 ​ Deferred tax liabilities ​ ​ ​ ​ ​ Unrealized gain on available-for-sale securities ​ (8) ​ ​ (8) ​ Right of use assets ​ (9,852) ​ (7,779) ​ Depreciation and amortization ​ (6,136) ​ ​ (1,878) ​ 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, and 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 $84.9 million and $90.9 million has been established at December 31, 2020 and 2019, respectively. The change in valuation allowance was $1.0 million for the year ended December 31, 2020. 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 general business credit carryovers. At December 31, 2020 and 2019, 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, 2020 and 2019, 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, 2020
Related party transactions
Related party transactions	15. Related-party transactions During the years ended December 31, 2020, 2019, and 2018, the Company received board and scientific advisory services from two of its prior executives, Steven M. Paul, M.D., the Company’s former President and Chief Executive Officer, and Dinah Sah, Ph.D., the Company’s former Chief Scientific Officer. The total amount of fees paid to Dr. Paul for services provided during the years ended December 31, 2020, 2019 and 2018 was $0.2 million, $0.2 million, and $0.1 million respectively. The total amount of fees paid to Dr. Sah for services provided during the years ended December 31, 2020 and December 31, 2019 was $0.4 million in each of the years. There were no fees for scientific advisory services paid to Dr. Sah in the year ended December 31, 2018. Under the collaboration agreement, the Company and Neurocrine have agreed to conduct research, development and commercialization of certain of the Company’s AAV gene therapy products (Note 9). Amounts due from Neurocrine are reflected as related party collaboration receivables. The Company recorded approximately $8.0 million and $18.5 million in related party collaboration receivables associated with Neurocrine as of December 31, 2020 and 2019, respectively.

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2020
Subsequent Events
Subsequent Events	16. Subsequent Events In February 2021, Neurocrine notified the Company that it had elected to terminate the Neurocrine Collaboration solely with regards to the VY-AADC Program, effective August 2, 2021 (the “Neurocrine VY-AADC Program Termination Effective Date”). The Neurocrine Collaboration Agreement remains in full force and effect for each other program thereunder. ​ As a result of the termination, as of the Neurocrine VY-AADC Program Termination Effective Date, the license granted by the Company to Neurocrine thereunder regarding the VY-AADC Program shall expire and the Company shall regain worldwide intellectual property rights regarding the VY-AADC Program, in each case in accordance with the terms of the Neurocrine Collaboration Agreement. The Company intends to support Neurocrine, the study sponsor and IND holder, on ongoing matters related to the completion of imaging and clinical assessments requested by the DSMB and the provision of other information requested by the FDA for the RESTORE-1 Phase 2 clinical trial. ​

Summary of significant accoun_2

Summary of significant accounting policies (Policies)	12 Months Ended
	Dec. 31, 2020
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”).
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 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 current marketable securities, and the warrants are included in current assets since they expire in September of 2021. 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 uses a forward-looking approach based on expected losses to estimate credit losses on certain types of financial instruments, including trade receivables and available-for-sale debt securities. No other than temporary losses have been recognized. Cash, cash equivalents, and marketable securities as of December 31, 2020 and 2019 consist of the following: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Amortized ​ Unrealized ​ Unrealized ​ Fair ​ Cost Gains Losses Value ​ ​ (in thousands) As of December 31, 2020 ​ ​ ​ ​ ​ Money market funds included in cash and cash equivalents ​ $ 103,992 ​ $ — ​ $ — ​ $ 103,992 ​ Marketable securities: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ U.S. Treasury notes ​ 70,348 ​ ​ — ​ ​ 6 ​ 70,342 ​ Equity securities ​ ​ 1,220 ​ ​ 5,136 ​ ​ — ​ ​ 6,356 ​ Total marketable securities ​ $ 71,568 ​ $ 5,136 ​ $ 6 ​ $ 76,698 ​ Total money market funds and marketable securities ​ $ 175,560 ​ $ 5,136 ​ $ 6 ​ $ 180,690 ​ As of December 31, 2019 ​ ​ ​ ​ ​ Money market funds included in cash and cash equivalents ​ $ 78,303 ​ $ — ​ $ — ​ $ 78,303 ​ Marketable securities: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ U.S. Treasury notes ​ 195,467 ​ ​ 52 ​ ​ 28 ​ 195,491 ​ Equity securities ​ ​ 1,220 ​ ​ 700 ​ ​ — ​ ​ 1,920 ​ Total marketable securities ​ $ 196,687 ​ $ 752 ​ $ 28 ​ $ 197,411 ​ Total money market funds and marketable securities ​ $ 274,990 ​ $ 752 ​ $ 28 ​ $ 275,714 ​ ​ All of the Company’s marketable debt securities at December 31, 2020 and 2019 have a contractual maturity of one year or less.
Restricted Cash	Restricted Cash As of December 31, 2020 and 2019, the Company maintained restricted cash totaling approximately $1.8 million and $0.7 million, respectively, held in the form of money market accounts as collateral for the Company’s facility lease obligations. 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 consolidated balance sheets that sum to the total of the same such amounts shown in the statements of cash flows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of December 31, ​ ​ ​ 2020 2019 2018 ​ ​ ​ (in thousands) ​ Cash and cash equivalents ​ $ 104,440 ​ $ 86,042 ​ $ 46,859 ​ Restricted cash included in deposits and other noncurrent assets ​ ​ 1,779 ​ ​ 735 ​ ​ 735 ​ Total cash, cash equivalents, and restricted cash ​ $ 106,219 ​ $ 86,777 ​ $ 47,594 ​ ​
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, 2020.
Revenue Recognition	Revenue Recognition As of December 31, 2020, all of the Company’s revenue has been generated from its collaboration agreements with Sanofi Genzyme, AbbVie, and Neurocrine The Company enters into collaboration agreements which are within the scope of ASC 606, Revenue from Contracts with Customers 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 contract liabilities within deferred revenue on the consolidated balance sheets 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, license fees, process development and facilities costs. Facilities costs primarily include the allocation of rent, utilities and depreciation.
Leases	Leases Under Accounting Standards Codification (ASC) 842 Leases Right-of-use assets represent the Company’s right to use an underlying asset for the lease term and lease liabilities represent the Company’s obligation to make lease payments arising from the lease. Operating lease right-of-use assets and liabilities are recognized at the lease commencement date based on the present value of lease payments over the lease term. As the Company’s leases typically do not provide an implicit rate, the Company uses an estimate of its incremental borrowing rate based on the information available at the lease commencement date in determining the present value of lease payments. Operating lease right-of-use assets also include the effect of any lease prepaid or deferred lease payments and are reduced by lease incentives. The lease terms may include options to extend or terminate the lease when it is reasonably certain that the Company will exercise that option. Lease expense is recognized on a straight-line basis over the lease term. The Company has lease agreements with lease and non-lease components, which are generally accounted for separately. Non-lease components as it pertains to the Company's leased premises generally refer to common area maintenance charges related to the premises. Prior to January 1, 2019, rent expense and lease incentives from operating leases were recognized on a straight-line basis over the lease term. The difference between rent expenses recognized and rental payments was recorded as deferred rent in the accompanying consolidated balance sheets.
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 The Black-Scholes option pricing model requires inputs based on certain subjective assumptions, including (i) the expected stock price volatility, (ii) the calculation of expected term of the award, (iii) the risk-free interest rate and (iv) 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 historical 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 The Company expenses the fair value of its stock-based compensation awards on a straight-line basis over the associated service period, which is generally the period in which the related services are received, adjusted for actual forfeitures of unvested awards as they occur. 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 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, 2020, the Company does not have any significant uncertain tax positions.
Comprehensive Income (Loss)	Comprehensive Income (Loss) Comprehensive income (loss) is comprised of net income (loss) and other comprehensive income or loss. Other comprehensive income or loss consists of unrealized gains or losses on marketable securities.
Net Income (Loss) Per Share	Net Income (Loss) Per Share Basic net income (loss) per share is calculated by dividing the net income (loss) by the weighted-average number of shares of common stock outstanding during the period, without consideration for potentially dilutive securities. Diluted net income (loss) per share is computed by dividing the net income (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 income (loss) per share, unvested restricted common stock and outstanding stock options are considered to be potentially dilutive securities. Unvested restricted common stock and outstanding stock options were excluded from the calculation of diluted net loss per share in the years ended December 31, 2019 and 2018, because their effect would be anti-dilutive and therefore, basic and diluted net loss per share were the same for the years ended December 31, 2019 and 2018. The following table sets forth the outstanding potentially dilutive securities that have been excluded in the calculation of diluted net income (loss) per share because to do so would be anti-dilutive: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of December 31, ​ ​ ​ 2020 ​ 2019 ​ 2018 ​ Unvested restricted common stock awards ​ 156,863 ​ 176,471 ​ 235,294 ​ Unvested restricted common stock units ​ ​ 527,625 ​ ​ 455,404 ​ ​ — ​ Outstanding stock options ​ 5,379,856 ​ 5,317,326 ​ 4,225,152 ​ Total ​ 6,064,344 ​ 5,949,201 ​ 4,460,446 ​ ​ Basic net income (loss) and diluted weighted-average shares outstanding are as follows for the year ended December 31, 2020, 2019, and 2018. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ ​ 2020 ​ 2019 ​ 2018 ​ ​ (in thousands, except share data) Numerator: ​ ​ ​ ​ ​ ​ ​ ​ ​ Net income (loss) ​ $ 36,741 ​ $ (43,597) ​ $ (88,288) Denominator for basic net income (loss) per share: ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted average shares outstanding-basic ​ ​ 37,132,447 ​ ​ 35,898,266 ​ ​ 32,065,781 Denominator for diluted net income (loss) per share: ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted average shares outstanding ​ ​ 37,132,447 ​ ​ 35,898,266 ​ ​ 32,065,781 Common stock options and restricted stock units ​ ​ 216,068 ​ ​ — ​ ​ — Weighted average shares outstanding-diluted ​ ​ 37,348,514 ​ ​ 35,898,266 ​ ​ 32,065,781 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
Concentrations of Credit Risk and Off Balance Sheet Risk & Concentration of Suppliers	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 financial institutions 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 third-party manufacturers 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 2016, the Financial Accounting Standards Board, or FASB, issued ASU 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial In August 2018, the FASB issued ASU No. 2018-13, Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement Recent Accounting Pronouncements In August 2020, the FASB issued ASU No. 2020-06, Debt – Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging—Contracts in Entity’s Own Equity (Subtopic 815-40) In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes

Summary of significant accoun_3

Summary of significant accounting policies (Tables)	12 Months Ended
	Dec. 31, 2020
Summary of significant accounting policies
Schedule of money market funds and marketable securities	Cash, cash equivalents, and marketable securities as of December 31, 2020 and 2019 consist of the following: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Amortized ​ Unrealized ​ Unrealized ​ Fair ​ Cost Gains Losses Value ​ ​ (in thousands) As of December 31, 2020 ​ ​ ​ ​ ​ Money market funds included in cash and cash equivalents ​ $ 103,992 ​ $ — ​ $ — ​ $ 103,992 ​ Marketable securities: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ U.S. Treasury notes ​ 70,348 ​ ​ — ​ ​ 6 ​ 70,342 ​ Equity securities ​ ​ 1,220 ​ ​ 5,136 ​ ​ — ​ ​ 6,356 ​ Total marketable securities ​ $ 71,568 ​ $ 5,136 ​ $ 6 ​ $ 76,698 ​ Total money market funds and marketable securities ​ $ 175,560 ​ $ 5,136 ​ $ 6 ​ $ 180,690 ​ As of December 31, 2019 ​ ​ ​ ​ ​ Money market funds included in cash and cash equivalents ​ $ 78,303 ​ $ — ​ $ — ​ $ 78,303 ​ Marketable securities: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ U.S. Treasury notes ​ 195,467 ​ ​ 52 ​ ​ 28 ​ 195,491 ​ Equity securities ​ ​ 1,220 ​ ​ 700 ​ ​ — ​ ​ 1,920 ​ Total marketable securities ​ $ 196,687 ​ $ 752 ​ $ 28 ​ $ 197,411 ​ Total money market funds and marketable securities ​ $ 274,990 ​ $ 752 ​ $ 28 ​ $ 275,714 ​
Reconciliation of cash, cash equivalents, and restricted cash	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of December 31, ​ ​ ​ 2020 2019 2018 ​ ​ ​ (in thousands) ​ Cash and cash equivalents ​ $ 104,440 ​ $ 86,042 ​ $ 46,859 ​ Restricted cash included in deposits and other noncurrent assets ​ ​ 1,779 ​ ​ 735 ​ ​ 735 ​ Total cash, cash equivalents, and restricted cash ​ $ 106,219 ​ $ 86,777 ​ $ 47,594 ​
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 income (loss) per share because to do so would be anti-dilutive: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of December 31, ​ ​ ​ 2020 ​ 2019 ​ 2018 ​ Unvested restricted common stock awards ​ 156,863 ​ 176,471 ​ 235,294 ​ Unvested restricted common stock units ​ ​ 527,625 ​ ​ 455,404 ​ ​ — ​ Outstanding stock options ​ 5,379,856 ​ 5,317,326 ​ 4,225,152 ​ Total ​ 6,064,344 ​ 5,949,201 ​ 4,460,446 ​ ​
Schedule of weighted average number of shares, basic and diluted	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ ​ 2020 ​ 2019 ​ 2018 ​ ​ (in thousands, except share data) Numerator: ​ ​ ​ ​ ​ ​ ​ ​ ​ Net income (loss) ​ $ 36,741 ​ $ (43,597) ​ $ (88,288) Denominator for basic net income (loss) per share: ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted average shares outstanding-basic ​ ​ 37,132,447 ​ ​ 35,898,266 ​ ​ 32,065,781 Denominator for diluted net income (loss) per share: ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted average shares outstanding ​ ​ 37,132,447 ​ ​ 35,898,266 ​ ​ 32,065,781 Common stock options and restricted stock units ​ ​ 216,068 ​ ​ — ​ ​ — Weighted average shares outstanding-diluted ​ ​ 37,348,514 ​ ​ 35,898,266 ​ ​ 32,065,781 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Fair value measurements (Tables

Fair value measurements (Tables)	12 Months Ended
	Dec. 31, 2020
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, 2020 and 2019 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, 2020 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Money market funds included in cash and cash equivalents ​ $ 103,992 ​ $ 103,992 ​ $ — ​ $ — ​ Marketable securities: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ U.S. Treasury notes ​ ​ 70,342 ​ ​ 70,342 ​ ​ — ​ ​ — ​ Equity securities ​ ​ 6,356 ​ ​ 6,356 ​ ​ — ​ ​ — ​ Total marketable securities ​ $ 76,698 ​ $ 76,698 ​ $ — ​ $ — ​ Warrants to purchase equity securities ​ ​ 3,816 ​ ​ — ​ ​ 3,816 ​ ​ — ​ Total ​ $ 184,506 ​ $ 180,690 ​ $ 3,816 ​ $ — ​ December 31, 2019 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Money market funds included in cash and cash equivalents $ 78,303 ​ $ 78,303 ​ $ — ​ $ — ​ Marketable securities: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ U.S. Treasury notes ​ ​ 195,491 ​ ​ 195,491 ​ ​ — ​ ​ — ​ Equity securities ​ ​ 1,920 ​ ​ 1,920 ​ ​ — ​ ​ — ​ Total marketable securities ​ $ 197,411 ​ $ 197,411 ​ $ — ​ $ — ​ Warrants to purchase equity securities ​ ​ 554 ​ ​ — ​ ​ 554 ​ ​ — ​ Total ​ $ 276,268 ​ $ 275,714 ​ $ 554 ​ $ — ​
Schedule of fair value assumptions	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of December 31, ​ ​ 2020 ​ 2019 ​ Risk-free interest rate ​ 0.1 % ​ 1.6 % ​ Expected dividend yield ​ — % ​ — % ​ Expected term (in years) ​ 0.7 ​ ​ 1.7 ​ ​ Expected volatility ​ 89.2 % ​ 71.6 % ​

Prepaid expenses and other cu_2

Prepaid expenses and other current assets (Tables)	12 Months Ended
	Dec. 31, 2020
Prepaid expense and other current assets
Schedule of prepaid expense and other current assets	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of December 31, ​ 2020 2019 ​ ​ (in thousands) ​ Warrants to purchase equity securities ​ $ 3,816 ​ $ 554 ​ Prepaid research and development contracts ​ 1,674 ​ ​ 1,999 ​ Other current assets ​ ​ 1,575 ​ ​ 1,398 ​ Prepaid insurance ​ 1,459 ​ 203 ​ Accrued interest receivable ​ ​ 95 ​ ​ 476 ​ Total ​ $ 8,619 ​ $ 4,630 ​

Property and equipment, net (Ta

Property and equipment, net (Tables)	12 Months Ended
	Dec. 31, 2020
Property and equipment, net
Schedule of property and equipment, net	Property and equipment, net consists of the following: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of December 31, ​ ​ 2020 2019 ​ ​ (in thousands) ​ Laboratory equipment ​ $ 18,093 ​ $ 13,748 ​ Leasehold improvements ​ ​ 15,469 ​ ​ 7,129 ​ Furniture and office equipment ​ ​ 2,410 ​ ​ 1,888 ​ Construction in progress ​ 866 ​ 2,188 ​ Other ​ ​ 167 ​ 690 ​ Total property and equipment ​ 37,005 ​ 25,643 ​ Less: accumulated depreciation ​ (11,570) ​ (7,657) ​ Property and equipment, net ​ $ 25,435 ​ $ 17,986 ​ ​ ​ ​ ​ ​ ​ ​ ​

Accrued expenses (Tables)

Accrued expenses (Tables)	12 Months Ended
	Dec. 31, 2020
Accrued expenses
Schedule of accrued expenses	Accrued expenses consist of the following: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of December 31, ​ ​ 2020 2019 ​ ​ (in thousands) ​ Research and development costs ​ $ 6,624 ​ $ 13,248 ​ Employee compensation costs ​ ​ 5,857 ​ ​ 5,733 ​ Professional services ​ 1,153 ​ 897 ​ Accrued goods and services ​ 496 ​ 1,386 ​ Other ​ ​ 75 ​ ​ 252 ​ Total ​ $ 14,205 ​ $ 21,516 ​

Lease obligation (Tables)

Lease obligation (Tables)	12 Months Ended
	Dec. 31, 2020
Lease obligation
Summary of significant contractual obligations under operating leases	The following table summarizes the Company’s significant contractual obligations under operating leases as of payment due date by period at December 31, 2020: ​ ​ ​ ​ ​ ​ Total Minimum ​ Lease Payments ​ ​ (in thousands) ​ 2021 ​ $ 7,822 ​ 2022 ​ 8,469 ​ 2023 ​ 8,723 ​ 2024 ​ ​ 8,985 ​ 2025 ​ ​ 9,644 ​ Thereafter ​ ​ 19,945 ​ Total future minimum lease payments ​ $ 63,588 ​ Less: imputed interest ​ ​ (15,981) ​ Total lease liabilities ​ $ 47,607 ​ Reported as: ​ ​ ​ ​ Other current liabilities ​ $ 4,198 ​ Other non-current liabilities ​ ​ 43,409 ​ Total lease liabilities ​ $ 47,607 ​

Other liabilities (Tables)

Other liabilities (Tables)	12 Months Ended
	Dec. 31, 2020
Other liabilities
Summary of other current and non-current liabilities	​ ​ ​ ​ ​ ​ ​ ​ ​ As of December 31, ​ ​ 2020 2019 ​ ​ (in thousands) ​ Other current liabilities ​ ​ ​ ​ ​ ​ Lease liabilities ​ 4,198 ​ ​ 3,193 ​ Total other current liabilities $ 4,198 ​ $ 3,193 ​ ​ ​ ​ ​ ​ ​ ​ Other non-current liabilities ​ ​ ​ ​ ​ ​ Lease liabilities $ 43,409 ​ $ 30,975 ​ Other ​ 1,001 ​ ​ 1,001 ​ Total other non-current liabilities $ 44,410 ​ $ 31,976 ​

Commitments and contingencies (

Commitments and contingencies (Tables)	12 Months Ended
	Dec. 31, 2020
Schedule of collaboration receivables and contract liabilities	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ ​ ​ Balance at ​ ​ ​ December 31, 2019 ​ Additions ​ Deductions ​ December 31, 2020 ​ ​ ​ ​ ( in thousands) ​ Related party collaboration receivable ​ $ 18,496 ​ $ 35,952 ​ $ (46,436) ​ $ 8,012 ​ Contract liabilities: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Deferred revenue ​ $ 70,040 ​ $ ​ ​ $ (26,351) ​ $ 43,689 ​
AbbVie Tau Collaboration Agreement
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 ​
AbbVie Alpha Synuclein Collaboration Agreement
Schedule of allocation of transaction price	​ ​ ​ ​ ​ ​ Performance Obligation ​ Amount ​ ​ ​ (in thousands) ​ ASN Research Services ​ $ 23,768 ​ ASN First Development Option Material Right ​ ​ 41,232 ​ Total ​ $ 65,000 ​
Neurocrine Collaboration Agreement
Schedule of allocation of variable consideration	​ ​ ​ ​ ​ ​ Performance Obligation ​ Amount ​ ​ ​ (in thousands) ​ Variable Consideration ​ ​ ​ VY-AADC Program ​ $ 86,480 ​ FA Program ​ ​ 87,991 ​ Discovery Program 1 ​ ​ 72,247 ​ Discovery Program 2 ​ ​ 69,515 ​ Total ​ $ 316,233 ​
Schedule of allocation of fixed consideration	​ ​ ​ ​ ​ ​ Performance Obligation ​ Amount ​ ​ ​ (in thousands) ​ Fixed Consideration ​ ​ ​ VY-AADC Program ​ $ 80,373 ​ FA Program ​ ​ 6,005 ​ Discovery Program 1 ​ ​ 3,002 ​ Discovery Program 2 ​ ​ 3,002 ​ Total ​ $ 92,382 ​

Common stock (Tables)

Common stock (Tables)	12 Months Ended
	Dec. 31, 2020
Common stock.
Schedule of shares reserved for future issuance	Shares Reserved For Future Issuance ​ ​ ​ ​ ​ ​ ​ ​ As of December 31, ​ ​ 2020 2019 Shares reserved for vesting of restricted stock awards under the Founder Agreements 156,863 ​ 176,471 ​ Shares reserved for exercise of outstanding stock options 5,485,078 ​ 5,317,326 Shares reserved for vesting of outstanding restricted stock units 638,471 ​ 455,404 Shares reserved for issuances under the 2015 Stock Option Plan ​ 2,827,185 ​ 1,875,078 ​ Shares reserved for issuances under the 2015 Employee Stock Purchase Plan ​ 1,484,923 ​ 1,218,876 ​ ​ 10,592,520 ​ 9,043,155

Stock-based compensation (Table

Stock-based compensation (Tables)	12 Months Ended
	Dec. 31, 2020
Stock-based compensation
Compensation cost recognized for all stock-based compensation awards	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year ended December 31, ​ 2020 2019 2018 ​ ​ (in thousands) Research and development ​ $ 6,357 ​ $ 7,383 ​ $ 4,717 ​ General and administrative ​ 8,577 ​ 8,257 ​ 10,993 ​ Total stock-based compensation expense ​ $ 14,934 ​ $ 15,640 ​ $ 15,710 ​
Summary of stock-based compensation expense by classification	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year ended December 31, ​ ​ ​ 2020 2019 2018 ​ ​ ​ (in thousands) ​ Stock options ​ $ 11,387 ​ $ 13,380 ​ $ 14,956 ​ Restricted stock awards and units ​ ​ 3,110 ​ ​ 1,935 ​ ​ 482 ​ Employee stock purchase plan awards ​ 437 ​ 325 ​ 272 ​ Total stock-based compensation expense ​ $ 14,934 ​ $ 15,640 ​ $ 15,710 ​
Summary of status and changes in unvested restricted stock	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted ​ ​ ​ ​ ​ Average ​ ​ ​ ​ ​ Grant Date ​ ​ ​ ​ ​ Fair Value ​ ​ ​ Units Per Unit ​ Unvested restricted stock units as of December 31, 2019 ​ 455,404 ​ $ 12.16 ​ Awarded ​ 494,604 ​ $ 12.67 ​ Vested ​ (150,759) ​ $ 12.12 ​ Forfeited ​ (160,778) ​ $ 11.47 ​ Unvested restricted stock units as of December 31, 2020 ​ 638,471 ​ $ 12.74 ​
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, 2019 5,317,326 ​ $ 15.98 ​ ​ ​ ​ ​ Granted ​ 1,560,575 ​ $ 11.87 ​ ​ ​ ​ ​ ​ Exercised ​ (228,436) ​ $ 10.15 ​ ​ ​ ​ ​ ​ Cancelled or forfeited ​ (1,164,387) ​ $ 17.33 ​ ​ ​ ​ ​ ​ Outstanding at December 31, 2020 ​ 5,485,078 ​ $ 14.77 ​ 7.5 ​ $ 1,313 ​ Exercisable at December 31, 2020 3,027,116 ​ $ 15.24 6.5 ​ $ — ​
Summary of weighted-average assumptions	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year ended December 31, ​ ​ 2020 2019 2018 ​ Risk-free interest rate 1.0 % 2.2 % ​ 2.8 % ​ Expected dividend yield — % — % ​ — % ​ Expected term (in years) 6.0 ​ 6.0 ​ ​ 6.0 ​ ​ Expected volatility 73.7 % 74.7 % ​ 74.4 % ​

Income taxes (Tables)

Income taxes (Tables)	12 Months Ended
	Dec. 31, 2020
Income taxes
Schedule of components of income expense (benefit)	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year ended December 31, ​ 2020 ​ 2019 ​ 2018 ​ (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	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year ended December 31, ​ ​ 2020 ​ 2019 ​ 2018 ​ Income tax computed at federal statutory tax rate ​ 21.0 % ​ 21.0 % ​ 21.0 % ​ Non-deductible expenses ​ 5.0 % ​ (2.2) % ​ (2.1) % ​ Other ​ 1.9 % ​ — % ​ — % ​ State taxes, net of federal benefit ​ (2.3) % ​ 8.0 % ​ 6.3 % ​ Change in valuation allowance ​ (5.0) % ​ (36.2) % ​ (28.1) % ​ General business credit carryovers ​ (20.6) % ​ 9.4 % ​ 3.1 % ​ Total ​ — % ​ — % ​ 0.2 % ​
Schedule of principal components of the Company's deferred tax assets and liabilities	​ ​ ​ ​ ​ ​ ​ ​ As of December 31, ​ ​ 2020 ​ 2019 ​ (in thousands) ​ Deferred tax assets: ​ ​ ​ ​ ​ ​ Net operating loss carryforwards $ 41,130 ​ $ 49,713 ​ Tax credit carryforwards 28,173 ​ 17,297 ​ Deferred revenue ​ 12,176 ​ ​ 16,805 ​ Lease liability ​ 13,006 ​ ​ 9,335 ​ Stock compensation ​ 4,368 ​ ​ 5,157 ​ Non-deductible accruals and reserves 1,288 ​ 1,502 ​ Intangibles 721 ​ 776 ​ Total deferred tax assets 100,862 ​ 100,585 ​ Less valuation allowance (84,866) ​ (90,920) ​ Net deferred tax assets 15,996 ​ 9,665 ​ Deferred tax liabilities ​ ​ ​ ​ ​ Unrealized gain on available-for-sale securities ​ (8) ​ ​ (8) ​ Right of use assets ​ (9,852) ​ (7,779) ​ Depreciation and amortization ​ (6,136) ​ ​ (1,878) ​ Net deferred taxes $ — ​ $ — ​ ​ ​ ​ ​ ​ ​ ​

Nature of business (Details)

Nature of business (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Nature of business
Accumulated deficit	$ (275,907)	$ (312,648)

Summary of significant accoun_4

Summary of significant accounting policies - Cash, Cash Equivalents, and Marketable Securities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Amortized Cost
Cash and cash equivalents	$ 104,440	$ 86,042	$ 46,859
Marketable securities, amortized cost	71,568	196,687
Total money market funds and marketable securities	175,560	274,990
Fair Value
Marketable securities, fair value	76,698	197,411
Total money market funds and marketable securities	180,690	275,714
Unrealized Gains and Losses
Marketable securities, unrealized gains	5,136	752
Total money market funds and marketable securities	5,136	752
Marketable securities, unrealized losses	6	28
Total money market funds and marketable securities	6	$ 28
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	$ 103,992	$ 78,303
Fair Value
Cash and cash equivalents	103,992	78,303
U.S. Treasury notes
Amortized Cost
Marketable securities, amortized cost	70,348	195,467
Fair Value
Marketable securities, fair value	70,342	195,491
Unrealized Gains and Losses
Marketable securities, unrealized gains		52
Marketable securities, unrealized losses	6	28
Equity securities
Amortized Cost
Marketable securities, amortized cost	1,220	1,220
Fair Value
Marketable securities, fair value	6,356	1,920
Unrealized Gains and Losses
Marketable securities, unrealized gains	$ 5,136	$ 700

Summary of significant accoun_5

Summary of significant accounting policies - Restricted Cash - (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Summary of significant accounting policies
Cash and cash equivalents	$ 104,440	$ 86,042	$ 46,859
Restricted cash included in deposits and other noncurrent assets	1,779	735	735
Total cash, cash equivalents, and restricted cash	$ 106,219	$ 86,777	$ 47,594	$ 32,265

Summary of significant accoun_6

Summary of significant accounting policies - Stock Based Compensation Expense - (Details)	12 Months Ended
	Dec. 31, 2020
Summary of significant accounting policies
Expected dividend yield	0.00%

Summary of significant accoun_7

Summary of significant accounting policies - Net income (loss) per share - (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities	6,064,344	5,949,201	4,460,446
Numerator:
Net income (loss)	$ 36,741	$ (43,597)	$ (88,288)
Denominator for basic net income (loss) per share:
Weighted average shares outstanding-basic	37,132,447	35,898,266	32,065,781
Denominator for diluted net income (loss) per share:
Weighted average shares outstanding-basic	37,132,447	35,898,266	32,065,781
Common stock options and restricted stock units	216,068
Weighted average shares outstanding-diluted	37,348,514	35,898,266	32,065,781
Financial instruments with off-balance sheet risk	$ 0
Unvested restricted common stock awards
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities	156,863	176,471	235,294
Unvested restricted common stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities	527,625	455,404
Outstanding stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities	5,379,856	5,317,326	4,225,152

Fair value measurements - Asset

Fair value measurements - Assets measured on a recurring basis - (Details) $ in Thousands	Dec. 31, 2020USD ($)item	Dec. 31, 2019USD ($)item	Dec. 31, 2018USD ($)
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	$ 104,440	$ 86,042	$ 46,859
Measurement Input, Risk-Free Interest Rate
Fair value assumptions
Warrants, measurement input | item	0.001	0.016
Measurement Input, Expected Dividend Yield
Fair value assumptions
Warrants, measurement input	0	0
Measurement Input, Expected Term
Fair value assumptions
Warrants, measurement input | item	0.7	1.7
Measurement Input, Expected Volatility
Fair value assumptions
Warrants, measurement input | item	0.892	0.716
Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Marketable securities	$ 76,698	$ 197,411
Total assets measured at fair value	184,506	276,268
Level 1 | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Marketable securities	76,698	197,411
Total assets measured at fair value	180,690	275,714
Level 2 | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Total assets measured at fair value	3,816	554
U.S. Treasury notes | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Marketable securities	70,342	195,491
U.S. Treasury notes | Level 1 | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Marketable securities	70,342	195,491
Equity securities | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Marketable securities	6,356	1,920
Equity securities | Level 1 | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Marketable securities	6,356	1,920
Warrants | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Warrants to purchase equity securities	3,816	554
Warrants | Level 2 | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Warrants to purchase equity securities	3,816	554
Money market funds
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	103,992	78,303
Money market funds | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	103,992	78,303
Money market funds | Level 1 | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	$ 103,992	$ 78,303

Prepaid expenses and other cu_3

Prepaid expenses and other current assets (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Prepaid expense and other current assets
Warrants to purchase equity securities	$ 3,816	$ 554
Prepaid research and development contracts	1,674	1,999
Other current assets	1,575	1,398
Prepaid insurance	1,459	203
Accrued interest receivable	95	476
Total	$ 8,619	$ 4,630

Property and equipment, net (De

Property and equipment, net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 37,005	$ 25,643
Less: accumulated depreciation	(11,570)	(7,657)
Property and equipment, net	25,435	17,986
Depreciation expense	3,817	2,765	$ 2,117
Laboratory equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	18,093	13,748
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	15,469	7,129
Furniture and office equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	2,410	1,888
Construction in progress
Property, Plant and Equipment [Line Items]
Property and equipment, gross	866	2,188
Other
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 167	$ 690

Accrued expenses (Details)

Accrued expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Accrued expenses
Research and development costs	$ 6,624	$ 13,248
Employee compensation costs	5,857	5,733
Professional services	1,153	897
Accrued goods and services	496	1,386
Other	75	252
Total	$ 14,205	$ 21,516

Lease obligation (Details)

Lease obligation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Lease expense	$ 6,200	$ 5,700	$ 4,000
Restricted cash included in deposits and other noncurrent assets	$ 1,779	$ 735	$ 735
Weighted average remaining lease term	6 years 9 months 18 days
Weighted average incremental borrowing rate	8.00%
Total minimum lease payments
2021	$ 7,822
2022	8,469
2023	8,723
2024	8,985
2025	9,644
Thereafter	19,945
Total future minimum lease payments	63,588
Less: imputed interest	(15,981)
Total lease liability	$ 47,607
Operating Lease, Liability, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesCurrent us-gaap:OtherLiabilitiesNoncurrent	us-gaap:OtherLiabilitiesCurrent us-gaap:OtherLiabilitiesNoncurrent
Current lease liabilities	$ 4,198	$ 3,193
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	Other Liabilities, Current	Other Liabilities, Current
Non-current lease liabilities	$ 43,409	$ 30,975
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	Other Liabilities, Noncurrent	Other Liabilities, Noncurrent
75 and 64 Sidney Street Leases
Leasehold improvements	$ 5,300
75 Hayden Avenue
Leasehold improvements	$ 5,600

Other liabilities (Details)

Other liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Other current liabilities
Lease liabilities	$ 4,198	$ 3,193
Total other current liabilities	4,198	3,193
Other non-current liabilities
Lease liabilities	43,409	30,975
Other	1,001	1,001
Total other non-current liabilities	$ 44,410	$ 31,976

Commitments and contingencies -

Commitments and contingencies - Neurocrine Collaboration Agreement - (Details) $ / shares in Units, $ in Thousands	1 Months Ended		3 Months Ended	12 Months Ended
	Jun. 30, 2019USD ($)shares	Mar. 31, 2019USD ($)item$ / sharesshares	Dec. 31, 2020USD ($)	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)
Allocation of Transaction Price
Revenue recognized				$ 26,351
Deferred revenue			$ 43,689	43,689	$ 70,040
Related party collaboration receivable			8,012	8,012	18,496
Neurocrine | FA Program
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Percentage of profit or loss under co-co option		60.00%
Neurocrine Collaboration Agreement
Allocation of Transaction Price
Related party collaboration receivable			8,000	8,000	18,500
Neurocrine Collaboration Agreement | VY-AADC
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Percentage of profit or loss under co-co option		50.00%
Percentage of profit or loss under co-co option after paying rate-shifting fee		45.00%
Neurocrine Collaboration Agreement | FA Program
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Percentage of profit or loss under co-co option		40.00%
Neurocrine Collaboration Agreement | Neurocrine
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of collaboration programs | item		4
Number of discovery programs | item		2
Programs added later | item		2
Number of targets | item		8
Upfront payment	$ 115,000	$ 115,000
Purchase of common stock, shares | shares	4,179,728	4,179,728
Purchase of common stock	$ 5,000	$ 50,000
Price per share | $ / shares		$ 11.9625
Aggregate maximum milestone payments to be received from collaborative partner		$ 170,000
Estimated cost reimbursement	431,100		316,200
Termination period		10 years
Period of advance notice for termination prior to first commercial sale		180 days
Period of advance notice for termination after first commercial sale		1 year
Discount related to equity investment	27,600
Equity investment	50,000
Allocation of Transaction Price
Allocation of variable consideration			316,233	316,233
Allocation of fixed consideration	92,400		92,382	92,382
Revenue recognized				56,700	$ 60,000
Deferred revenue			43,800	43,800
Reimbursement costs expected to be received				8,000
Costs to obtain collaboration agreement			800	800
Consideration received	$ 5,000
Neurocrine Collaboration Agreement | Neurocrine | Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Percentage of reduction in royalty payments		50.00%
Neurocrine Collaboration Agreement | Neurocrine | VY-AADC
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Percentage of profit or loss under co-co option		50.00%
Rate-shifting fee		$ 35,000
Percentage of profit or loss under co-co option after paying rate-shifting fee		55.00%
Allocation of Transaction Price
Allocation of variable consideration			86,480	86,480
Allocation of fixed consideration			80,373	80,373
Neurocrine Collaboration Agreement | Neurocrine | FA Program
Allocation of Transaction Price
Allocation of variable consideration			87,991	87,991
Allocation of fixed consideration			6,005	6,005
Neurocrine Collaboration Agreement | Neurocrine | Discovery program 1
Allocation of Transaction Price
Allocation of variable consideration			72,247	72,247
Allocation of fixed consideration			3,002	3,002
Neurocrine Collaboration Agreement | Neurocrine | Discovery program 2
Allocation of Transaction Price
Allocation of variable consideration			69,515	69,515
Allocation of fixed consideration			$ 3,002	$ 3,002
Neurocrine Collaboration Agreement | Neurocrine | Development and regulatory milestones | FA Program
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Aggregate maximum milestone payments to be received from collaborative partner		$ 195,000
Neurocrine Collaboration Agreement | Neurocrine | Development and regulatory milestones | Discovery program 1
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Aggregate maximum milestone payments to be received from collaborative partner		130,000
Neurocrine Collaboration Agreement | Neurocrine | Commercial Milestone
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Per Milestone, maximum milestone payments to be received from collaborative partner		275,000
Aggregate maximum milestone payments to be received from collaborative partner		$ 1,100,000

Commitments and contingencies_2

Commitments and contingencies - Collaboration receivables and contract liabilities - (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)
Commitments and contingencies
Related party collaboration receivable, beginning balance	$ 18,496
Additions	35,952
Deductions	(46,436)
Related party collaboration receivable, ending balance	8,012
Deferred revenue, beginning balance	70,040
Deductions	(26,351)
Deferred revenue, ending balance	$ 43,689

Commitments and contingencies_3

Commitments and contingencies - Significant Agreements - (Details) - Collaborative Arrangement - Sanofi Genzyme $ in Millions	1 Months Ended
	Feb. 28, 2015USD ($)shares
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Non-refundable upfront payment received	$ 65
Redeemable Convertible Preferred Stock Series B
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Temporary equity issued (in shares) | shares	10,000,000
Temporary equity issued	$ 30
Maximum potential in-kind services to be provided by collaborative partner	$ 5

Commitments and contingencies_4

Commitments and contingencies - Termination of Agreement (Details) - Termination Agreement - Sanofi Genzyme $ in Millions	Jun. 14, 2019USD ($)item
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of non central nervous system | item	2
Up-front payment made	$ 10
Milestone payment agreed to be paid	$ 10
Period in which milestone payment will be made of filing of investigational new drug	15 days
Percentage of threshold sublicensing income agreed to be paid	50.00%
Unused in-kind services waived	$ 0.4
Threshold number of capsids having exclusive options to evaluate for no consideration | item	4
Threshold additional number of capsids having options to evaluate for low six-figure payment per capsid | item	2
Threshold number of capsids that has right to obtain an exclusive license | item	2
Option exercise fee	$ 1
Development and regulatory milestones
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payable upon achievement	15
Sales milestone
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payable upon achievement	$ 20

Commitments and contingencies_5

Commitments and contingencies - Accounting Analysis - (Details) $ in Thousands	Jun. 14, 2019USD ($)	Feb. 28, 2018USD ($)item	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Feb. 28, 2015USD ($)
Income Statement Related Disclosures [Abstract]
Revenue recognized			$ 26,351
Deferred revenue			43,689	$ 70,040
Total payable amount	$ 20,000
Sanofi Genzyme | Collaborative Arrangement
Allocable arrangement consideration
Non-refundable upfront payment received						$ 65,000
Income Statement Related Disclosures [Abstract]
Revenue recognized	28,700			32,600	$ 900
Deferred revenue	48,700
Up-front fee paid on date of termination	10,000
Milestone payable upon achievement	10,000
Sanofi Genzyme | Termination Agreement
Allocable arrangement consideration
Non-refundable upfront payment received	$ 1,000
AbbVie | AbbVie Tau Collaboration Agreement
Allocable arrangement consideration
Non-refundable upfront payment received		$ 69,000
Income Statement Related Disclosures [Abstract]
Revenue recognized			46,300
Number of development options | item		3
AbbVie | AbbVie Tau Collaboration Agreement | Research Services
Income Statement Related Disclosures [Abstract]
Revenue recognized			$ 4,500	$ 11,300	$ 6,900

Commitments and contingencies_6

Commitments and contingencies - AbbVie Tau Collaboration Agreement - (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Feb. 28, 2018USD ($)item	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Deferred revenue		$ 43,689	$ 70,040
Revenue recognized		26,351
AbbVie Tau Collaboration Agreement | AbbVie
Collaborative Arrangement and Arrangement Other than Collaborative [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 development options | item	3
Number of research antibodies | item	1
Revenue recognized		46,300
Allocation of Transaction Price
Allocation of Transaction Price		69,000
AbbVie Tau Collaboration Agreement | AbbVie | Minimum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of development options | item	1
Antibodies to include in Research Period | item	2
The number of Licensed Products | item	1
AbbVie Tau Collaboration Agreement | AbbVie | Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of antibodies encoded by virus | item	5
Antibodies to include in Research Period | item	3
AbbVie Tau Collaboration Agreement | AbbVie | Research Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue recognized		4,500	$ 11,300	$ 6,900
Allocation of Transaction Price
Allocation of Transaction Price		34,482
AbbVie Tau Collaboration Agreement | AbbVie | First Development Option Material Right
Allocation of Transaction Price
Allocation of Transaction Price		$ 34,518

Commitments and contingencies_7

Commitments and contingencies - AbbVie Alpha Synuclein Collaboration Agreement - (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Feb. 28, 2019USD ($)item	Dec. 31, 2020USD ($)item	Dec. 31, 2019USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Deferred revenue		$ 43,689	$ 70,040
Revenue recognized		26,351
AbbVie Alpha Synuclein Collaboration Agreement | AbbVie
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of antibodies encoded by virus | item	1
Number of research antibodies | item	1
Non-refundable upfront payment received	$ 65,000
First Research Compound payment	80,000
Three additional research compounds payment to be received for each	$ 30,000
Number of additional research compounds | item	3
Milestone payments to be received for licensed compound in case Parkinson's disease indication	$ 450,000
Milestone payments to be received for licensed compound in case first indication other than Parkinson's disease	185,000
Milestone payments to be received for licensed compound for subsequent non-Parkinson's disease indication	92,500
Revenue recognized		$ 58,900
Allocation of Transaction Price
Allocation of Transaction Price	$ 65,000
AbbVie Alpha Synuclein Collaboration Agreement | AbbVie | Minimum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Antibodies to include in Research Period | item	1
The number of Licensed Products | item	1
AbbVie Alpha Synuclein Collaboration Agreement | AbbVie | Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Antibodies to include in Research Period | item	4	4
AbbVie Alpha Synuclein Collaboration Agreement | AbbVie | Research Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue recognized		$ 4,800	$ 1,300
Allocation of Transaction Price
Allocation of Transaction Price	$ 23,768
AbbVie Alpha Synuclein Collaboration Agreement | AbbVie | First Development Option Material Right
Allocation of Transaction Price
Allocation of Transaction Price	41,232
AbbVie Alpha Synuclein Collaboration Agreement | Commercial Milestone | AbbVie
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Specified commercial milestone payments	$ 500,000

Commitments and contingencies_8

Commitments and contingencies - Other Agreements - (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2016USD ($)multiple	Dec. 31, 2017USD ($)	Sep. 30, 2016USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Patent costs incurred	$ 0.5
Loss Contingency [Abstract]
Contingency reserves for litigation liabilities	0	$ 0
General and administrative
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Patent costs incurred	0.5	$ 0.6	$ 0.8
Various contract research organizations and institutions
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payable upon achievement	$ 0.1
Various contract research organizations and institutions | Minimum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of days written notice required by reporting entity to terminate agreement	30 days
Various contract research organizations and institutions | Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of days written notice required by reporting entity to terminate agreement	90 days
Non-profit organization agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Maximum funding				$ 4
Funding multiple | multiple				2.6
Milestone payment liability					$ 1
MRIC
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Amount of investment						$ 2

Preferred stock (Details)

Preferred stock (Details) - shares	Dec. 31, 2020	Dec. 31, 2019
Preferred stock
Preferred stock, authorized	5,000,000	5,000,000

Common stock (Details)

Common stock (Details)	12 Months Ended
	Dec. 31, 2020Vote$ / sharesshares	Jan. 31, 2021shares	Dec. 31, 2019$ / 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)	10,592,520		9,043,155
2015 Stock Option Plan
Class of Stock [Line Items]
Shares available for future issuance (in shares)	2,827,185	7,707,888		1,311,812
Vesting of restricted stock awards | Founder Awards
Class of Stock [Line Items]
Shares available for future issuance (in shares)	156,863		176,471
Exercise of stock options
Class of Stock [Line Items]
Shares available for future issuance (in shares)	5,485,078		5,317,326
Vesting of restricted stock units
Class of Stock [Line Items]
Shares available for future issuance (in shares)	638,471		455,404
Issuance under share based compensation plans | 2015 Stock Option Plan
Class of Stock [Line Items]
Shares available for future issuance (in shares)	2,827,185		1,875,078
Issuance under share based compensation plans | 2015 Employee Stock Purchase Plan
Class of Stock [Line Items]
Shares available for future issuance (in shares)	1,484,923		1,218,876

Stock-based compensation (Detai

Stock-based compensation (Details) $ / shares in Units, $ in Thousands	1 Months Ended			12 Months Ended				61 Months Ended
	Dec. 31, 2019$ / sharesshares	Oct. 31, 2015shares	Jan. 31, 2014$ / sharesshares	Dec. 31, 2020USD ($)item$ / sharesshares	Dec. 31, 2019USD ($)item$ / sharesshares	Dec. 31, 2018USD ($)itemshares	Dec. 31, 2016item	Jan. 31, 2021shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future issuance (in shares)	9,043,155			10,592,520	9,043,155
Stock-based compensation. | $				$ 14,934	$ 15,640	$ 15,710
Stock options granted (in shares)				1,560,575
Exercise price | $ / shares	$ 15.98			$ 14.77	$ 15.98
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)			823,529
2015 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future issuance (in shares)				1,484,923				1,926,969
Percentage of outstanding common stock								1.00%
New stock options issued				104,108	70,217
Number of shares authorized for stock-based compensation plan (in shares)		262,362
Period to purchase shares				6 months
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		2,827,185				7,707,888
Percentage limit of issued and outstanding shares up to which additional shares may be issued		4.00%
Percentage of outstanding common stock								4.00%
Outstanding stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation. | $				$ 11,387	$ 13,380	$ 14,956
Outstanding stock options | Inducement Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
New stock options issued				29,000	58,125	650,000
Number of executives issued awards | item				3	2	1
Vesting period				4 years
Outstanding stock options | Inducement Awards | Vesting period, tranche one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage				25.00%
Outstanding stock options | Inducement Awards | Vesting period, tranche two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period				36 months
Vesting percentage				75.00%
Unvested restricted common stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation. | $				$ 3,110	$ 1,935	$ 482
Unrecognized stock-based compensation expense | $				$ 5,600
Vesting period				3 years
Restricted stock granted, original issued price (in dollars per share) | $ / shares				$ 12.67
Unvested restricted common stock units | Inducement Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period				3 years
Unvested restricted common stock units | Inducement Awards | Vesting period, tranche one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage				33.00%
Unvested restricted common stock units | Inducement Awards | Vesting period, tranche two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage				33.00%
Performance based milestone awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Probable performance milestones | item							1
Performance milestones | item							3
Stock Repurchase Program, Remaining Number of Shares Authorized to be Repurchased	58,823				58,823
Stock-based compensation. | $						300
Vesting period	3 years
Founders restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
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
Non-statutory stock options | Inducement Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
New stock options issued				172,500	338,750
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)				1,388,075
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
Share-based Payment Arrangement, Employee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation. | $				$ 11,200	$ 13,300	14,600
Share-based Payment Arrangement, Employee | Unvested restricted common stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation. | $				2,800	1,900	100
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation. | $				6,357	7,383	4,717
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation. | $				$ 8,577	$ 8,257	$ 10,993

Stock-based compensation - Comp

Stock-based compensation - Compensation expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation.	$ 14,934	$ 15,640	$ 15,710
Outstanding stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation.	11,387	13,380	14,956
Unvested restricted common stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation.	3,110	1,935	482
Employee stock purchase plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation.	437	325	272
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation.	6,357	7,383	4,717
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation.	$ 8,577	8,257	10,993
Dr. Sah
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation.		$ 2,200
Consultant duration	3 years
Dr. Paul
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation.			$ 5,400
Consultant duration	3 years

Stock-based compensation - Rest

Stock-based compensation - Restricted Stock - (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Restricted stock disclosures
Stock-based compensation.	$ 14,934	$ 15,640	$ 15,710
Unvested restricted common stock units
Shares
Balance, beginning (in shares)	455,404
Granted (in shares)	494,604
Vested (in shares)	(150,759)
Forfeited (in shares)	(160,778)
Balance, ending (in shares)	638,471	455,404
Weighted Average Grant Date Fair Value Per Share
Balance, beginning (in dollars per share)	$ 12.16
Granted (in dollars per share)	12.67
Vested (in dollars per share)	12.12
Forfeited (in dollars per share)	11.47
Balance, ending (in dollars per share)	$ 12.74	$ 12.16
Restricted stock disclosures
Vesting period	3 years
Stock-based compensation.	$ 3,110	$ 1,935	482
Unrecognized stock-based compensation expense	$ 5,600
Remaining weighted-average remaining vesting period	1 year 9 months 18 days
Share-based Payment Arrangement, Employee
Restricted stock disclosures
Stock-based compensation.	$ 11,200	13,300	14,600
Share-based Payment Arrangement, Employee | Unvested restricted common stock units
Restricted stock disclosures
Stock-based compensation.	$ 2,800	$ 1,900	$ 100

Stock-based compensation - Stoc

Stock-based compensation - Stock Options - (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding, beginning balance (in shares)	5,317,326
Granted (in shares)	1,560,575
Exercised (in shares)	(228,436)
Cancelled or forfeited (in shares)	(1,164,387)
Outstanding, ending balance (in shares)	5,485,078	5,317,326
Exercisable (in shares)	3,027,116
Weighted Average Exercise Price
Outstanding (in dollars per share)	$ 15.98
Granted (in dollars per share)	11.87
Exercised (in dollars per share)	10.15
Cancelled or forfeited (in dollars per share)	17.33
Outstanding (in dollars per share)	14.77	$ 15.98
Exercisable (in dollars per share)	$ 15.24
Remaining Contractual Life
Outstanding	7 years 6 months
Exercisable	6 years 6 months
Aggregate Intrinsic Value
Outstanding	$ 1,313
Options disclosures
Stock-based compensation.	14,934	$ 15,640	$ 15,710
Outstanding stock options
Options disclosures
Stock-based compensation.	$ 11,387	13,380	14,956
Share-based Payment Arrangement, Employee
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 7.64
Stock-based compensation.	$ 11,200	$ 13,300	$ 14,600

Stock-based compensation - Unre

Stock-based compensation - Unrecognized expense and weighted-average period (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Fair value assumptions
Expected dividend yield	0.00%
Employees and directors stock options
Fair value assumptions
Risk-free interest rate	1.00%	2.20%	2.80%
Expected term	6 years	6 years	6 years
Expected volatility	73.70%	74.70%	74.40%
Unvested stock options
Options disclosures
Unrecognized stock-based compensation expense	$ 20
Remaining weighted-average remaining vesting period	2 years 6 months
Share-based Payment Arrangement, Employee
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 7.64

401(k) Savings Plan (Details)

401(k) Savings Plan (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
401(k) Savings Plan
Employer contribution expense	$ 1.1	$ 1	$ 0.8

Income taxes (Details)

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

Income taxes - Net operating lo

Income taxes - Net operating loss carryforwards (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2016item
Operating Loss Carryforwards [Line Items]
Maximum NOL carryforwards (as a percent)	80.00%
Valuation Allowance	$ 84,866	$ 90,920	$ 90,900
Change in the valuation allowance	$ (1,000)
Number of ownership changes | item				3
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Income tax computed at federal statutory tax rate	21.00%	21.00%	21.00%
Nondeductible expenses	5.00%	(2.20%)	(2.10%)
Other	1.90%
State taxes, net of federal benefit	(2.30%)	8.00%	6.30%
Change in valuation allowance	(5.00%)	(36.20%)	(28.10%)
General business credit carryovers	(20.60%)	9.40%	3.10%
Total			0.20%
Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 153,400
State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	140,900
Investment tax credit	900
Research and development tax credits carryforwards | Federal
Operating Loss Carryforwards [Line Items]
Tax credits carryforwards	21,100
Research and development tax credits carryforwards | State
Operating Loss Carryforwards [Line Items]
Tax credits carryforwards	$ 8,300

Income taxes - Deferred tax ass

Income taxes - Deferred tax assets (liabilities) and unrecognized tax benefits (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income taxes
Unrecognized tax benefits	$ 0	$ 0
Amounts in uncertain tax position	0	0
Accrued interest	0	0
Deferred tax assets:
Net operating loss carryforwards	41,130	49,713
Tax credit carryforwards	28,173	17,297
Deferred revenue	12,176	16,805
Lease liability	13,006	9,335
Stock compensation	4,368	5,157
Non-deductible accruals and reserves	1,288	1,502
Intangibles	721	776
Total deferred tax assets	100,862	100,585
Less valuation allowance	(84,866)	(90,920)	$ (90,900)
Net deferred tax assets	15,996	9,665
Deferred tax liabilities
Unrealized gain on available-for-sale securities	(8)	(8)
Right of use assets	(9,852)	(7,779)
Depreciation and amortization	$ (6,136)	$ (1,878)

Related party transactions (Det

Related party transactions (Details)	12 Months Ended
	Dec. 31, 2020USD ($)item	Dec. 31, 2019USD ($)item	Dec. 31, 2018USD ($)item
Related party transactions
Related party collaboration receivable	$ 8,012,000	$ 18,496,000
Former Executives | Board and scientific advisory services
Related party transactions
Number of individuals providing services | item	2	2	2
Dr. Paul | Board and scientific advisory services
Related party transactions
Total amount of services received	$ 200,000	$ 200,000	$ 100,000
Dr. Sah | Board and scientific advisory services
Related party transactions
Total amount of services received	400,000	400,000	$ 0
Neurocrine Collaboration Agreement
Related party transactions
Related party collaboration receivable	$ 8,000,000	$ 18,500,000