Prevail Therapeutics (PRVL) 10-K 26 Mar 20 2019 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Mar. 26, 2020	Jun. 28, 2019
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Trading Symbol	PRVL
Entity Registrant Name	Prevail Therapeutics Inc.
Entity Central Index Key	0001714798
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Well-known Seasoned Issuer	No
Entity Address, State or Province	NY
Entity Common Stock, Shares Outstanding		34,170,881
Entity Public Float			$ 214.5
Title of 12(b) Security	Common Stock, par value $0.0001 per share
Security Exchange Name	NASDAQ
Entity Interactive Data Current	Yes
Entity File Number	001-38939
Entity Tax Identification Number	82-2129632
Entity Address, Address Line One	430 East 29th Street
Entity Address, Address Line Two	Suite 1520
Entity Address, City or Town	New York
Entity Address, Postal Zip Code	10016
City Area Code	917
Local Phone Number	336-9310
Document Annual Report	true
Document Transition Report	false
Entity Incorporation, State or Country Code	DE
Documents Incorporated by Reference	Part III of this Annual Report on Form 10-K incorporates by reference certain information from the registrant’s definitive Proxy Statement for its 2020 annual meeting of shareholders, which the registrant intends to file pursuant to Regulation 14A with the Securities and Exchange Commission not later than 120 days after the registrant’s fiscal year end of December 31, 2019. Except with respect to information specifically incorporated by reference in this Form 10-K, the Proxy Statement is not deemed to be filed as part of this Form 10-K.

Balance Sheets

Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 168,051	$ 63,014
Prepaid expenses and other current assets	6,410	563
Total current assets	174,461	63,577
Property and equipment, net	2,549	678
Operating lease right-of-use assets	10,001	8,534
Restricted cash	91	91
Total assets	187,102	72,880
Current liabilities:
Accounts payable	5,162	1,241
Accrued expenses and other current liabilities	5,330	1,477
Operating lease liabilities	1,341	917
Total current liabilities	11,833	3,635
Long-term operating lease liabilities	9,927	7,952
Total liabilities	21,760	11,587
Commitments and contingencies (see Note 15) Redeemable convertible preferred stock
Stockholders' equity (deficit)
Common stock - $0.0001 par value, 200,000,000 and 28,398,600 shares authorized as of December 31, 2019 and 2018, respectively, 34,138,750 and 7,209,000 shares issued and outstanding as of December 31, 2019 and 2018, respectively	3	1
Additional paid-in capital	249,441	2,496
Accumulated deficit	(84,102)	(20,914)
Total stockholders’ equity (deficit)	165,342	(18,417)
Total liabilities, redeemable convertible preferred stock, and stockholders' equity (deficit)	187,102	72,880
Series Seed Preferred Stock
Temporary equity:
Preferred stock	0	3,524
Series A Preferred Stock
Temporary equity:
Preferred stock	$ 0	$ 76,186

Balance Sheets (Parenthetical)

Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2019	Dec. 31, 2018
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	200,000,000	28,398,600
Common stock, shares issued	34,138,750	7,209,000
Common stock, shares outstanding	34,138,750	7,209,000
Series Seed Preferred Stock
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	0	6,480,000
Preferred stock, shares issued	0	6,480,000
Preferred stock, shares outstanding	0	6,480,000
Series A Preferred Stock
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	0	9,072,000
Preferred stock, shares issued	0	8,997,085
Preferred stock, shares outstanding	0	8,997,085

Statements of Operations

Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Operating Expenses:
Research and development	$ 48,798	$ 14,127
General and administrative	17,005	4,682
Operating loss	(65,803)	(18,809)
Change in fair value of derivative liabilities		(781)
Other income		87
Interest income	2,615	887
Interest expense		(471)
Total other income (expense), net	2,615	(278)
Net loss	$ (63,188)	$ (19,087)
Net loss per share
Basic and diluted	$ (2.22)	$ (3.71)
Weighted average shares outstanding:
Basic and diluted	28,494,950	5,145,469

Statements of Changes in Redeem

Statements of Changes in Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Series Seed Preferred Stock	Series A Preferred Stock	Series B Preferred Stock	Common Stock	Additional Paid-In Capital	Accumulated Deficit
Balance at Dec. 31, 2017	$ (940)				$ 1	$ 886	$ (1,827)
Balance, temporary equity, shares at Dec. 31, 2017		6,480,000
Balance, temporary equity at Dec. 31, 2017		$ 3,524
Balance, shares at Dec. 31, 2017					7,209,000
Issuance of Preferred Stock			$ 64,907
Issuance of Preferred Stock, shares			7,666,716
Series A Preferred Stock issued as a result of conversion of convertible note - related party			$ 11,279
Series A Preferred Stock shares issued as a result of conversion of convertible note - related party, shares			1,330,369
Stock-based compensation	1,610					1,610
Net loss	(19,087)						(19,087)
Balance at Dec. 31, 2018	$ (18,417)				$ 1	2,496	(20,914)
Balance, temporary equity, shares at Dec. 31, 2018		6,480,000	8,997,085
Balance, temporary equity at Dec. 31, 2018		$ 3,524	$ 76,186
Balance, shares at Dec. 31, 2018	7,209,000				7,209,000
Issuance of Preferred Stock				$ 49,834
Issuance of Preferred Stock, shares				3,958,046
Stock-based compensation	$ 4,311					4,311
Conversion of convertible preferred stock into common stock upon the closing of initial public offering	129,543	$ (3,524)	$ (76,186)	$ (49,834)	$ 1	129,542
Conversion of convertible preferred stock into common stock upon the closing of initial public offering, shares		(6,480,000)	(8,997,085)	(3,958,046)	19,435,131
Issuance of common stock upon closing of initial public offering, net of issuance costs	113,041				$ 1	113,040
Issuance of common stock upon closing of initial public offering, net of issuance costs, shares					7,353,000
Exercise of stock options	52					52
Exercise of stock options, shares					141,619
Net loss	(63,188)						(63,188)
Balance at Dec. 31, 2019	$ 165,342				$ 3	$ 249,441	$ (84,102)
Balance, temporary equity, shares at Dec. 31, 2019		0	0
Balance, temporary equity at Dec. 31, 2019		$ 0	$ 0
Balance, shares at Dec. 31, 2019	34,138,750				34,138,750

Statements of Changes in Rede_2

Statements of Changes in Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Initial Public Offering
Issuance costs	$ 3,211
Series A Preferred Stock
Issuance costs		$ 93
Series B Preferred Stock
Issuance costs	$ 166

Statements of Cash Flows

Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities
Net loss	$ (63,188)	$ (19,087)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation	265	55
Stock-based compensation	4,311	1,610
Operating lease right-of-use asset	1,637	485
Amortization of convertible note discount, issuance costs and other non-cash interest		471
Change in fair value of derivative liabilities		781
Other		12
Changes in operating assets and liabilities
Prepaid expenses and other current assets	(5,847)	(459)
Accounts payable	3,921	976
Accrued expenses and other current liabilities	3,853	1,328
Operating lease liabilities	(706)	(183)
Net cash used in operating activities	(55,754)	(14,011)
Cash flows from investing activities
Purchases of property and equipment	(2,136)	(627)
Net cash used in investing activities	(2,136)	(627)
Cash flows from financing activities
Proceeds from issuance of common stock	116,251
Payment of issuance costs for preferred stock	(166)	(93)
Payment of issuance costs for common stock	(3,210)
Proceeds from exercise of stock options	52
Net cash provided by financing activities	162,927	64,907
Net increase in cash, cash equivalents and restricted cash	105,037	50,269
Cash, cash equivalents, and restricted cash at beginning of period	63,105	12,836
Cash, cash equivalents, and restricted cash at end of period	168,142	63,105
Supplemental disclosure of non-cash investing and financing activities
Conversion of convertible note plus accrued interest into 1,330,369 shares of Series A Preferred Stock		11,279
Right-of-use asset obtained in exchange for operating lease obligation	3,104	7,740
Conversion of preferred stock to common stock upon the initial public offering	129,543
Series A Preferred Stock
Cash flows from financing activities
Proceeds from issuance of Preferred Stock		$ 65,000
Series B Preferred Stock
Cash flows from financing activities
Proceeds from issuance of Preferred Stock	$ 50,000

Statements of Cash Flows (Paren

Statements of Cash Flows (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Reconciliation of cash, cash equivalents and restricted cash reported within the Balance Sheets:
Cash and cash equivalents	$ 168,051	$ 63,014
Restricted cash	91	91
Total cash, cash equivalents and restricted cash	$ 168,142	$ 63,105
Series A Preferred Stock
Conversion of convertible note	1,330,369

Nature of the Business

Nature of the Business	12 Months Ended
	Dec. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Nature of the Business	1. NATURE OF THE BUSINESS Prevail Therapeutics Inc., or the Company, was incorporated in the State of Delaware on July 6, 2017. The Company is a gene therapy company leveraging breakthroughs in human genetics with the goal of developing and commercializing disease-modifying AAV-based gene therapies for patients with devastating neurodegenerative diseases. We are applying a precision medicine approach to neurodegeneration by studying our gene therapies in genetically defined patient populations GBA1 GRN The Company is subject to a number of risks common to early-stage companies in the biotechnology industry. Principal among these risks are the uncertainties in the development process, development of the same or similar technological innovations by competitors, protection of proprietary technology, dependence on key personnel, compliance with government regulations and approval requirements, and the need to obtain additional financing to fund operations. Product candidates currently under development will require significant additional research and development efforts, including extensive pre-clinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure, and extensive compliance-reporting capabilities. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s technology will be obtained, that any products developed will obtain necessary government regulatory approval, or that any approved products will be commercially viable. The Company operates in an environment of rapid change in technology and substantial competition from pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees, consultants and contractors. Stock Split - In June 2019, the Board of Directors of the Company approved a 1.62-for-one forward stock-split of the Company’s outstanding shares of common stock, convertible preferred stock and options outstanding and available for future issuance. The stock split became effective on June 7, 2019. Accordingly, all share and per share amounts for all periods presented in the accompanying financial statements and notes thereto have been retroactively adjusted, where applicable, to reflect this forward stock split. Shares of common stock underlying outstanding stock options and other equity instruments were proportionately increased and the respective per share value and exercise prices, if applicable, were proportionately decreased in accordance with the terms of the agreements governing such securities. Initial Public Offering - In June 2019, the Company completed its initial public offering, or IPO, whereby the Company sold an aggregate of 7,353,000 shares of its common stock at a price of $17.00 per share. The shares began trading on The Nasdaq Global Select Market on June 20, 2019. The aggregate net proceeds received by the Company from the offering were approximately $113.0 million, after deducting underwriting discounts and commissions and offering expenses payable by the Company of $12.0 million. Upon the closing of the IPO, all outstanding shares of redeemable convertible preferred stock converted into 19,435,131 shares of common stock. Additionally, the Company is authorized to issue 200,000,000 shares of common stock and 10,000,000 shares of preferred stock. On June 24, 2019, the Company filed an Amended and Restated Certificate of Incorporation with the Secretary of State of the State of Delaware, in connection with the closing of the IPO. Liquidity and Capital Resources- Since its inception, the Company has incurred operating losses and has consistently used cash in operations. The Company has not recognized any revenue to date, devoting its efforts and capital resources to research and development of its product candidates. The Company’s activities have been primarily funded by the sale of shares of convertible preferred stock and common stock (see Note 8). The Company manages its capital resources to ensure the Company will continue as a going concern while maximizing the return to stockholders through the optimization of the debt and equity balances. The Company’s cash and cash equivalents as of December 31, 2019 were $168.1 million. In March 2019, the Company raised an aggregate of $49.8 million of net proceeds from its Series B Preferred Stock financing. In June 2019, the Company completed its IPO whereby the Company sold an aggregate of 7,353,000 shares of its common stock for aggregate net proceeds of approximately $113.0 million. Based on the Company’s cash and cash equivalents balance as of December 31, 2019, the Company estimates that its cash and cash equivalents balance will be sufficient to enable it to fund its operating expenses and capital expenditure requirements for at least the next 12 months from the issuance date of these financial statements. This estimate is based on assumptions that may prove to be incorrect, and the Company could use its available capital resources sooner than currently expected. Changing circumstances could cause the Company to consume capital resources sooner than currently anticipated, and the Company may need to spend more than currently planned due to circumstances beyond its control.

Significant Accounting Policies

Significant Accounting Policies	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Significant Accounting Policies	2. SIGNIFICANT ACCOUNTING POLICIES Basis of Presentation— The accompanying financial statements have been prepared by the Company in accordance with accounting principles generally accepted in the United States, or U.S. GAAP, and pursuant to the rules and regulations of the U.S. Securities and Exchange Commission, or the SEC, for reporting on Form 10-K. In April 2012, the Jumpstart Our Business Startups Act of 2012, or the JOBS Act, was enacted. Section 107(b) of the JOBS Act provides that an emerging growth company can take advantage of an extended transition period for complying with new or revised accounting standards. Thus, an emerging growth company can delay the adoption of certain accounting standards until those standards would otherwise apply to private companies. The Company has irrevocably elected not to avail itself of this extended transition period, and, as a result, the Company will adopt new or revised accounting standards on the relevant dates on which adoption of such standards is required for other public companies. Use of Estimates —The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements, and the reported amounts of expenses during the reporting period. Significant estimates in the financial statements include, but are not limited to stock-based compensation, fair value of common stock, fair value of the derivative liabilities, operating lease right-of-use assets and liabilities, the recoverability of the Company’s net deferred tax assets and related valuation allowance, and accrued liabilities related to expenses incurred for research and development from external vendors. The Company bases its estimates on historical experience, known trends and other market-specific or other relevant factors that it believes to be reasonable under the circumstances. The Company tracks the progress of its various research and development studies and manufacturing projects to ensure related prepaid expenses and accrued expenses are in line with progress of each. On an ongoing basis, management evaluates its estimates, as there are changes in circumstances, facts and experience. Actual results may differ materially from those estimates or assumptions. Cash, Cash Equivalents and Restricted Cash —The Company’s cash and cash equivalents include short-term highly liquid investments which are readily convertible into cash. These investments include money market securities and commercial paper with maturities of three months or less when acquired. The Company’s institutional money market accounts permit daily redemption and the fair values of these investments are based upon the quoted prices in active markets provided by the holding financial institutions, which are considered Level 1 inputs in the fair value hierarchy, as described below. The Company had cash and cash equivalents of $168.1 million as of December 31, 2019. Restricted cash represents cash on deposit with a financial institution as collateral in support of a letter of credit outstanding in favor of the Company’s landlord for office space. The restricted cash balance has been excluded from the cash balance and is classified as non-current restricted cash on the balance sheets as the lease expires after December 31, 2020. Concentration of Credit Risk —The Company maintains cash deposits in excess of government-provided insurance limits. The Company maintains its cash balances with one high quality, accredited financial institution, and accordingly, such funds are not exposed to significant credit risk. Leases —The Company determines if an arrangement is a lease at inception. Operating leases are included in operating lease right-of-use, or ROU, assets, operating lease liabilities, and long-term operating lease liabilities in the Company’s balance sheets. ROU 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 ROU assets and liabilities are recognized at commencement date based on the present value of lease payments over the lease term. As the Company’s leases do not provide a readily determinable implicit rate, the Company’s uses its incremental borrowing rate based on the information available at commencement date in determining the present value of lease payments. The operating lease ROU asset also includes any lease prepayments, offset by lease incentives. The Company’s facilities operating leases have lease and non-lease components for which the Company has elected to apply the practical expedient and account for each lease component and related non-lease component as one single component. Operating lease cost is recognized on a straight-line basis over the lease term. Property and Equipment, Net —Property and equipment are stated at cost less accumulated depreciation. Depreciation expense is recognized using the straight-line method over the estimated useful life of the asset, ranging from 3-7 years as follows: Fixed Asset Type Estimated useful life Laboratory equipment 7 years Leasehold improvements Lesser of useful life or remaining lease term Computer equipment 3 years Furniture and fixtures 7 years Expenditures for repairs and maintenance of assets are charged to expense as incurred, while major betterments are capitalized. Upon retirement or sale, the cost and related accumulated depreciation of assets disposed of are removed from the accounts and any resulting gain or loss is included in the statements of operations. Impairment of Long-Lived Assets —Long-lived assets, comprised of property and equipment, to be held and used and the right-of-use asset associated with the Company’s leased office space are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long-lived asset for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset to its carrying value. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset over its current fair value. To date, the Company has not recorded any impairment losses on long-lived assets. Comprehensive Loss —The Company does not have items of other comprehensive loss for the years ended December 31, 2019 and 2018, and therefore does not present a statement of comprehensive loss. The Company’s comprehensive loss equals its net loss Fair Value Measurements —Certain assets and liabilities are carried at fair value under U.S. GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. A framework is used for measuring fair value utilizing a three-tier hierarchy that prioritizes the inputs to valuation techniques used to measure fair value. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: • Level 1 —Unadjusted quoted prices in active markets for identical assets or liabilities. • Level 2 —Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. • Level 3 —Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. The following table summarizes assets measured at fair value on a recurring basis at December 31, 2019 (and does not include cash of $0.2 million as of December 31, 2019): December 31, 2019 Active Markets (Level 1) Observable Inputs (Level 2) Unobservable Inputs (Level 3) (in thousands) Cash and cash equivalents: Money market funds 167,805 — — Restricted cash: Money market funds 91 — — Total $ 167,896 $ — $ — The following table summarizes assets measured at fair value on a recurring basis at December 31, 2018 (and does not include cash of less than $0.1 million as of December 31, 2018): December 31, 2018 Active Markets (Level 1) Observable Inputs (Level 2) Unobservable Inputs (Level 3) (in thousands) Cash and cash equivalents: Money market funds 63,007 — — Restricted cash: Money market funds 91 — — Total $ 63,098 $ — $ — There have been no changes to the valuation methods utilized by the Company, nor were there transfers between Level 1, Level 2 and Level 3 investments during the year ended December 31, 2019. there were no financial instruments classified as Level 3 investments. Derivative Liabilities —From time to time, the Company may issue certain financial instruments with embedded features which, dependent on their specific contractual terms or other conditions, may be required to be accounted for as separate derivative assets or liabilities. These instruments are required to be measured at fair value. In determining the appropriate fair value, the Company’s uses a discounted cash flow analysis because these instruments are not quoted on an active market. These instruments are then adjusted to reflect fair value at each period end. Any increase or decrease in the fair value is recorded in the statement of operations as change in fair value of derivative liabilities. Research and Development Costs —Research and development costs are expensed as incurred. Research and development expenses consist principally of personnel costs, including salaries, stock-based compensation, and benefits for research and development employees, costs related to third-party contract research, contract development and manufacturing organizations, other third-party research service providers, third-party license fees, other direct and indirect operational costs related to the Company’s research and development activities, including facility-related expenses. Non-refundable research and development advance payments are capitalized and expensed as the related goods are delivered or services are performed. Stock-Based Compensation —The Company measures all stock options and other stock-based awards granted to employees, directors, consultants and other nonemployees based on the fair value on the date of the grant and recognizes compensation expense of those awards over the requisite service period, which is generally the vesting period of the respective award. The Company recognizes forfeitures at the time forfeitures occur. The Company classifies stock-based compensation expense in its statement of operations in the same way the payroll costs or service payments are classified for the related stock-based award recipient. The fair value of each stock option grant is estimated on the date of grant using the Black-Scholes option pricing model. The Company lacks company specific historical and implied volatility information. Therefore, it estimates its expected stock volatility based on the historical volatility of a publicly-traded set of peer companies and expects to continue to do so until it has adequate historical data regarding the volatility of its own traded stock price. Debt Issuance Costs— The Company incurred third-party costs in connection with the Company’s convertible note as described in Note 7. The Company amortizes these costs over the term of its agreement as interest expense in the statement of operations. Income Taxes —The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the financial statements or the Company’s tax returns. Under this method, deferred tax assets and liabilities are determined on the basis of the differences between the financial statements and tax basis of the assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. The effect of a change in tax rates on deferred tax assets and liabilities is recognized in income in the period that includes the enactment date. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company recognizes deferred tax assets to the extent that the Company believes that these assets are more likely than not to be realized. In making such a determination, management considers all available positive and negative evidence, including future reversals of existing taxable temporary differences, projected future taxable income, tax-planning strategies, and results of recent operations. Valuation allowances are provided, if based upon the weight of available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized. If management determines that the Company would be able to realize its deferred tax assets in the future in excess of their net recorded amount, management would make an adjustment to the deferred tax asset valuation allowance, which would reduce the provision for income taxes. The Company records uncertain tax positions in accordance with ASC 740 on the basis of a two-step process in which (1) management determines whether it is more likely than not that the tax positions will be sustained on the basis of the technical merits of the position and (2) for those tax positions that meet the more-likely-than-not recognition threshold, management recognizes the largest amount of tax benefit that is more than 50 percent likely to be realized upon ultimate settlement with the related tax authority. The Company provides reserves for potential payments of tax to various tax authorities related to uncertain tax positions. These reserves are based on a determination of whether and how much of a tax benefit taken by the Company in its tax filings or positions is more likely than not to be realized following resolution of any potential contingencies related to the tax benefit. Potential interest related to the underpayment of income taxes will be classified as a component of interest expense and any related penalties will be classified in operating expenses in the statement of operations. Net Loss per Common Share— Basic net loss per share is computed using the “two-class” method which includes the weighted average number of shares of common stock outstanding during the period and other securities that participate in dividends (a participating security). As of December 31, 2018, the Company’s convertible preferred stock were participating securities as defined by ASC 260-10, Earnings per Share. This stock converted into common stock upon the completion of the IPO in June 2019. During the periods where the Company incurs net losses, the Company allocates no loss to participating securities because these securities have no contractual obligation to share in the losses of the Company. Under the two-class method, basic net loss per share applicable to common stockholders is computed by dividing the net loss applicable to common stockholders by the weighted average number of common shares outstanding for the period. Diluted net loss per share is computed similar to basic net loss per share except that the denominator is increased to include the number of additional shares for the potential dilutive effects of convertible debt, convertible preferred stock and stock options outstanding during the period calculated in accordance with the treasury stock method, or the two-class method, whichever is more dilutive. The Company allocates net earnings on a pari passu (equal) basis to both common and preferred stockholders. Net losses are not allocated to preferred stockholders as they do not have an obligation to share in the Company’s net losses. For all periods presented, basic and diluted net loss per share are the same, as any additional share equivalents would be anti-dilutive (Note 14). Segment Reporting— Operating segments are identified as components of an enterprise for which separate discrete financial information is available for evaluation by the Company’s Chief Operating Decision Maker in making decisions regarding resource allocation and assessing performance. To date, the Company has viewed its operations and manages its business as one operating segment. Recently Issued Accounting Pronouncements (Adopted) —In November 2018, the SEC’s release, Disclosure Update and Simplification, became effective. The Company adopted this amendment on January 1, 2019. The included amendments are intended to simplify and update the SEC’s disclosure requirements and eliminate duplicative disclosures between the SEC rules and U.S. GAAP. The amendments included new interim financial statement disclosures to reconcile the beginning balance to the ending balance in stockholders’ equity for each period for which an income statement is required to be filed. The Company adopted the rule on January 1, 2019 and included its first presentation of changes in stockholders’ equity in its Form 10-Q for the three-month period ended March 31, 2019. In July 2018, the FASB issued ASU 2018-09, Codification Improvements, Recently Issued Accounting Pronouncements (Not Yet Adopted) —In August 2018, the FASB issued ASU 2018-13 Fair Value Measurement – Disclosure Framework-Changes to the Disclosure Requirement for Fair Value Measurement, or ASU 2018-13. The amendments in ASU 2018-13 modify the disclosure requirements on fair value measurements in ASC 820, Fair Value Measurement, based on the concepts in the FASB Concepts Statement, including the consideration of costs and benefits. The amendments under ASU 2018-13 are effective for interim and annual fiscal periods beginning after December 15, 2019, with early adoption permitted. The Company does not expect the adoption of ASU 2018-13 to have a material impact on its financial statements. In August 2018, the FASB issued ASU No. 2018-15, Intangible-Goodwill and Other Internal-Use Software (Subtopic 350-40)

License Agreements

License Agreements	12 Months Ended
	Dec. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
License Agreements	3. LICENSE AGREEMENTS REGENXBIO Inc. In August 2017, the Company entered into a License Agreement, or the REGENXBIO GBA1 License, with REGENXBIO Inc., or REGENXBIO. Under the terms of the REGENXBIO GBA1 License, REGENXBIO granted the Company an exclusive, worldwide license under certain patents and patent applications to make, have made, use, import, sell and offer for sale products for the treatment of disease, including but not limited to Parkinson’s disease and Gaucher disease, whether or not caused by mutations in the gene that produces the GBA1 enzyme in humans by in vivo gene therapy using AAV9 delivering the gene (or any portion thereof) encoding for GBA1. As consideration for the licensed rights under the REGENXBIO GBA1 License, the Company issued 2,430,000 shares of its common stock with an aggregate fair value of $0.4 million in a concurrent private placement to REGENXBIO. The Company is obligated, pursuant to the REGENXBIO GBA1 License, to pay REGENXBIO: (1) an annual maintenance fee; (2) mid-to high-single digit royalty percentages on net sales of licensed products, subject to reduction in specified circumstances; and (3) mid-teen to low-twenties royalty percentages of any sublicense fees the Company receives from sublicensees for the licensed intellectual property rights. The initial license fee paid in connection with the REGENXBIO GBA1 License, including the fair value of the common stock issued to REGENXBIO, was recognized as research and development expense in the year ended December 31, 2017. The initial annual maintenance fee of $0.1 million was recognized as research and development expense for the year ended December 31, 2018. The REGENXBIO GBA1 License will expire on a country-by-country, licensed product-by-licensed product basis upon the later of (1) the expiration, lapse, abandonment or invalidation of the last valid claim of the In May 2018, the Company entered into a license agreement, or the REGENXBIO Option Genes License, with REGENXBIO pursuant to which REGENXBIO granted the Company three distinct exclusive options for specified genes, or the Option Genes, exercisable at the Company’s sole discretion through May 10, 2019. Each option represented the right to obtain an exclusive, worldwide license under certain patents and patent applications to make, have made, use, import, sell and offer for sale products for the treatment or prevention of disease, including but not limited to Parkinson’s disease, whether or not caused by mutations in any Option Gene that is the subject of the applicable license, in humans by in vivo gene therapy using AAV9 delivering the applicable licensed Option Gene and/or RNA interference or antisense modalities that target the applicable licensed Option Gene. The Company also received a non-exclusive, royalty-free, worldwide research license to perform research and development activities for each Option Gene solely for purposes of evaluating whether to exercise the applicable option. Under the terms of the REGENXBIO Option Genes License, the Company paid REGENXBIO an initial fee of $0.6 million. In connection with the exercise of each option, the Company is required to pay REGENXBIO: (1) an additional up-front fee of $0.6 million; (2) an annual maintenance fee; (3) mid- to high-single digit royalty percentages on net sales of the licensed product, subject to reduction in specified circumstances; and (4) mid-teen to low-twenties royalty percentages of any sublicense fees the Company receives from sublicensees for the licensed intellectual property rights. If a licensed product includes the GBA1 gene and otherwise would be subject to royalties under the REGENXBIO GBA1 License, then royalties for that licensed product will only be due under the REGENXBIO Option Genes License. The initial $0.6 million paid for the REGENXBIO Option Genes License, was recognized as research and development expense for the year ended December 31, 2018. The REGENXBIO Option Genes License will expire on a country-by-country, licensed product-by-licensed product basis upon the later of (1) the expiration, lapse, abandonment or invalidation of the last valid claim of the licensed intellectual property and (2) seven years from the first commercial sale of each licensed product. The Company has the right to terminate the REGENXBIO Option Genes License upon a specified period of prior written notice. REGENXBIO may terminate the REGENXBIO Option Genes License immediately if the Company becomes insolvent, if the Company is late by a specified number of days in paying money due under the REGENXBIO Option Genes License, or if the Company or its affiliates commence any action against REGENXBIO or its licensors to declare or render any claim of the licensed patent rights invalid or unenforceable. Either party may terminate the REGENXBIO Option Genes License for material breach if such breach is not cured within a specified number of days. In April 2019, the Company exercised all of the options under the REGENXBIO Option Genes License and paid the additional up-front fee of $0.6 million per option for an aggregate of $1.8 million to REGENXBIO. In August 2019, in accordance with the REGENXBIO GBA1 License, the Company paid an annual maintenance fee of approximately $0.1 million. As a result, the Company recognized approximately $1.9 million in research and development expense related to these license agreements for the year ended December 31, 2019.

Fair Value of Financial Instrum

Fair Value of Financial Instruments	12 Months Ended
	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Fair Value of Financial Instruments	4. FAIR VALUE OF FINANCIAL INSTRUMENTS The Company’s financial instruments consist of cash equivalents, the derivative liabilities related to certain redemption rights pursuant to the issuance of the convertible note to OrbiMed Private Investments VI, LP, or OrbiMed, and corresponding convertible note (Note 7), accounts payable and accrued expenses. Fair value estimates of these instruments are made at a specific point in time, based on relevant market information. The fair value of cash equivalents, which consisted of money-market funds, were determined using Level 1 inputs reflecting quoted prices in active markets. The carrying amount of accounts payable and accrued expenses as reported on the balance sheets as of December 31, 2019 and 2018, approximates fair value, due to the short-term duration of these instruments. To derive the fair value of the convertible note derivative liabilities, the Company estimated the fair value of the convertible notes with and without the derivative liabilities using a discounted cash flow approach. The difference between the “with” and “without” convertible note prices represents the fair value of the derivative liabilities at issuance and immediately prior to conversion. Key inputs for this valuation were the stated interest rate of the convertible notes, the assumed cost of debt, an assessment of the likelihood and timing of conversion, and the discount upon conversion of the note into equity. The convertible note derivative liabilities were settled in March 2018 for $3.0 million. The Company recognized a loss of $0.8 million related to the settlement during the year ended December 31, 2018 as change in fair value of derivative liabilities. As of December 31, 2019 and 2018, there were no financial instruments classified as Level 3 investments. Further, during the years ended December 31, 2019 and 2018 there were no transfers between Level 1, Level 2, and Level 3.

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2019
Property Plant And Equipment [Abstract]
Property and Equipment, Net	5. PROPERTY AND EQUIPMENT, NET Property and equipment, net, consisted of the following: As of December 31, 2019 2018 (in thousands) Laboratory equipment $ 1,730 $ 677 Leasehold improvements 887 — Computer equipment 36 57 Furniture and fixtures 218 1 Gross property and equipment 2,871 735 Less: accumulated depreciation (322 ) (57 ) Property and equipment, net $ 2,549 $ 678 Depreciation expense was $0.3 million and $0.1 million for the years ended December 31, 2019 and 2018, respectively.

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	12 Months Ended
	Dec. 31, 2019
Accrued Expenses And Other Current Liabilities [Abstract]
Accrued Expenses and Other Current Liabilities	6. ACCRUED EXPENSES AND OTHER CURRENT LIABILITIES Accrued expenses consisted of the following: As of December 31, 2019 2018 (in thousands) Accrued compensation $ 2,577 $ 840 Accrued research and development 1,595 273 Accrued professional fees 634 — Other 524 364 Total accrued expenses $ 5,330 $ 1,477

Convertible Note - Related Part

Convertible Note - Related Party	12 Months Ended
	Dec. 31, 2019
Debt Disclosure [Abstract]
Convertible Note - Related Party	7. CONVERTIBLE NOTE – RELATED PARTY In December 2017, the Company entered into a Convertible Note Purchase Agreement with OrbiMed, an existing investor of the Company (Note 13). Under the terms of the agreement, the Company issued a convertible note, or the Note, with a principal amount of $10.0 million, which accrued interest at 8.0% per annum, and had a maturity date of December 31, 2018. Under the terms of the agreement, the Note would be convertible in two scenarios: (i) if the Company closes a sale of preferred stock in an equity financing with aggregate proceeds of at least $25.0 million, or a Qualified Financing, and (ii) upon a Corporate Transaction, which is defined as a deemed liquidation event such as a merger or consolidation, or sale of the Company. In the first scenario, the Note automatically converts into preferred stock of the Company at a conversion price equal to 90% of the price per share paid in the financing, or the Automatic Conversion Upon a Qualified Financing. In the second scenario, OrbiMed could elect to receive either (i) the outstanding balance of the note immediately prior to the Corporate Transaction, or (ii) the amount that OrbiMed would have received if OrbiMed converted the outstanding balance of the Note immediately prior to such Corporate Transaction into the number of shares of Series Seed Preferred Stock determined by dividing the outstanding balance by the Original Issue Price of Series Seed Preferred Stock, which becomes due and payable in cash as and when such amounts are paid to holders of the Series Seed Preferred Stock, or the Automatic Redemption Based Upon Series Seed Price. In connection with the issuance of the Note, the Company incurred issuance costs of less than $0.1 million. The debt issuance costs are recorded as a discount on the debt and presented net of the principal balance on the balance sheet. The costs are amortized to interest expense over the life of the debt using the effective interest method. Derivative Liabilities The Note was considered a hybrid financial instrument consisting of a fixed interest rate debt host with certain embedded features requiring evaluation for bifurcation and separate accounting. The Company determined that the Automatic Conversion Upon a Qualified Financing and Automatic Redemption Based Upon Series Seed Price features were considered freestanding financial instruments which required bifurcation from the host debt instrument. At the issuance date of the Note, the Company bifurcated from the respective host debt instrument the automatic conversion and automatic redemption features and recorded derivative liabilities of $2.2 million. The derivative liabilities were revalued at each reporting date and immediately prior to conversion with changes in fair value recorded to Change in fair value of derivative liabilities in the statement of operations. The Note derivative liabilities were settled in March 2018 for $3.0 million. The Company recognized a loss of $0.8 million related to the settlement as change in fair value of derivative liabilities. The resulting debt discount from the derivative liabilities was presented as a direct deduction from the carrying amount of the Note payable and was amortized to interest expense using the effective interest rate method. The Company recognized $0.1 million of coupon interest and $0.3 million of discount amortization for the year ended December 31, 2018. In March and April 2018, the Company issued 7,666,716 shares of Series A Preferred Stock (Note 8) for total net proceeds of $64.9 million. In connection with the issuance, the derivative liabilities of $3.0 million were settled and the principal amount of the Note of $10.0 million together with $0.2 million of accrued interest thereon, was automatically converted into 1,330,369

Redeemable Convertible Preferre

Redeemable Convertible Preferred Stock	12 Months Ended
	Dec. 31, 2019
Temporary Equity Disclosure [Abstract]
REDEEMABLE CONVERTIBLE PREFERRED STOCK	8. REDEEMABLE CONVERTIBLE PREFERRED STOCK In August 2017, the Company authorized the sale and issuance of up to 6,480,000 shares of Series Seed preferred stock, or Series Seed Preferred Stock, with a par value per share of $0.0001. On the same date, the Company issued 6,480,000 shares of Series Seed Preferred Stock to OrbiMed and the Silverstein Foundation, or the Initial Investors, at a price of $0.62 per share for aggregate proceeds of $4.0 million. Issuance costs were $0.1 million. In March 2018, the Company authorized the sale and issuance of up to 9,072,000 shares of Series A preferred stock, or Series A Preferred Stock, with a par value per share of $0.0001. During March and April 2018, the Company issued 7,666,716 shares of Series A Preferred Stock at a price of $8.48 per share for aggregate proceeds of $65.0 million. Issuance costs were $0.1 million. In connection with the Series A Preferred Stock issuance, 1,330,369 shares of Series A Preferred Stock were issued upon the conversion of the convertible note with OrbiMed, resulting in a total Series A Preferred Stock issuance of 8,997,085 shares. In March 2019, the Company authorized the sale and issuance of 3,958,046 shares of Series B preferred stock, or Series B Preferred Stock, with a par value per share of $0.0001 at a price of $12.63 per share for aggregate net proceeds of $49.8 million. Issuance costs were $0.2 million. Series Seed Preferred Stock, Series A Preferred Stock and Series B Preferred Stock are collectively referred to as “Preferred Stock”. As of December 31, 2019, the Company had 10,000,000 shares of preferred stock authorized and none issued and outstanding. As of December 31, 2018, the Company had 15,552,000 shares of Preferred Stock authorized, of which 6,480,000 shares were issued and outstanding and were designated as Series Seed Preferred Stock and 8,997,085 shares were issued and outstanding and were designated as Series A Preferred Stock. The rights, privileges, and preferences of the shares of redeemable convertible Preferred Stock are summarized as follows: Voting At every meeting of the shareholders of the corporation, each holder of the Preferred Stock was entitled to the same number of votes as the number of whole shares of common stock into which such holder’s shares of preferred stock could be converted on the record date for the determination of shareholders entitled to vote at any such meeting. Except as otherwise expressly provided in the stock purchase agreement or as required by law, the common stock and the preferred stock will vote together as a single class. Dividends The holders of the Preferred Stock were entitled to receive dividends, out of any assets legally available, prior to and in preference to any other dividend, at a rate of 6% of the original issue price. The right to receive preferred dividends shall not be cumulative, and therefore, if not declared in any year, the right to receive such dividends shall not carry forward into the next year. The holders of the Preferred Stock are also entitled to receive dividends payable when, as and if declared by the Board of Directors of the Corporation, with the holders of common stock, paid out of any assets or on the common stock of the Company, on an as-converted to common stock basis. No dividends had been declared as of December 31, 2019 and 2018. Conversion Each share of Preferred Stock was eligible to be converted into common stock at a one-to-one conversion ratio, subject to adjustments based on certain events specified in the Certificate of Incorporation, or COI, including anti-dilution adjustments, at the option of the holder, at any time and from time to time, and without the payment of additional consideration by the holder, into such number of fully paid and nonassessable shares of common stock as is determined by dividing the Series Seed Original Issue Price by the Series Seed Conversion Price, the Series A Original Issue Price by the Series A Conversion Price, or the Series B Original Issue Price by the Series B Conversion Price in effect at the time of conversion. At December 31, 2018, the conversion price of the Series Seed and Series A preferred shares were $0.62 and $8.48 per share, respectively. As a result of the IPO in June 2019, each share of Preferred Stock converted into such number of shares of common stock at the then effective conversion rate. Redemption Prior to the IPO, the Preferred Stock was eligible to be redeemed upon a Deemed Liquidation Event. The Series B stock was eligible to be redeemed at $12.63 per share, or the holders of the Series B stock received an amount equal to the amount entitled if the Series B stock converted into shares of common stock on the redemption date. The Series A stock was eligible to be redeemed at $8.48 per share, or the holders of Series A stock received an amount equal to the amount entitled if the Series A stock converted into shares of common stock on the redemption date. The Series Seed stock was eligible to be redeemed at $0.62 per share, or the Series Seed stock received an amount equal to the amount entitled if the Series Seed stock converted into shares of common stock on the redemption date. Right of First Offer Upon the proposal or sale of any New Securities, the Company would have first offered such New Securities to each investor of the Preferred Stock, and each investor would have been entitled to apportion the right of first offer in such proportions as it deemed appropriate. Upon the occurrence of a Trigger Event as defined in the COI, each share of Series A stock would have been automatically converted into such number of shares of common stock at the then effective conversion rate. Liquidation In the event of any voluntary or involuntary liquidation, dissolution or winding up of the Corporation or Deemed Liquidation Event, the holders of shares of Preferred Stock then outstanding were entitled to be paid out of the assets of the Corporation available for distribution to its stockholders before any payment shall be made to the holders of Common Stock, according to their priority, based on the greater of the Preferred Stock original issue price or amounts that would be payable had the shares been converted into common stock immediately prior to such liquidation event. If upon any such liquidation event, the assets of the Corporation available for distribution to its stockholders were insufficient to pay the holders of shares of Series A and Series B stock the full amount to which they were entitled, distributions would first have been made to holders of the Series B stock equal to their original issuance price plus any declared but unpaid dividends. After distribution to the holders Series B stock, the holders of Series A stock as a class would receive a distribution equal to their original issue price plus any declared but unpaid dividends. After payment of all preferential amounts required to be paid to the holders of preferred stock, the remaining funds and assets available for distribution to the shareholders of the Company would have been distributed among the holders of Series B stock, Series A stock, Series Seed stock, and Common Stock, pro rata based on the number of shares held by each such holder on an as-converted basis.

Common Stock

Common Stock	12 Months Ended
	Dec. 31, 2019
Equity [Abstract]
COMMON STOCK	9. COMMON STOCK General As of July 6, 2017, the date of incorporation, the Company was authorized to issue 6,480,000 shares of common stock at a per share par value of $0.0001, which was subsequently amended in August 2017 to a total amount of 16,200,000 authorized shares of common stock issued at a par value of $0.0001. Total authorized shares were subsequently amended to 27,540,000 authorized shares of common stock in March 2018, 28,398,600 authorized shares of common stock in December 2018 and 35,640,000 authorized shares of common stock in March 2019. In August 2017, OrbiMed purchased 2,430,000 shares of common stock for $150 per share. Proceeds of $0.4 million were allocated to these shares based on relative fair value. In August 2017, the Company’s founder and chief executive officer purchased 2,349,000 shares of common stock for $145 pursuant to a restricted stock purchase agreement. Of the stock purchased, 25% of the restricted shares shall vest on the one-year anniversary of the effective date of the purchase agreement; and an additional 1/48th of the restricted shares shall vest at the end of each successive one-month period for 36 months thereafter in equal installments, provided that the founder and chief executive officer remains in continuing service to the Company. In August 2017, 2,430,000 shares of common stock were sold to REGENXBIO in exchange for license rights (Note 3). In June 2019, the Board of Directors of the Company approved a 1.62-for-one forward stock split of the Company’s outstanding shares of common stock and convertible preferred stock. Accordingly, all share and per share amounts for all periods presented in the accompanying financial statements and notes thereto have been retroactively adjusted, where applicable, to reflect this forward stock split. Shares of common stock underlying outstanding stock options and other equity instruments were proportionately increased and the respective per share value and exercise prices, if applicable, were proportionately decreased in accordance with the terms of the agreements governing such securities. In June 2019, the Company completed an IPO in which 7,353,000 shares of common stock were sold. The voting, dividend and liquidation rights of the holders of common stock are subject to and qualified by the rights, powers, and preferences of the holders of the shares of preferred stock. The common stock has the following characteristics: Voting The holders of the 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, provided, however, that except as otherwise required by law, holders of common stock as such shall not be entitled to vote on any amendment to the Company’s Certificate of Incorporation that relates solely to the terms of one or more outstanding series of Preferred Stock if the holders of such affected series are entitled, either separately or together with the holders of one or more other such series, to vote thereon pursuant to the Company’s Certificate of Incorporation or pursuant to Delaware General Corporation Law. There shall be no cumulative voting. Dividends The holders of shares of common stock are entitled to receive dividends, if and when declared by the Board of Directors. Cash dividends may not be declared or paid to the holders of common stock until paid on the preferred stock. As of December 31, 2019 and 2018, no dividends have been declared or paid since the Company’s inception. Liquidation After payment to the holders of shares of preferred stock of their liquidation preference, the holders of the common stock are entitled to share ratably in the Company’s assets available for distribution to stockholders, in the event of any voluntary or involuntary liquidation, dissolution or winding up of the Company or upon the occurrence of a Deemed Liquidation Event. Reserve for future issuance As of December 31, 2019 and 2018 the Company has reserved the following number of shares of common stock for future issuance upon the conversion of preferred stock, exercise of options or grant of equity As of December 31, 2019 2018 Redeemable convertible preferred stock outstanding, as converted — 15,477,085 Options and restricted stock issued and outstanding 7,051,663 6,366,739 Shares available for future stock option grants 2,762,749 844,287 Total 9,814,412 22,688,111

Leases

Leases	12 Months Ended
	Dec. 31, 2019
Leases [Abstract]
Leases	10. LEASES Lease Agreements —The Company leases office and laboratory space in its New York City location under an operating lease agreement entered in September 2017, with an original term of 49 months. In connection with the lease, the Company paid a security deposit of approximately $0.1 million in the form of an unconditional and irrevocable letter of credit, which is secured with cash on deposit classified as restricted cash. The original lease was initially modified in October 2018, which extended the original term of the lease and included space on two additional floors. The Company accounted for the new agreement as a modification of the original agreement and recorded an additional right-of-use asset and corresponding lease liability based on the incremental borrowing rate determined as of the effective date of the modified lease. The additional floors were recognized as additional lease components. One of these lease components provided an incentive allowance to reimburse the Company for the cost of qualified leasehold improvement. During the year ended December 31, 2019, the Company incurred qualified costs of $0.4 million, of which $0.3 million were payable to the Company as of December 31, 2019 and will be recognized over the remaining lease term. In July 2019, the Company entered into a second modification to further extend the term to 76 months, rent additional space and surrender a portion of the originally leased space. The Company accounted for the new agreement as a modification of the existing agreement with the additional space recognized as a separate lease component. The Company recorded an additional right-of-use asset and a corresponding lease liability calculated based on the incremental borrowing rate determined as of the effective date of the second lease modification. The agreement does not include any options to extend or terminate the lease, and no restrictions or covenants are imposed by the lease agreement. The Company identified and assessed the following significant assumption in recognizing the right-of-use assets and corresponding liabilities: • Incremental borrowing rate— The Company’s lease agreement does not provide a readily determinable implicit rate. As the Company does not have any external borrowings for comparable terms of the lease, the Company estimated the incremental borrowing rate based on the credit quality of the Company and by comparing interest rates available in the market for similar borrowings adjusted for the impact of collateral over the term of the lease. The Company is required to pay for operating costs, including insurance, maintenance, and taxes, which are billed annually based on the Company’s share of the total rentable square footage. These additional charges are considered variable lease cost and are recognized in the period in which the costs are incurred. The components of the lease expense were as follows: Years ended December 31, 2019 2018 (in thousands) Operating lease cost $ 2,033 $ 752 Variable lease cost 731 226 Total lease cost $ 2,764 $ 978 Weighted-average remaining lease term 5.8 years 6.1 years Weighted-average discount rate 9.25 % 9.00 % Cash paid for amounts included in the measurement of the lease liabilities, net of qualified costs, were $1.1 million for the year ended December 31, 2019. Cash paid for amounts included in the measurement of the lease liabilities were $0.6 million for the year ended December 31, 2018. As of December 31, 2019 the maturities of the Company’s remaining operating lease liabilities were as follows: December 31, 2019 (in thousands) 2020 $ 2,299 2021 2,379 2022 2,463 2023 2,549 2024 2,638 Thereafter 2,263 Present value adjustment (3,322 ) Present value of lease payments $ 11,269

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	11. STOCK-BASED COMPENSATION In August 2017, the Company adopted the Prevail Therapeutics Inc. 2017 Equity Incentive Plan, or the 2017 Plan, under which the Company granted incentive stock options, nonqualified stock options, stock appreciate rights, or SARs, restricted stock, unrestricted stock, restricted stock awards, performance awards, or other awards that are convertible into or based on Company stock. In March 2018, the Company amended the 2017 Plan and increased In June 2019, the Company adopted the Prevail Therapeutics Inc. 2019 Equity Incentive Plan or the 2019 Plan, under which the Company may grant incentive stock options, nonqualified stock options, SARs, restricted stock, unrestricted stock, restricted stock awards, performance awards or other awards that are convertible into or based on Company stock. The aggregate number of shares of common stock that may be issued pursuant to stock awards under the 2019 Plan was 2,762,749 as of December 31, 2019. In addition, the number of shares of common stock reserved for issuance under the 2019 Plan will automatically increase on January 1 of each year, beginning on January 1, 2020 and continuing through and including January 1, 2029, by 4% of the total number of shares of the Company’s common stock outstanding on December 31 of the preceding calendar year, or a lesser number of shares of common stock determined by the Company’s Board of Directors. In June 2019, the Company adopted the 2019 Employee Stock Purchase Plan or the 2019 ESPP, under which the Company may permit employees to purchase shares of common stock. As of December 31, 2019, the maximum number of shares of common stock that may be issued under the 2019 ESPP was 330,000 shares. In addition, the number of shares of common stock reserved for issuance under the 2019 ESPP will automatically increase on January 1 of each year, beginning on January 1, 2020 and continuing through and including January 1, 2027, by the lesser of (1) 1% of the total number of shares of the Company’s common stock outstanding on December 31 of the preceding calendar year, (2) 1,500,000 shares or (3) such lesser number of shares as determined by the Company’s Board of Directors. No purchases were made pursuant to this plan as of December 31, 2019. The exercise price for a stock option awarded under the 2019 Plan shall not be less than 100% of the fair market value of the Company’s common stock on the date of grant. Options granted under the 2019 Plan vest 25% after the first year and monthly thereafter over the following three years and expire ten years from the date of grant. Stock Options The following tables summarize stock option activity under the 2017 Plan and the 2019 Plan: Number of Awards Weighted Average Exercise Price Weighted Average Remaining Contractual Life (Years) Aggregate Intrinsic Value (in thousands, except share data) Outstanding, December 31, 2018 4,017,736 $ 0.83 9.3 $ 7,939 Granted 1,939,642 9.08 Exercised (141,619 ) 0.37 Cancelled/forfeited (134,337 ) 2.05 Outstanding, December 31, 2019 5,681,422 $ 3.60 8.6 $ 69,219 Exercisable, December 31, 2019 1,527,587 0.70 8.3 23,108 Vested and expected to vest, December 31, 2019 5,681,422 3.60 8.6 69,219 As of December 31, 2019, the total unrecognized compensation expense related to unvested employee and non-employee options was $14.8 million, which the Company expects to recognize over an estimated weighted-average period of 2.9 years. As of December 31, 2018, the total unrecognized compensation expense related to unvested employee and non-employee options was $7.1 million, which the Company expects to recognize over an estimated weighted-average period of 3.3 years. The Company uses the Black-Scholes option pricing model to estimate the fair value of stock options on the date of grant. The Company determined the assumptions for the Black-Scholes option-pricing valuation model as discussed below. Each of these inputs is subjective and generally requires significant judgment to determine. The weighted average fair value and assumptions used to determine the fair value of stock options granted was as follows: As of December 31, 2019 2018 Weighted average grant date fair value of common stock $ 6.29 $ 2.39 Expected term 6.0 6.1 Risk-free interest rate 2.3 % 2.8 % Expected volatility 79.2 % 74.6 % Dividend rate — — Expected Term — The expected term represents the period that the stock-based awards are expected to be outstanding. As the Company does not have sufficient historical experience for determining the expected term of the stock option awards granted, the Company based its expected term for awards issued to employees and non-employees using the simplified method, which is presumed to be the midpoint between the vesting date and the end of the contractual term. Risk-Free Interest Rate — The risk-free interest rate is based on the U.S. Treasury yield curve in effect at the date of grant for zero-coupon U.S. Treasury constant maturity notes with terms approximately equal to the stock-based awards’ expected term. Expected Volatility — Since the Company does not have a trading history of common stock, the expected volatility was derived from the average historical stock volatilities of the common stock of several public companies within the industry that the Company considers to be comparable to its business over a period equivalent to the expected term of the stock-based awards. Dividend Rate — The expected dividend rate is zero as the Company has not paid and does not anticipate paying any dividends in the foreseeable future. Fair Value of Common Stock — Prior to the IPO, the fair value of the shares of common stock underlying the stock-based awards were determined by the Company’s Board of Directors with input from management. Since there was no public market for the common stock prior to June 20, 2019, the Company’s Board of Directors had determined the fair value of the common stock at the time of grant of the stock-based award by considering a number of objective and subjective factors, including having valuations of the common stock performed by a third-party valuation specialist. The fair value of the common stock is now determined by the public market. Restricted Stock As of each of December 31, 2019 and December 31, 2018, 2,349,000 shares of common stock were subject to a repurchase right by the Company. Non-vested Restricted Stock Award (RSA) Outstanding The following table presents a summary of the Company’s non-vested restricted stock award activity under all plans and related information for the years ended December 31, 2019 and December 31, 2018: Number of Restricted Stock Awards Outstanding Weighted Average Grant Date Fair Value Per Share Non-vested restricted stock awards outstanding as of December 31, 2018 1,566,001 $ 0.18 Restricted stock awards vested (587,242 ) 0.18 Non-vested restricted stock awards outstanding as of December 31, 2019 978,759 $ 0.18 Years ended December 31, 2019 2018 (in thousands) Aggregate grant date fair value of restricted stock awards vested $ 101 $ 135 Restricted stock awards are generally granted at the fair market value of the Company’s common stock on the date of grant and vest 25% after the first year and monthly thereafter over the following three years. Forfeitures are based on actual forfeitures in the given period. There were $0.2 million of total unrecognized compensation cost related to non-vested restricted stock awards granted under the Company’s equity incentive plans as of December 31, 2019. This cost is expected to be recognized over a weighted-average period of 1.6 years. Total stock-based compensation expense is recognized for restricted stock and stock options granted to employees and non-employees and has been reported in the Company’s statements of operations as follows: Years ended December 31, 2019 2018 (in thousands) Research and development $ 2,399 $ 1,454 General and administrative 1,912 156 Total stock-based compensation expense $ 4,311 $ 1,610 Convertible Preferred Stock Immediately prior to the closing of the Company’s IPO, 19,435,131 shares of outstanding Preferred Stock converted into 19,435,131 shares of common stock. Preferred Stock The Company’s amended and restated certificate of incorporation, which became effective upon the completion of the IPO, authorizes 10,000,000 shares of preferred stock, of which no shares were issued or outstanding as of December 31, 2019.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Income Taxes	12. INCOME TAXES The effective tax rate for the Company for the years ended December 31, 2019 and December 31, 2018 was zero percent. A reconciliation of income tax computed at the statutory federal income tax rate to the provision (benefit) for income taxes included in the accompanying statements of operations for the Company is as follows: Years ended December 31, 2019 2018 Expected provision at statutory federal rate 21.0 % 21.0 % State tax – net of federal benefit 11.6 % 10.1 % Nondeductible interest expense 0.0 % (1.4 )% Tax credits 0.8 % 1.2 % Changes in valuation allowance (32.8 )% (29.3 )% Other (0.6 )% (1.6 )% Effective Income Tax Rate 0.0 % 0.0 % Deferred income taxes reflect the net tax effects of temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes, and operating losses and tax credit carryforwards. Significant components of the Company’s deferred tax assets and liability are as As of December 31, 2019 2018 (in thousands) Deferred Tax Assets: Net operating loss carryforwards $ 18,837 $ 4,177 Accruals and reserves 847 275 Lease liability 3,238 2,458 Stock compensation 552 76 Tax credits 806 289 Section 195 start up cost 5,558 1,288 Other 1 — Gross deferred tax assets $ 29,839 $ 8,563 Less valuation allowance (26,918 ) (6,212 ) Net deferred tax assets $ 2,921 $ 2,351 Deferred tax liabilities: Fixed assets $ (99 ) $ (22 ) Right of use assets $ (2,822 ) $ (2,329 ) Gross deferred tax liability $ (2,921 ) $ (2,351 ) Net deferred tax assets $ — $ — The Company has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets. Based upon the Company’s history of operating losses, the Company has concluded that it is more likely than not that the benefit of its deferred tax assets will not be realized. Accordingly, the Company has provided a full valuation allowance for its deferred tax assets as of December 31, 2019 and December 31, 2018. The change in the valuation allowance was $20.7 million from December 31, 2018 to December 31, 2019 As of December 31, 2019 and 2018, the Company’s federal net operating loss carryforwards were approximately $57.3 million and $12.8 million, respectively. The Company had federal research and development credit carryforwards as of December 31, 2019 and December 31, 2018 of approximately $0.8 million and $0.3 million, respectively. The federal net operating loss incurred prior to January 1, 2018 and tax credit carryforwards will begin to expire in 2037 if not utilized. Federal net operating losses incurred after December 31, 2017 will not expire. The Company has not completed a formal research credit and development analysis, therefore when this analysis is finalized, the Company plans to update its research and development credit carryforward. As of December 31, 2019, and December 31, 2018, the Company had state net operating loss carryforwards of approximately $115.5 million and $25.5 million, respectively. The state net operating loss carryforwards will begin to expire in 2037, if not utilized. Future utilization of the Company’s net operating loss and research and development credit carryforwards to offset future taxable income may be subject to an annual limitation, pursuant to IRC Sections 382 and 383, as a result of ownership changes that may have occurred or that could occur in the future. An ownership change occurs when a cumulative change in ownership of more than 50% occurs within a three-year period. The Company has not completed an IRC Section 382/383 analysis regarding the limitation of net operating loss and research and development credit carryforwards. In accordance with ASC 740, Income Taxes In The Tax Cuts and Jobs Act (“the Act”) was enacted in December 2017. The Act includes a number of changes to then-existing U.S. tax laws that impact the Company, most notably a reduction of the U.S. federal corporate tax rate from a maximum of 35% to a flat 21%, effective January 1, 2018. As a result of the new law, as of December 31, 2017, the Company remeasured its deferred tax assets based on the rates at which they are expected to reverse in the future, resulting in a reduction in the deferred tax asset balance of $0.2 million, which was The Company is subject to taxation in the United States federal and state jurisdictions. The Company’s federal income tax and state income tax returns are subject to examination by tax authorities. The Company is currently under examination by the New York City Department of Finance for tax years 2017 and 2018 and by the New York State Department of Taxation and Finance regarding its 2018 R&D biotechnology tax credit request. No related reserve has been recorded for this position.

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2019
Related Party Transactions [Abstract]
Related Party Transactions	13. RELATED PARTY TRANSACTIONS Since the Company’s inception in July 2017, the Company has engaged in transactions with related parties including OrbiMed, the Company’s largest shareholder, and REGENXBIO. In August 2017, OrbiMed purchased the initial Common Stock of the entity. In August 2017, the Company entered into a Series Seed Preferred Stock Purchase Agreement with OrbiMed. In August 2017, the Company entered into a Patent License Agreement and Stock Purchase Agreement with REGENXBIO Inc. See Note 3. During the year ended December 31, 2019, the Company exercised all of the options under the REGENXBIO Option Gene License and paid the additional up-front fee of $0.6 million per option, or an aggregate of $1.8 million, to REGENXBIO which was recorded as research and development expense. In addition, in accordance with the REGENXBIO GBA1 License, the Company paid an annual maintenance fee of approximately $0.1 million, which was also recorded as research and development expense and incurred expenses of approximately $0.2 million in board member fees during the year ended December 31, 2019 which was recorded as general and administrative expense. Aggregate payments to related parties for the years ended December 31, 2019 and December 31, 2018 totaled $2.2 million and $0.7 million, respectively, which was primarily recorded as research and development expense.

Net Loss Per Share

Net Loss Per Share	12 Months Ended
	Dec. 31, 2019
Earnings Per Share [Abstract]
Net Loss Per Share	14. NET LOSS PER SHARE The following outstanding potentially dilutive common stock equivalents have been excluded from the calculation of diluted net loss per share for the periods presented due to their anti-dilutive effect: Years ended December 31, 2019 2018 Redeemable convertible preferred stock (as converted) — 15,477,085 Common stock options issued and outstanding 5,681,422 4,017,739 Restricted stock subject to future vesting 978,759 1,566,001 Total 6,660,181 21,060,825 Neither the Company’s redeemable convertible preferred stock nor restricted stock subject to future vesting participates in losses. Years ended December 31, 2019 2018 (in thousands, except share data) Net loss $ (63,188 ) $ (19,087 ) Weighted-average number of shares-basic and diluted 28,494,950 5,145,469 Net loss per share—basic and diluted (2.22 ) (3.71 )

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2019
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	15. COMMITMENTS AND CONTINGENCIES Contingencies —From time to time, the Company may be involved in disputes or regulatory inquiries that arise in the ordinary course of business. When the Company determines that a loss is both probable and reasonably estimable, a liability is recorded and disclosed if the amount is material to the financial statements taken as a whole. When a material loss contingency is only reasonably possible, the Company does not record a liability, but instead discloses the nature and the amount of the claim, and an estimate of the loss or range of loss, if such an estimate can reasonably be made. The Company does not have accruals established for any litigation liabilities as of December 31, 2019 and December 31, 2018. In June 2019, the Company received a letter on behalf of Alector, a biopharmaceutical company employing antibodies for the treatment of neurodegeneration, stating concerns regarding whether confidential information of Alector was used in connection with work on behalf of the Company and patents and patent applications filed on behalf of the Company, as well as alleging that Alector has certain rights to the Company’s patents and patent applications. The Company believes these allegations of wrongdoing and Alector’s claims of rights to any of the Company’s intellectual property are without basis or merit, as the Company’s gene therapy programs and underlying patents and patent applications were based on work done by Dr. Abeliovich derived from publicly available information or from work outside of and wholly separate from any matters on which he consulted for Alector or information he received while consulting for Alector. Dr. Abeliovich intends to vigorously defend any claim or lawsuit making allegations relating to these matters, however, there can be no assurance regarding any resolution or the outcome of these matters. If the Company becomes party to any demand, claim or allegations related to these matters, the Company also intends to vigorously defend any such proceedings. The Company records accruals for such contingencies when it is probable that a liability will be incurred and the amount of loss can be reasonably estimated. The Company is unable to estimate the probability and amount of potential loss as of December 31, 2019 and has not recorded an accrual. The Company is also party to various agreements, principally relating to licensed technology, that require future payments relating to milestones, or royalties on future sales of specified products. No material milestone or royalty payments under these agreements are expected to be payable in the immediate future. See Note 3 for further details of these agreements.

Employee Benefit Plan

Employee Benefit Plan	12 Months Ended
	Dec. 31, 2019
Compensation And Retirement Disclosure [Abstract]
Employee Benefit Plan	16. EMPLOYEE BENEFIT PLAN The Company maintains a 401(k) plan that provides eligible U.S. employees with an opportunity to save for retirement on a tax advantaged basis. Eligible employees are able to defer eligible compensation up to certain Code limits, which are updated annually. The Company has the ability to make matching and discretionary contributions to the 401(k) plan. In 2019, the Company contributed 100% of the first 6% of contributions. Total employer matching contributions to the Company’s 401(k) Plan for the years ended December 31, 2019 and December 31, 2018 were $0.3 million and $0.1 million, respectively.

Significant Accounting Polici_2

Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation— The accompanying financial statements have been prepared by the Company in accordance with accounting principles generally accepted in the United States, or U.S. GAAP, and pursuant to the rules and regulations of the U.S. Securities and Exchange Commission, or the SEC, for reporting on Form 10-K. In April 2012, the Jumpstart Our Business Startups Act of 2012, or the JOBS Act, was enacted. Section 107(b) of the JOBS Act provides that an emerging growth company can take advantage of an extended transition period for complying with new or revised accounting standards. Thus, an emerging growth company can delay the adoption of certain accounting standards until those standards would otherwise apply to private companies. The Company has irrevocably elected not to avail itself of this extended transition period, and, as a result, the Company will adopt new or revised accounting standards on the relevant dates on which adoption of such standards is required for other public companies.
Use of Estimates	Use of Estimates —The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements, and the reported amounts of expenses during the reporting period. Significant estimates in the financial statements include, but are not limited to stock-based compensation, fair value of common stock, fair value of the derivative liabilities, operating lease right-of-use assets and liabilities, the recoverability of the Company’s net deferred tax assets and related valuation allowance, and accrued liabilities related to expenses incurred for research and development from external vendors. The Company bases its estimates on historical experience, known trends and other market-specific or other relevant factors that it believes to be reasonable under the circumstances. The Company tracks the progress of its various research and development studies and manufacturing projects to ensure related prepaid expenses and accrued expenses are in line with progress of each. On an ongoing basis, management evaluates its estimates, as there are changes in circumstances, facts and experience. Actual results may differ materially from those estimates or assumptions.
Cash, Cash Equivalents and Restricted Cash	Cash, Cash Equivalents and Restricted Cash —The Company’s cash and cash equivalents include short-term highly liquid investments which are readily convertible into cash. These investments include money market securities and commercial paper with maturities of three months or less when acquired. The Company’s institutional money market accounts permit daily redemption and the fair values of these investments are based upon the quoted prices in active markets provided by the holding financial institutions, which are considered Level 1 inputs in the fair value hierarchy, as described below. The Company had cash and cash equivalents of $168.1 million as of December 31, 2019. Restricted cash represents cash on deposit with a financial institution as collateral in support of a letter of credit outstanding in favor of the Company’s landlord for office space. The restricted cash balance has been excluded from the cash balance and is classified as non-current restricted cash on the balance sheets as the lease expires after December 31, 2020.
Concentration of Credit Risk	Concentration of Credit Risk —The Company maintains cash deposits in excess of government-provided insurance limits. The Company maintains its cash balances with one high quality, accredited financial institution, and accordingly, such funds are not exposed to significant credit risk.
Leases	Leases —The Company determines if an arrangement is a lease at inception. Operating leases are included in operating lease right-of-use, or ROU, assets, operating lease liabilities, and long-term operating lease liabilities in the Company’s balance sheets. ROU 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 ROU assets and liabilities are recognized at commencement date based on the present value of lease payments over the lease term. As the Company’s leases do not provide a readily determinable implicit rate, the Company’s uses its incremental borrowing rate based on the information available at commencement date in determining the present value of lease payments. The operating lease ROU asset also includes any lease prepayments, offset by lease incentives. The Company’s facilities operating leases have lease and non-lease components for which the Company has elected to apply the practical expedient and account for each lease component and related non-lease component as one single component. Operating lease cost is recognized on a straight-line basis over the lease term.
Property and Equipment, Net	Property and Equipment, Net —Property and equipment are stated at cost less accumulated depreciation. Depreciation expense is recognized using the straight-line method over the estimated useful life of the asset, ranging from 3-7 years as follows: Fixed Asset Type Estimated useful life Laboratory equipment 7 years Leasehold improvements Lesser of useful life or remaining lease term Computer equipment 3 years Furniture and fixtures 7 years
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets —Long-lived assets, comprised of property and equipment, to be held and used and the right-of-use asset associated with the Company’s leased office space are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long-lived asset for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset to its carrying value. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset over its current fair value. To date, the Company has not recorded any impairment losses on long-lived assets.
Comprehensive Loss	Comprehensive Loss —The Company does not have items of other comprehensive loss for the years ended December 31, 2019 and 2018, and therefore does not present a statement of comprehensive loss. The Company’s comprehensive loss equals its net loss
Fair Value Measurements	Fair Value Measurements —Certain assets and liabilities are carried at fair value under U.S. GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. A framework is used for measuring fair value utilizing a three-tier hierarchy that prioritizes the inputs to valuation techniques used to measure fair value. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: • Level 1 —Unadjusted quoted prices in active markets for identical assets or liabilities. • Level 2 —Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. • Level 3 —Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. The following table summarizes assets measured at fair value on a recurring basis at December 31, 2019 (and does not include cash of $0.2 million as of December 31, 2019): December 31, 2019 Active Markets (Level 1) Observable Inputs (Level 2) Unobservable Inputs (Level 3) (in thousands) Cash and cash equivalents: Money market funds 167,805 — — Restricted cash: Money market funds 91 — — Total $ 167,896 $ — $ — The following table summarizes assets measured at fair value on a recurring basis at December 31, 2018 (and does not include cash of less than $0.1 million as of December 31, 2018): December 31, 2018 Active Markets (Level 1) Observable Inputs (Level 2) Unobservable Inputs (Level 3) (in thousands) Cash and cash equivalents: Money market funds 63,007 — — Restricted cash: Money market funds 91 — — Total $ 63,098 $ — $ — There have been no changes to the valuation methods utilized by the Company, nor were there transfers between Level 1, Level 2 and Level 3 investments during the year ended December 31, 2019. there were no financial instruments classified as Level 3 investments.
Derivative Liabilities	Derivative Liabilities —From time to time, the Company may issue certain financial instruments with embedded features which, dependent on their specific contractual terms or other conditions, may be required to be accounted for as separate derivative assets or liabilities. These instruments are required to be measured at fair value. In determining the appropriate fair value, the Company’s uses a discounted cash flow analysis because these instruments are not quoted on an active market. These instruments are then adjusted to reflect fair value at each period end. Any increase or decrease in the fair value is recorded in the statement of operations as change in fair value of derivative liabilities.
Research and Development Costs	Research and Development Costs —Research and development costs are expensed as incurred. Research and development expenses consist principally of personnel costs, including salaries, stock-based compensation, and benefits for research and development employees, costs related to third-party contract research, contract development and manufacturing organizations, other third-party research service providers, third-party license fees, other direct and indirect operational costs related to the Company’s research and development activities, including facility-related expenses. Non-refundable research and development advance payments are capitalized and expensed as the related goods are delivered or services are performed.
Stock-Based Compensation	Stock-Based Compensation —The Company measures all stock options and other stock-based awards granted to employees, directors, consultants and other nonemployees based on the fair value on the date of the grant and recognizes compensation expense of those awards over the requisite service period, which is generally the vesting period of the respective award. The Company recognizes forfeitures at the time forfeitures occur. The Company classifies stock-based compensation expense in its statement of operations in the same way the payroll costs or service payments are classified for the related stock-based award recipient. The fair value of each stock option grant is estimated on the date of grant using the Black-Scholes option pricing model. The Company lacks company specific historical and implied volatility information. Therefore, it estimates its expected stock volatility based on the historical volatility of a publicly-traded set of peer companies and expects to continue to do so until it has adequate historical data regarding the volatility of its own traded stock price.
Debt Issuance Costs	Debt Issuance Costs— The Company incurred third-party costs in connection with the Company’s convertible note as described in Note 7. The Company amortizes these costs over the term of its agreement as interest expense in the statement of operations.
Income Taxes	Income Taxes —The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the financial statements or the Company’s tax returns. Under this method, deferred tax assets and liabilities are determined on the basis of the differences between the financial statements and tax basis of the assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. The effect of a change in tax rates on deferred tax assets and liabilities is recognized in income in the period that includes the enactment date. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company recognizes deferred tax assets to the extent that the Company believes that these assets are more likely than not to be realized. In making such a determination, management considers all available positive and negative evidence, including future reversals of existing taxable temporary differences, projected future taxable income, tax-planning strategies, and results of recent operations. Valuation allowances are provided, if based upon the weight of available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized. If management determines that the Company would be able to realize its deferred tax assets in the future in excess of their net recorded amount, management would make an adjustment to the deferred tax asset valuation allowance, which would reduce the provision for income taxes. The Company records uncertain tax positions in accordance with ASC 740 on the basis of a two-step process in which (1) management determines whether it is more likely than not that the tax positions will be sustained on the basis of the technical merits of the position and (2) for those tax positions that meet the more-likely-than-not recognition threshold, management recognizes the largest amount of tax benefit that is more than 50 percent likely to be realized upon ultimate settlement with the related tax authority. The Company provides reserves for potential payments of tax to various tax authorities related to uncertain tax positions. These reserves are based on a determination of whether and how much of a tax benefit taken by the Company in its tax filings or positions is more likely than not to be realized following resolution of any potential contingencies related to the tax benefit. Potential interest related to the underpayment of income taxes will be classified as a component of interest expense and any related penalties will be classified in operating expenses in the statement of operations.
Net Loss per Common Share	Net Loss per Common Share— Basic net loss per share is computed using the “two-class” method which includes the weighted average number of shares of common stock outstanding during the period and other securities that participate in dividends (a participating security). As of December 31, 2018, the Company’s convertible preferred stock were participating securities as defined by ASC 260-10, Earnings per Share. This stock converted into common stock upon the completion of the IPO in June 2019. During the periods where the Company incurs net losses, the Company allocates no loss to participating securities because these securities have no contractual obligation to share in the losses of the Company. Under the two-class method, basic net loss per share applicable to common stockholders is computed by dividing the net loss applicable to common stockholders by the weighted average number of common shares outstanding for the period. Diluted net loss per share is computed similar to basic net loss per share except that the denominator is increased to include the number of additional shares for the potential dilutive effects of convertible debt, convertible preferred stock and stock options outstanding during the period calculated in accordance with the treasury stock method, or the two-class method, whichever is more dilutive. The Company allocates net earnings on a pari passu (equal) basis to both common and preferred stockholders. Net losses are not allocated to preferred stockholders as they do not have an obligation to share in the Company’s net losses. For all periods presented, basic and diluted net loss per share are the same, as any additional share equivalents would be anti-dilutive (Note 14).
Segment Reporting	Segment Reporting— Operating segments are identified as components of an enterprise for which separate discrete financial information is available for evaluation by the Company’s Chief Operating Decision Maker in making decisions regarding resource allocation and assessing performance. To date, the Company has viewed its operations and manages its business as one operating segment.
Recently Issued Accounting Pronouncements (Adopted)	Recently Issued Accounting Pronouncements (Adopted) —In November 2018, the SEC’s release, Disclosure Update and Simplification, became effective. The Company adopted this amendment on January 1, 2019. The included amendments are intended to simplify and update the SEC’s disclosure requirements and eliminate duplicative disclosures between the SEC rules and U.S. GAAP. The amendments included new interim financial statement disclosures to reconcile the beginning balance to the ending balance in stockholders’ equity for each period for which an income statement is required to be filed. The Company adopted the rule on January 1, 2019 and included its first presentation of changes in stockholders’ equity in its Form 10-Q for the three-month period ended March 31, 2019. In July 2018, the FASB issued ASU 2018-09, Codification Improvements,
Recently Issued Accounting Pronouncements (Not Yet Adopted)	Recently Issued Accounting Pronouncements (Not Yet Adopted) —In August 2018, the FASB issued ASU 2018-13 Fair Value Measurement – Disclosure Framework-Changes to the Disclosure Requirement for Fair Value Measurement, or ASU 2018-13. The amendments in ASU 2018-13 modify the disclosure requirements on fair value measurements in ASC 820, Fair Value Measurement, based on the concepts in the FASB Concepts Statement, including the consideration of costs and benefits. The amendments under ASU 2018-13 are effective for interim and annual fiscal periods beginning after December 15, 2019, with early adoption permitted. The Company does not expect the adoption of ASU 2018-13 to have a material impact on its financial statements. In August 2018, the FASB issued ASU No. 2018-15, Intangible-Goodwill and Other Internal-Use Software (Subtopic 350-40)

Significant Accounting Polici_3

Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Schedule of Estimated Useful Lives of Property and Equipment	Depreciation expense is recognized using the straight-line method over the estimated useful life of the asset, ranging from 3-7 years as follows: Fixed Asset Type Estimated useful life Laboratory equipment 7 years Leasehold improvements Lesser of useful life or remaining lease term Computer equipment 3 years Furniture and fixtures 7 years
Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis	The following table summarizes assets measured at fair value on a recurring basis at December 31, 2019 (and does not include cash of $0.2 million as of December 31, 2019): December 31, 2019 Active Markets (Level 1) Observable Inputs (Level 2) Unobservable Inputs (Level 3) (in thousands) Cash and cash equivalents: Money market funds 167,805 — — Restricted cash: Money market funds 91 — — Total $ 167,896 $ — $ — The following table summarizes assets measured at fair value on a recurring basis at December 31, 2018 (and does not include cash of less than $0.1 million as of December 31, 2018): December 31, 2018 Active Markets (Level 1) Observable Inputs (Level 2) Unobservable Inputs (Level 3) (in thousands) Cash and cash equivalents: Money market funds 63,007 — — Restricted cash: Money market funds 91 — — Total $ 63,098 $ — $ —

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2019
Property Plant And Equipment [Abstract]
Summary of Property and Equipment, Net	Property and equipment, net, consisted of the following: As of December 31, 2019 2018 (in thousands) Laboratory equipment $ 1,730 $ 677 Leasehold improvements 887 — Computer equipment 36 57 Furniture and fixtures 218 1 Gross property and equipment 2,871 735 Less: accumulated depreciation (322 ) (57 ) Property and equipment, net $ 2,549 $ 678

Accrued Expenses and Other Cu_2

Accrued Expenses and Other Current Liabilities (Tables)	12 Months Ended
	Dec. 31, 2019
Accrued Expenses And Other Current Liabilities [Abstract]
Schedule of Accrued Expenses and Other Current Liabilities	Accrued expenses consisted of the following: As of December 31, 2019 2018 (in thousands) Accrued compensation $ 2,577 $ 840 Accrued research and development 1,595 273 Accrued professional fees 634 — Other 524 364 Total accrued expenses $ 5,330 $ 1,477

Common Stock (Tables)

Common Stock (Tables)	12 Months Ended
	Dec. 31, 2019
Equity [Abstract]
Schedule of Number of Shares of Common Stock Reserved for Future Issuance	As of December 31, 2019 and 2018 the Company has reserved the following number of shares of common stock for future issuance upon the conversion of preferred stock, exercise of options or grant of equity As of December 31, 2019 2018 Redeemable convertible preferred stock outstanding, as converted — 15,477,085 Options and restricted stock issued and outstanding 7,051,663 6,366,739 Shares available for future stock option grants 2,762,749 844,287 Total 9,814,412 22,688,111

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2019
Leases [Abstract]
Summary of Components of Operating Lease Expense	The components of the lease expense were as follows: Years ended December 31, 2019 2018 (in thousands) Operating lease cost $ 2,033 $ 752 Variable lease cost 731 226 Total lease cost $ 2,764 $ 978 Weighted-average remaining lease term 5.8 years 6.1 years Weighted-average discount rate 9.25 % 9.00 %
Summary of Maturities of Remaining Operating Lease Liabilities	As of December 31, 2019 the maturities of the Company’s remaining operating lease liabilities were as follows: December 31, 2019 (in thousands) 2020 $ 2,299 2021 2,379 2022 2,463 2023 2,549 2024 2,638 Thereafter 2,263 Present value adjustment (3,322 ) Present value of lease payments $ 11,269

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of Stock Option Activity	The following tables summarize stock option activity under the 2017 Plan and the 2019 Plan: Number of Awards Weighted Average Exercise Price Weighted Average Remaining Contractual Life (Years) Aggregate Intrinsic Value (in thousands, except share data) Outstanding, December 31, 2018 4,017,736 $ 0.83 9.3 $ 7,939 Granted 1,939,642 9.08 Exercised (141,619 ) 0.37 Cancelled/forfeited (134,337 ) 2.05 Outstanding, December 31, 2019 5,681,422 $ 3.60 8.6 $ 69,219 Exercisable, December 31, 2019 1,527,587 0.70 8.3 23,108 Vested and expected to vest, December 31, 2019 5,681,422 3.60 8.6 69,219
Schedule of Assumptions to Estimate Fair Value of Stock Options Using Black-Scholes Option Pricing Model	The weighted average fair value and assumptions used to determine the fair value of stock options granted was as follows: As of December 31, 2019 2018 Weighted average grant date fair value of common stock $ 6.29 $ 2.39 Expected term 6.0 6.1 Risk-free interest rate 2.3 % 2.8 % Expected volatility 79.2 % 74.6 % Dividend rate — —
Summary of Non-vested Restricted Stock Award Activity	The following table presents a summary of the Company’s non-vested restricted stock award activity under all plans and related information for the years ended December 31, 2019 and December 31, 2018: Number of Restricted Stock Awards Outstanding Weighted Average Grant Date Fair Value Per Share Non-vested restricted stock awards outstanding as of December 31, 2018 1,566,001 $ 0.18 Restricted stock awards vested (587,242 ) 0.18 Non-vested restricted stock awards outstanding as of December 31, 2019 978,759 $ 0.18 Years ended December 31, 2019 2018 (in thousands) Aggregate grant date fair value of restricted stock awards vested $ 101 $ 135
Summary of Stock-Based Compensation Expense	Total stock-based compensation expense is recognized for restricted stock and stock options granted to employees and non-employees and has been reported in the Company’s statements of operations as follows: Years ended December 31, 2019 2018 (in thousands) Research and development $ 2,399 $ 1,454 General and administrative 1,912 156 Total stock-based compensation expense $ 4,311 $ 1,610

Income Taxes (Table)

Income Taxes (Table)	12 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Schedule of Effective Income Tax Rate Reconciliation	A reconciliation of income tax computed at the statutory federal income tax rate to the provision (benefit) for income taxes included in the accompanying statements of operations for the Company is as follows: Years ended December 31, 2019 2018 Expected provision at statutory federal rate 21.0 % 21.0 % State tax – net of federal benefit 11.6 % 10.1 % Nondeductible interest expense 0.0 % (1.4 )% Tax credits 0.8 % 1.2 % Changes in valuation allowance (32.8 )% (29.3 )% Other (0.6 )% (1.6 )% Effective Income Tax Rate 0.0 % 0.0 %
Schedule of Deferred Tax Assets and Liabilities	Significant components of the Company’s deferred tax assets and liability are as As of December 31, 2019 2018 (in thousands) Deferred Tax Assets: Net operating loss carryforwards $ 18,837 $ 4,177 Accruals and reserves 847 275 Lease liability 3,238 2,458 Stock compensation 552 76 Tax credits 806 289 Section 195 start up cost 5,558 1,288 Other 1 — Gross deferred tax assets $ 29,839 $ 8,563 Less valuation allowance (26,918 ) (6,212 ) Net deferred tax assets $ 2,921 $ 2,351 Deferred tax liabilities: Fixed assets $ (99 ) $ (22 ) Right of use assets $ (2,822 ) $ (2,329 ) Gross deferred tax liability $ (2,921 ) $ (2,351 ) Net deferred tax assets $ — $ —

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	12 Months Ended
	Dec. 31, 2019
Earnings Per Share [Abstract]
Schedule of Potentially Dilutive Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share	The following outstanding potentially dilutive common stock equivalents have been excluded from the calculation of diluted net loss per share for the periods presented due to their anti-dilutive effect: Years ended December 31, 2019 2018 Redeemable convertible preferred stock (as converted) — 15,477,085 Common stock options issued and outstanding 5,681,422 4,017,739 Restricted stock subject to future vesting 978,759 1,566,001 Total 6,660,181 21,060,825
Schedule of Basic and Diluted Net Loss Per Share	Neither the Company’s redeemable convertible preferred stock nor restricted stock subject to future vesting participates in losses. Years ended December 31, 2019 2018 (in thousands, except share data) Net loss $ (63,188 ) $ (19,087 ) Weighted-average number of shares-basic and diluted 28,494,950 5,145,469 Net loss per share—basic and diluted (2.22 ) (3.71 )

Nature of the Business - Additi

Nature of the Business - Additional Information (Details) $ / shares in Units, $ in Thousands	1 Months Ended		12 Months Ended
	Jun. 30, 2019USD ($)$ / sharesshares	Mar. 31, 2019USD ($)$ / sharesshares	Dec. 31, 2019USD ($)shares	Dec. 31, 2018USD ($)shares	Mar. 31, 2018shares	Aug. 31, 2017shares	Jul. 06, 2017shares
Nature Of Business [Line Items]
Entity incorporation date			Jul. 6, 2017
Description of stock split			1.62-for-one forward stock-split
Stock split ratio	1.62
Stock issued during period, shares, new issues | shares	7,353,000
Underwriting discounts, commissions and estimated offering expenses | $			$ 3,210
Common stock, shares authorized | shares		35,640,000	200,000,000	28,398,600	27,540,000	16,200,000	6,480,000
Preferred stock, shares authorized | shares			10,000,000	15,552,000
Cash and cash equivalents | $			$ 168,051	$ 63,014
Proceeds from issuance of common stock | $			$ 116,251
Series B Preferred Stock
Nature Of Business [Line Items]
Shares issued, price per share | $ / shares		$ 12.63
Underwriting discounts, commissions and estimated offering expenses | $		$ 200
Net proceeds from issuance of convertible preferred stock | $		$ 49,800
Common Stock
Nature Of Business [Line Items]
Stock issued during period, shares, new issues | shares			7,353,000
Initial Public Offering
Nature Of Business [Line Items]
Common stock, shares authorized | shares	200,000,000
Preferred stock, shares authorized | shares	10,000,000
Proceeds from issuance of common stock | $	$ 113,000
Initial Public Offering | Common Stock
Nature Of Business [Line Items]
Stock issued during period, shares, new issues | shares	7,353,000
Shares issued, price per share | $ / shares	$ 17
Aggregate net proceeds received from offering | $	$ 113,000
Underwriting discounts, commissions and estimated offering expenses | $	$ 12,000
Redeemable convertible preferred stock converted into share | shares	19,435,131

Significant Accounting Polici_4

Significant Accounting Policies - Additional Information (Details)	12 Months Ended
	Dec. 31, 2019USD ($)Segment	Dec. 31, 2018USD ($)
Significant Accounting Policies [Line Items]
Cash and cash equivalents	$ 168,051,000	$ 63,014,000
Fair value of cash	200,000
Transfers of assets between Level 1 and Level 2	0	0
Transfers of assets between Level 2 and Level 1	0	0
Transfers of liabilities between Level 1 and Level 2	0	0
Transfers of liabilities between Level 2 and Level 1	0	0
Transfers of assets between Level 1 and Level 3	0	0
Transfers of assets between Level 3 and Level 1	0	0
Transfers of assets between Level 2 and Level 3	0	0
Transfers of assets between Level 3 and Level 2	0	0
Transfers of liabilities between Level 1 and Level 3	0	0
Transfers of liabilities between Level 3 and Level 1	0	0
Transfers of liabilities between Level 2 and Level 3	0	0
Transfers of liabilities between Level 3 and Level 2	$ 0	0
Number of operating segment | Segment	1
Fair Value, Inputs, Level 3
Significant Accounting Policies [Line Items]
Financial instruments, owned, at fair value	$ 0	0
Minimum
Significant Accounting Policies [Line Items]
Property and equipment useful life	3 years
Largest amount of tax benefit	50.00%
Maximum
Significant Accounting Policies [Line Items]
Property and equipment useful life	7 years
Fair value of cash		$ 100,000

Significant Accounting Polici_5

Significant Accounting Policies - Schedule of Estimated Useful Lives of Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2019
Laboratory Equipment
Property Plant And Equipment [Line Items]
Property and equipment useful life	7 years
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment useful life	Lesser of useful life or remaining lease term
Computer Equipment
Property Plant And Equipment [Line Items]
Property and equipment useful life	3 years
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment useful life	7 years

Significant Accounting Polici_6

Significant Accounting Policies - Schedule of Assets Measured at Fair Value (Details) - Active Markets (Level 1) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total	$ 167,896	$ 63,098
Money Market Funds
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	167,805	63,007
Restricted cash	$ 91	$ 91

License Agreements - Additional

License Agreements - Additional Information (Details) - USD ($) $ in Thousands	Aug. 31, 2019	Jun. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Research And Development Arrangement Contract To Perform For Others [Line Items]
Stock issued during period, shares, new issues		7,353,000
Common stock, fair value			$ 113,041
Research and development			$ 48,798	$ 14,127
REGENXBIO GBA1
Research And Development Arrangement Contract To Perform For Others [Line Items]
Licenses to be expired			7 years
License
Research And Development Arrangement Contract To Perform For Others [Line Items]
Research and development			$ 1,900
Maintenance fee	$ 100
License | REGENXBIO GBA1
Research And Development Arrangement Contract To Perform For Others [Line Items]
Stock issued during period, shares, new issues					2,430,000
Common stock, fair value					$ 400
Research and development				100
Initial fee			600
Additional up-front fee			600
License | REGENXBIO Option Genes License
Research And Development Arrangement Contract To Perform For Others [Line Items]
Research and development			1,800	$ 600
Additional upfront fee paid			$ 600

Fair Value of Financial Instr_2

Fair Value of Financial Instruments - Additional Information (Details) - USD ($)	1 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2019
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Transfers of assets between Level 1 and Level 2		$ 0	$ 0
Transfers of assets between Level 2 and Level 1		0	0
Transfers of liabilities between Level 1 and Level 2		0	0
Transfers of liabilities between Level 2 and Level 1		0	0
Transfers of assets between Level 1 and Level 3		0	0
Transfers of assets between Level 3 and Level 1		0	0
Transfers of assets between Level 2 and Level 3		0	0
Transfers of assets between Level 3 and Level 2		0	0
Transfers of liabilities between Level 1 and Level 3		0	0
Transfers of liabilities between Level 3 and Level 1		0	0
Transfers of liabilities between Level 2 and Level 3		0	0
Transfers of liabilities between Level 3 and Level 2		0	0
Fair Value, Inputs, Level 3
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Financial instruments, owned, at fair value		0	$ 0
Convertible Note
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Settlement of convertible note	$ 3,000,000
Loss recognized related to convertible note settlement as change in fair value of derivative liabilities		$ 800,000

Property and Equipment, Net - S

Property and Equipment, Net - Summary of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Gross property and equipment	$ 2,871	$ 735
Less: accumulated depreciation	(322)	(57)
Property and equipment, net	2,549	678
Laboratory Equipment
Property Plant And Equipment [Line Items]
Gross property and equipment	1,730	677
Leasehold Improvements
Property Plant And Equipment [Line Items]
Gross property and equipment	887
Computer Equipment
Property Plant And Equipment [Line Items]
Gross property and equipment	36	57
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Gross property and equipment	$ 218	$ 1

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Property Plant And Equipment [Abstract]
Depreciation	$ 265	$ 55

Accrued Expenses and Other Cu_3

Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Accrued Expenses And Other Current Liabilities [Abstract]
Accrued compensation	$ 2,577	$ 840
Accrued research and development	1,595	273
Accrued professional fees	634
Other	524	364
Total accrued expenses	$ 5,330	$ 1,477

Convertible Note - Related Pa_2

Convertible Note - Related Party - Additional Information (Details) - USD ($)	1 Months Ended		12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Apr. 30, 2018
Debt Instrument [Line Items]
Debt instrument principal amount					$ 10,000,000
Convertible Note
Debt Instrument [Line Items]
Derivative liabilities settlement amount	$ 3,000,000				$ 3,000,000
Change in fair value of derivative liability	$ 800,000
Coupon interest				$ 100,000
Discount amortization				$ 300,000
Maximum | Convertible Note
Debt Instrument [Line Items]
Debt issuance costs			$ 100,000
Automatic Conversion Upon Qualified Financing
Debt Instrument [Line Items]
Conversion price equal to price per share paid, percentage			90.00%
Derivative liabilities			$ 2,200,000
Convertible Note Purchase Agreement | OrbiMed Private Investments VI, LP ("the Holder")
Debt Instrument [Line Items]
Debt instrument principal amount		$ 10,000,000
Debt instrument interest rate		8.00%
Debt instrument maturity date		Dec. 31, 2018
Convertible Note Purchase Agreement | OrbiMed Private Investments VI, LP ("the Holder") | Qualified Financing | Minimum
Debt Instrument [Line Items]
Proceeds from issuance of Preferred Stock		$ 25,000,000

Convertible Note - Related Pa_3

Convertible Note - Related Party - Series A Preferred Stock - Additional Information (Details) - USD ($) $ in Thousands	2 Months Ended	12 Months Ended
	Apr. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Temporary Equity [Line Items]
Preferred stock, issued		0
Debt instrument principal amount	$ 10,000
Accrued interest	200
Convertible Note
Temporary Equity [Line Items]
Derivative liabilities settlement amount	$ 3,000			$ 3,000
Series A Preferred Stock
Temporary Equity [Line Items]
Preferred stock, issued	7,666,716		8,997,085	7,666,716
Proceeds from issuance of Preferred Stock	$ 64,900		$ 65,000
Conversion of convertible note	1,330,369	1,330,369

Redeemable Convertible Prefer_2

Redeemable Convertible Preferred Stock - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended				12 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Mar. 31, 2018	Aug. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Apr. 30, 2018
Temporary Equity [Line Items]
Issuance costs					$ 3,210
Preferred stock, issued					0
Conversion of convertible preferred stock into common stock upon the closing of initial public offering, shares	19,435,131
Issuance of common stock upon closing of initial public offering, net of issuance costs, shares	7,353,000
Preferred stock, shares authorized					10,000,000	15,552,000
Preferred stock, shares outstanding					0
Preferred stock, dividend rate					6.00%
Series Seed Preferred Stock
Temporary Equity [Line Items]
Preferred stock, shares authorized				6,480,000	0	6,480,000
Preferred stock, par value				$ 0.0001	$ 0.0001	$ 0.0001
Preferred stock, shares issued					0	6,480,000
Preferred stock, issued						6,480,000
Conversion of convertible preferred stock into common stock upon the closing of initial public offering, shares					(6,480,000)
Preferred stock, shares outstanding						6,480,000
Preferred stock, conversion price per share						$ 0.62
Preferred Stock, Redemption Price Per Share					$ 0.62
Series Seed Preferred Stock | OrbiMed Private Investments VI LP and the Silverstein Foundation
Temporary Equity [Line Items]
Preferred stock, shares issued				6,480,000
Shares issued, price per share				$ 0.62
Aggregate proceeds from issuance of stock				$ 4,000
Issuance costs				$ 100
Series A Preferred Stock
Temporary Equity [Line Items]
Preferred stock, shares authorized			9,072,000		0	9,072,000
Preferred stock, par value			$ 0.0001		$ 0.0001	$ 0.0001
Preferred stock, shares issued					0	8,997,085
Shares issued, price per share			$ 8.48
Aggregate proceeds from issuance of stock			$ 65,000
Issuance costs			$ 100
Preferred stock, issued			7,666,716			8,997,085	7,666,716
Conversion of convertible preferred stock into common stock upon the closing of initial public offering, shares					(8,997,085)
Issuance of common stock upon closing of initial public offering, net of issuance costs, shares			8,997,085
Preferred stock, shares outstanding						8,997,085
Preferred stock, conversion price per share						$ 8.48
Preferred Stock, Redemption Price Per Share					$ 8.48
Series A Preferred Stock | OrbiMed Private Investments VI, LP ("the Holder")
Temporary Equity [Line Items]
Conversion of convertible preferred stock into common stock upon the closing of initial public offering, shares			1,330,369
Series B Preferred Stock
Temporary Equity [Line Items]
Preferred stock, shares authorized		3,958,046
Preferred stock, par value		$ 0.0001
Shares issued, price per share		$ 12.63
Aggregate proceeds from issuance of stock		$ 49,800
Issuance costs		$ 200
Conversion of convertible preferred stock into common stock upon the closing of initial public offering, shares					(3,958,046)
Preferred Stock, Redemption Price Per Share					$ 12.63

Common Stock - Additional Infor

Common Stock - Additional Information (Details) $ / shares in Units, $ in Thousands	1 Months Ended		12 Months Ended
	Jun. 30, 2019shares	Aug. 31, 2017USD ($)$ / sharesshares	Dec. 31, 2019USD ($)$ / sharesshares	Dec. 31, 2018USD ($)$ / sharesshares	Mar. 31, 2019shares	Mar. 31, 2018shares	Jul. 06, 2017$ / sharesshares
Class Of Stock [Line Items]
Common stock, shares authorized		16,200,000	200,000,000	28,398,600	35,640,000	27,540,000	6,480,000
Common stock, par value | $ / shares		$ 0.0001	$ 0.0001	$ 0.0001			$ 0.0001
Description of stock split			1.62-for-one forward stock-split
Stock split ratio	1.62
Conversion of convertible preferred stock into common stock upon the closing of initial public offering, shares	19,435,131
Issuance of common stock upon closing of initial public offering, net of issuance costs, shares	7,353,000
Dividend declared or unpaid | $			$ 0	$ 0
Restricted Stock [Member]
Class Of Stock [Line Items]
Other stock issued before IPO, shares		2,349,000
Other stock issued before IPO, value | $		$ 145
Vesting period		36 months
Restricted Stock [Member] | Vest on One-year Anniversary [Member]
Class Of Stock [Line Items]
Vesting percentage		25.00%
Restricted Stock [Member] | Vest on each successive One-month period [Member]
Class Of Stock [Line Items]
Vesting percentage		0.02%
OrbiMed Private Investments VI, LP ("the Holder")
Class Of Stock [Line Items]
Other stock issued before IPO, shares		2,430,000
Other stock issued before IPO, value | $		$ 150
Other stock issued before IPO, relative fair value | $		$ 400
REGENXBIO [Member]
Class Of Stock [Line Items]
Other stock issued before IPO, shares		2,430,000

Common Stock - Schedule of Numb

Common Stock - Schedule of Number of Shares of Common Stock Reserved for Future Issuance (Details) - shares	Dec. 31, 2019	Dec. 31, 2018
Class Of Stock [Line Items]
Number of shares of common stock reserved for future issuance	9,814,412	22,688,111
Redeemable Convertible Preferred Stock Outstanding, as Converted [Member]
Class Of Stock [Line Items]
Number of shares of common stock reserved for future issuance		15,477,085
Options And Restricted Stock Issued And Outstanding [Member]
Class Of Stock [Line Items]
Number of shares of common stock reserved for future issuance	7,051,663	6,366,739
Shares Available For Future Stock Option Grants [Member]
Class Of Stock [Line Items]
Number of shares of common stock reserved for future issuance	2,762,749	844,287

Leases - Additional Information

Leases - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Sep. 30, 2017
Leases [Abstract]
Operating lease arrangement, original term			49 months
Security deposit			$ 0.1
Qualified costs incurred	$ 0.4
Reimbursement of qualified costs	0.3
Cash paid for amounts included in measurement of lease liabilities	$ 1.1	$ 0.6

Leases - Summary of Components

Leases - Summary of Components of Lease Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Leases [Abstract]
Operating lease cost	$ 2,033	$ 752
Variable lease cost	731	226
Total lease cost	$ 2,764	$ 978
Weighted-average remaining lease term	5 years 9 months 18 days	6 years 1 month 6 days
Weighted-average discount rate	9.25%	9.00%

Leases - Summary of Maturities

Leases - Summary of Maturities of Remaining Operating Lease Liabilities (Details) $ in Thousands	Dec. 31, 2019USD ($)
Leases [Abstract]
2020	$ 2,299
2021	2,379
2022	2,463
2023	2,549
2024	2,638
Thereafter	2,263
Present value adjustment	(3,322)
Present value of lease payments	$ 11,269

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Details) - USD ($) $ in Millions	1 Months Ended					12 Months Ended
	Jun. 30, 2019	Apr. 30, 2019	Dec. 31, 2018	Mar. 31, 2018	Aug. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Conversion of convertible preferred stock into common stock upon the closing of initial public offering, shares	19,435,131
Preferred stock, shares authorized			15,552,000			10,000,000	15,552,000
Preferred stock, issued						0
Preferred stock, shares outstanding						0
Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Conversion of convertible preferred stock into common stock upon the closing of initial public offering, shares						19,435,131
Convertible Preferred Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Preferred stock, shares outstanding						19,435,131
Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized compensation expense			$ 7.1			$ 14.8	$ 7.1
Unrecognized compensation expense, estimated weighted-average period for recognition						2 years 10 months 24 days	3 years 3 months 18 days
Dividend rate						0.00%
Restricted Stock [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period					36 months
Repurchase of common stock			2,349,000			2,349,000	2,349,000
Restricted Stock Awards
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period						3 years
Unrecognized compensation expense, estimated weighted-average period for recognition						1 year 7 months 6 days
Unrecognized stock-based compensation expense						$ 0.2
Vest on One-year Anniversary [Member] | Restricted Stock [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting percentage					25.00%
Vest on One-year Anniversary [Member] | Restricted Stock Awards
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting percentage						25.00%
2017 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Increase in number of issuance of shares		6,029,733	4,862,027	4,003,427
2019 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Annual shares increase for future issuance by percentage						4.00%
Vesting period						3 years
Expiration period						10 years
2019 Equity Incentive Plan | Vest on One-year Anniversary [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting percentage						25.00%
2019 Equity Incentive Plan | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of issuance of shares						2,762,749
Exercise price as a percentage of grant date fair value						100.00%
2019 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Annual shares increase for future issuance by percentage						1.00%
Annual increase in ordinary shares for available for future issuance						1,500,000
Number of shares Purchase						0
2019 Employee Stock Purchase Plan | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of issuance of shares						330,000

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Details) - Stock Option - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Number ofAwards
Outstanding beginning balance, Number of Awards	4,017,736
Granted, Number of Awards	1,939,642
Exercised, Number of Awards	(141,619)
Cancelled/Forfeited, Number of Awards	(134,337)
Outstanding ending balance, Number of Awards	5,681,422	4,017,736
Exercisable, Number of Awards	1,527,587
Vested and expected to vest, Number of Awards	5,681,422
Weighted Average Exercise Price
Outstanding beginning balance, Weighted Average Exercise Price	$ 0.83
Granted, Weighted Average Exercise Price	9.08
Exercised, Weighted Average Exercise Price	0.37
Cancelled/Forfeited, Weighted Average Exercise Price	2.05
Outstanding ending balance, Weighted Average Exercise Price	3.60	$ 0.83
Exercisable, Weighted Average Exercise Price	0.70
Vested and expected to vest, Weighted Average Exercise Price	$ 3.60
Weighted Average Remaining Contractual Life (Years)
Outstanding, Weighted-Average Remaining Contractual Life (Years)	8 years 7 months 6 days	9 years 3 months 18 days
Exercisable, Weighted-Average Remaining Contractual Life (Years)	8 years 3 months 18 days
Vested and expected to vest, Weighted-Average Remaining Contractual Life (Years)	8 years 7 months 6 days
Outstanding beginning balance, Weighted Average Exercise Price	$ 69,219	$ 7,939
Exercisable, Aggregate Intrinsic Value	23,108
Vested and expected to vest, Aggregate Intrinsic Value	$ 69,219

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Assumptions to Estimate Fair Value of Stock Options Using Black-Scholes Option Pricing Model (Details) - Stock Option - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted average grant date fair value of common stock	$ 6.29	$ 2.39
Expected term	6 years	6 years 1 month 6 days
Risk-free interest rate	2.30%	2.80%
Expected volatility	79.20%	74.60%
Dividend rate	0.00%

Stock-Based Compensation - Su_2

Stock-Based Compensation - Summary of Non-vested Restricted Stock Award Activity (Details) - Restricted Stock Awards - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Number of Restricted Stock Awards Outstanding
Non-vested restricted stock awards outstanding, Beginning	1,566,001
Restricted stock awards vested	(587,242)
Non-vested restricted stock awards outstanding, Ending	978,759	1,566,001
Weighted Average Grant Date Fair Value Per Share
Non-vested restricted stock awards outstanding, Beginning	$ 0.18
Restricted stock awards vested	0.18
Non-vested restricted stock awards outstanding, Ending	$ 0.18	$ 0.18
Aggregate grant date fair value of restricted stock awards vested	$ 101	$ 135

Stock-Based Compensation - Su_3

Stock-Based Compensation - Summary of Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation expense	$ 4,311	$ 1,610
Research and Development | Employees and Non-Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation expense	2,399	1,454
General and Administrative | Employees and Non-Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation expense	$ 1,912	$ 156

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($)	6 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating Loss Carryforwards [Line Items]
Effective tax rate	0.00%	0.00%	0.00%
Change in valuation allowance		$ 20,700,000
Cumulative change in ownership percentage		50.00%
Period for cumulative change in ownership		3 years
Reserves or related accruals for interest and penalties		$ 0	$ 0
Reduction in deferred tax asset		$ 26,918,000	$ 6,212,000
Expected provision at statutory federal rate		21.00%	21.00%
Maximum
Operating Loss Carryforwards [Line Items]
Reduction in deferred tax asset	$ 200,000			$ 200,000
Expected provision at statutory federal rate				35.00%
Minimum
Operating Loss Carryforwards [Line Items]
Expected provision at statutory federal rate		21.00%
Federal [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward		$ 57,300,000	$ 12,800,000
Net operating loss carryforward, expire		2037
Federal [Member] | Research and Development [Member]
Operating Loss Carryforwards [Line Items]
Research and development credit carryforwards		$ 800,000	300,000
State [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward		$ 115,500,000	$ 25,500,000
Net operating loss carryforward, expire		2037

Income Taxes - Schedule of Effe

Income Taxes - Schedule of Effective Income Tax Rate Reconciliation (Details)	6 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Expected provision at statutory federal rate		21.00%	21.00%
State tax – net of federal benefit		11.60%	10.10%
Nondeductible interest expense		0.00%	(1.40%)
Tax credits		0.80%	1.20%
Changes in valuation allowance		(32.80%)	(29.30%)
Other		(0.60%)	(1.60%)
Effective Income Tax Rate	0.00%	0.00%	0.00%

Income Taxes - Schedule of Defe

Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred Tax Assets:
Net operating loss carryforwards	$ 18,837	$ 4,177
Accruals and reserves	847	275
Lease liability	3,238	2,458
Stock compensation	552	76
Tax credits	806	289
Section 195 start up cost	5,558	1,288
Other	1
Gross deferred tax assets	29,839	8,563
Less valuation allowance	(26,918)	(6,212)
Net deferred tax assets	2,921	2,351
Deferred tax liabilities:
Fixed assets	(99)	(22)
Right of use assets	(2,822)	(2,329)
Gross deferred tax liability	$ (2,921)	$ (2,351)

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Research and Development
Related Party Transaction [Line Items]
Aggregate payments to related parties	$ 2,200,000	$ 700,000
Expenses incurred for annual maintenance fee	100,000
REGENXBIO Option Genes License | Research and Development
Related Party Transaction [Line Items]
Additional upfront fee paid	600,000
Aggregate payments to related parties	1,800,000
REGENXBIO Option Genes License | Maximum | General and Administrative
Related Party Transaction [Line Items]
Payment for purchase of materials pursuant to material transfer agreement	$ 200,000

Net Loss Per Share - Schedule o

Net Loss Per Share - Schedule of Potentially Dilutive Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share (Details) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	6,660,181	21,060,825
Redeemable Convertible Preferred Stock Outstanding, as Converted [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)		15,477,085
Common stock options issued and outstanding
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	5,681,422	4,017,739
Restricted stock subject to future vesting
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	978,759	1,566,001

Net Loss Per Share - Schedule_2

Net Loss Per Share - Schedule of Basic and Diluted Net Loss per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Numerator:
Net loss	$ (63,188)	$ (19,087)
Denominator:
Weighted-average number of shares-basic and diluted	28,494,950	5,145,469
Net loss per share—basic and diluted	$ (2.22)	$ (3.71)

Commitments And Contingencies -

Commitments And Contingencies - Additional Information (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Estimated litigation liability	$ 0	$ 0

Employee Benefit Plan - Additio

Employee Benefit Plan - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Compensation And Retirement Disclosure [Abstract]
Employer matching contribution, percent of match	100.00%
Company matching contributions to maximum employees eligible contribution	6.00%
Employers contributions to defined contribution plan	$ 0.3	$ 0.1