4D Molecular Therapeutics (FDMT) 10-K 25 Mar 21 2020 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2020	Mar. 15, 2021	Jun. 30, 2020
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Document Annual Report	true
Document Transition Report	false
Entity Registrant Name	4D Molecular Therapeutics, Inc.
Entity Central Index Key	0001650648
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
ICFR Auditor Attestation Flag	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		26,692,879
Entity Public Float			$ 0
Entity Interactive Data Current	Yes
Entity File Number	001-39782
Entity Tax Identification Number	47-3506994
Entity Address, Address Line One	5858 Horton Street #455
Entity Address, City or Town	Emeryville
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94608
City Area Code	(510)
Local Phone Number	505-2680
Entity Incorporation, State or Country Code	DE
Title of 12(b) Security	Common Stock, par value $0.0001 per share
Trading Symbol	FDMT
Security Exchange Name	NASDAQ
Documents Incorporated by Reference	DOCUMENTS INCORPORATED BY REFERENCE Portions of the registrant’s definitive Proxy Statement relating to the 2021 Annual Meeting of Stockholders are incorporated herein by reference in Part III of this Annual Report on Form 10-K to the extent stated herein. The proxy statement will be filed with the Securities and Exchange Commission within 120 days of the registrant’s fiscal year ended December 31, 2020.

Balance Sheets

Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 276,726	$ 49,652
Accounts receivable	1,486	978
Prepaid expenses and other current assets (includes $169 and $149 at December 31, 2020 and 2019, respectively, attributable to related parties)	4,444	1,878
Total current assets	282,656	52,508
Property and equipment, net	5,073	5,049
Other assets	602	677
Total assets	288,331	58,234
Current liabilities
Accounts payable	1,787	1,744
Accrued and other current liabilities	8,371	5,347
Deferred revenue (includes $0 and $1,122 at December 31, 2020 and 2019, respectively, attributable to related parties)	6,586	5,864
Total current liabilities	16,744	12,955
Deferred revenue, net of current portion (includes $0 and $4,015 at December 31, 2020 and 2019, respectively, attributable to related parties)	13,226	13,603
Derivative liability	122	101
Other liabilities	1,852	1,565
Total liabilities	31,944	28,224
Commitments and contingencies (Note 8)	0	0
Redeemable convertible preferred stock, $0.0001 par value; 0 and 7,375,638 shares authorized shares at December 31, 2020 and 2019, respectively; 0 and 7,375,631 shares issued and outstanding at December 31, 2020 and 2019, respectively. Liquidation value of $0 and $108,596 at December 31, 2020 and 2019, respectively.	0	102,980
Stockholders’ equity (deficit)
Preferred stock, $0.0001 par value, 10,000,000 shares authorized as of December 31, 2020 and no shares authorized as of December 31, 2019; no shares issued and outstanding as of December 31, 2020 and 2019, respectively.	0	0
Common stock, $0.0001 par value, 300,000,000 and 50,000,000 shares authorized at December 31, 2020 and 2019, respectively; 26,681,983 and 5,178,955 shares issued and outstanding at December 31, 2020 and 2019, respectively.	3	1
Additional paid-in-capital	392,063	6,054
Accumulated deficit	(135,679)	(79,025)
Total stockholders’ equity (deficit)	256,387	(72,970)
Total liabilities, redeemable convertible preferred stock and stockholders’ equity (deficit)	$ 288,331	$ 58,234

Balance Sheets (Parenthetical)

Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Statement Of Financial Position [Abstract]
Due from related parties, current	$ 169	$ 149
Due to related parties, current	0	1,122
Due to related parties, noncurrent	$ 0	$ 4,015
Temporary equity, Par Value	$ 0.0001	$ 0.0001
Temporary equity, shares authorized	0	7,375,638
Temporary equity, shares issued	0	7,375,631
Temporary equity, shares outstanding	0	7,375,631
Temporary equity, liquidation preference	$ 0	$ 108,596
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	0
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	300,000,000	50,000,000
Common stock, shares, issued	26,681,983	5,178,955
Common stock, shares, outstanding	26,681,983	5,178,955

Statements of Operations and Co

Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Revenue:
Collaboration and license revenue	$ 13,363	$ 6,960
Collaboration and license revenue, related parties	249	26
Total revenue	13,612	6,986
Operating expenses:
Research and development (includes $579 and $350 for the years ended December 31, 2020 and 2019, respectively, attributable to related parties)	53,038	38,718
Acquired in-process research and development, related parties	0	5,137
General and administrative	17,238	13,895
Total operating expenses	70,276	57,750
Loss from operations	(56,664)	(50,764)
Other income (expense):
Interest income	152	1,504
Other income (expense), net	(181)	(46)
Total other income (expense)	(29)	1,458
Net loss and comprehensive loss	$ (56,693)	$ (49,306)
Net loss per share attributable to common stockholders, basic and diluted	$ (8.82)	$ (9.59)
Weighted-average shares outstanding used in computing net loss per share attributable to common stockholders, basic and diluted	6,430,555	5,142,560

Statements of Operations and _2

Statements of Operations and Comprehensive Loss (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Income Statement [Abstract]
Research and development expense, related party	$ 579	$ 350

Statements of Redeemable Conver

Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Cumulative Effect, Period of Adoption, Adjustment	Common Stock	Additional Paid-in Capital	Additional Paid-in CapitalCumulative Effect, Period of Adoption, Adjustment	Accumulated Deficit	Accumulated DeficitCumulative Effect, Period of Adoption, Adjustment
Stockholders’ Equity (Deficit), beginning balances at Dec. 31, 2018	$ (27,587)	$ 307	$ 1	$ 2,438		$ (30,026)	$ 307
Redeemable Convertible Preferred Stock, beginning balances (in shares) at Dec. 31, 2018	7,375,631
Redeemable Convertible Preferred Stock, beginning balances at Dec. 31, 2018	$ 102,980
Stockholders’ Equity (Deficit), beginning balances (in shares) at Dec. 31, 2018			5,126,344
Accounting Standards Update [Extensible List]	ASU 2014-09
Issuance of common stock upon exercise of stock options	$ 75			75
Issuance of common stock upon exercise of stock options (in shares)			52,611
Stock-based compensation	3,541			3,541
Net loss	(49,306)					(49,306)
Stockholders’ Equity (Deficit), ending balances at Dec. 31, 2019	$ (72,970)		$ 1	6,054	$ (39)	(79,025)	$ 39
Redeemable Convertible Preferred Stock, ending balances (in shares) at Dec. 31, 2019	7,375,631
Redeemable Convertible Preferred Stock, ending balances at Dec. 31, 2019	$ 102,980
Stockholders’ Equity (Deficit), ending balances (in shares) at Dec. 31, 2019			5,178,955
Accounting Standards Update [Extensible List]	ASU 2018-07
Issuance of redeemable convertible preferred stock, net of $3,138 of issuance cost	$ 72,468
Issuance of redeemable convertible preferred stock, net of $3,138 of issuance cost (in shares)	4,200,353
Conversion of redeemable convertible preferred stock into common stock	$ 175,448		$ 1	175,447
Conversion of redeemable convertible preferred stock into common stock (in shares)			11,575,984
Conversion of redeemable convertible preferred stock into common stock	$ (175,448)
Conversion of redeemable convertible preferred stock into common stock (in shares)	(11,575,984)
Issuance of common stock upon initial public offering, net of $17,468 issuance cost	$ 204,713		$ 1	204,712
Issuance of common stock upon initial public offering, net of $17,468 issuance cost (in shares)			9,660,000
Issuance of common stock upon exercise of stock options	$ 857			857
Issuance of common stock upon exercise of stock options (in shares)	267,044		267,044
Stock-based compensation	$ 4,984			4,984
Vesting of common stock warrant issued for services	48			48
Net loss	(56,693)					(56,693)
Stockholders’ Equity (Deficit), ending balances at Dec. 31, 2020	$ 256,387		$ 3	$ 392,063		$ (135,679)
Redeemable Convertible Preferred Stock, ending balances (in shares) at Dec. 31, 2020	0
Redeemable Convertible Preferred Stock, ending balances at Dec. 31, 2020	$ 0
Stockholders’ Equity (Deficit), ending balances (in shares) at Dec. 31, 2020			26,681,983

Statements of Redeemable Conv_2

Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)
Issuance of redeemable convertible preferred stock, issuance cost	$ 3,138
Initial Public Offering
Issuance of common stock upon initial public offering, issuance cost	$ 17,468

Statements of Cash Flows

Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Cash flows from operating activities
Net loss	$ (56,693)	$ (49,306)
Adjustments to reconcile net loss to net cash used in operating activities
Stock-based compensation expense	4,984	3,541
Vesting of common stock warrant in return for services	48	0
Change in fair value of derivative liability	21	37
Depreciation and amortization	1,443	1,004
(Gain) loss on disposition of property and equipment	(1)	7
Write-off of public offering costs	0	2,610
In-process research and development acquired and expensed in non-monetary related party transaction	0	5,137
Changes in operating assets and liabilities
Accounts receivable	(508)	146
Prepaid expenses and other current assets	(2,566)	(695)
Other assets	75	(220)
Accounts payable	43	533
Accrued and other liabilities	1,900	3,841
Deferred revenue	345	(3,346)
Net cash used in operating activities	(50,909)	(36,711)
Cash flows from investing activities
Acquisition of property and equipment	(1,000)	(3,203)
Net cash used in investing activities	(1,000)	(3,203)
Cash flows from financing activities
Issuance of redeemable convertible preferred stock, net of issuance costs	72,468	(10)
Issuance of common stock in initial public offering, net of issuance costs	205,658	(2,260)
Issuance of common stock upon exercise of stock options	857	75
Net cash provided by (used in) financing activities	278,983	(2,195)
Net increase (decrease) in cash and cash equivalents	227,074	(42,109)
Cash and cash equivalents, beginning of period	49,652	91,761
Cash and cash equivalents, end of period	276,726	49,652
Supplemental disclosures of non-cash investing and financing information
Conversion of redeemable convertible preferred stock into common stock	175,448	0
Purchases of property and equipment in accounts payable and accrued and other liabilities	466	385
Unpaid offering costs	$ 946	$ 368

The Company

The Company	12 Months Ended
	Dec. 31, 2020
Organization Consolidation And Presentation Of Financial Statements [Abstract]
The Company	1. The Company 4D Molecular Therapeutics, Inc. (the “Company”) was formed as a limited liability company in September 2013 under the name 4D Molecular Therapeutics, LLC. The Company changed its name and converted into a corporation which was incorporated in the state of Delaware in March 2015. The Company is a clinical-stage gene therapy company pioneering the development of product candidates using its targeted and evolved adeno-associated viruses (“AAV”) vectors. Initial Public Offering In December 2020, the Company sold and issued 9,660,000 shares of common stock at a price to the public of $23.00 per share, which included shares sold upon the underwriters’ exercise of their overallotment option to purchase 1,260,000 additional shares. The Company received an aggregate of $204.7 million in net proceeds, after deducting underwriting discounts and commissions and offering costs. Upon the closing of the IPO, all outstanding shares of redeemable convertible preferred stock automatically converted into 11,575,984 shares of common stock. Subsequent to the closing of the IPO, there were no shares of redeemable convertible preferred stock outstanding. Liquidity The Company has incurred significant losses and negative cash flows from operations and had an accumulated deficit of $135.7 million as of December 31, 2020. The Company believes that its cash and cash equivalents as of December 31, 2020 are sufficient for the Company to fund planned operations for at least one year from the issuance date of these financial statements for the year ended December 31, 2020. The Company has historically financed its operations primarily through the sale of equity securities and, to a lesser extent from cash received pursuant to its collaboration and license agreements. To date, none of the Company’s product candidates have been approved for sale and therefore, the Company has not generated any revenue from product sales. Management expects operating losses and negative cash flows from operations to continue for the foreseeable future. The Company currently plans to raise additional funding as required based on the status of its clinical trials and projected cash flows. There can be no assurance that, in the event the Company requires additional financing, such financing will be available at terms acceptable to the Company, if at all. Failure to generate sufficient cash flows from operations, raise additional capital and reduce discretionary spending should additional capital not become available could have a material adverse effect on the Company’s ability to achieve its business objectives.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The accompanying financial statements have been prepared in accordance with United States Generally Accepted Accounting Principles (“U.S. GAAP”). Use of Estimates and Judgements The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and judgements that affect the reported amounts of assets, liabilities, revenue and expenses; and disclosure of contingent assets and liabilities as of the date of the financial statements. Such estimates include the determination of useful lives for property and equipment, the contract term, transaction price and costs of collaboration agreements, as well as estimates of the fair value of common stock (prior to the IPO) , stock options and derivative instruments and income tax uncertainties. Actual results could differ from those estimates. Due to the coronavirus (“COVID-19”) pandemic, there has been uncertainty and disruption in the global economy and financial markets. The Company is not aware of any specific event or circumstance that would require an update to its estimates or judgments or a revision of the carrying value of its assets or liabilities as of December 31, 2020. While there was not a material impact to the Company’s financial statements as of December 31, 2020, these estimates may change, as new events occur and additional information is obtained, as well as other factors related to the COVID-19 pandemic that could result in material impacts to the financial statements in future reporting periods. Segment Information The Company operates and manages its business as one reportable and operating segment. The Company’s chief executive officer, who is the chief operating decision maker, reviews financial information on a company-wide basis for purposes of allocating resources and assessing financial performance. As of and for the years ended December 31, 2020 and 2019, all of the Company’s long-lived assets were located in the United States and all revenue was earned in the United States. Concentration of Credit Risk Financial instruments that potentially subject the Company to a concentration of credit risk consist of cash and cash equivalents and accounts receivable. The Company’s cash is held at two financial institutions in the United States of America. The Company’s cash equivalents are invested in money market funds. The Company has not experienced any losses on its deposits of cash and cash equivalents. Such deposits may, at times, exceed federally insured limits. The Company’s partners in collaboration and license agreements who represent 10% or more of the Company’s total revenue are as follows: Year Ended December 31, 2020 Year Ended December 31, 2019 Customer A 95% 90% Customer B * * Total 95% 90% * Less than 10% The Company’s partners in collaboration and license agreements who represent 10% or more of the Company’s total accounts receivable are as follows: December 31, 2020 December 31, 2019 Customer A 74% 64% Customer B 26% 36% Total 100% 100% The Company’s total revenues by geographic region, based on the location of the customer, are as follows (in thousands): Year Ended December 31, 2020 Year Ended December 31, 2019 Australia $ — $ 7 Netherlands 742 26 Switzerland 12,897 6,287 United States (27 ) 666 $ 13,612 $ 6,986 Cash and Cash Equivalents The Company considers all highly liquid investments with original maturities of three months or less at the date of purchase to be cash equivalents. Cash equivalents consist of money market funds. Other Risks and Uncertainties The Company is subject to risks and uncertainties common to early-stage companies in the biotechnology industry, including, but not limited to, development by competitors of new technological innovations, protection of proprietary technology, dependence on key personnel, suppliers for key raw materials, contract manufacturing organizations (“CMOs”) and contract research organizations (“CROs”), compliance with government regulations 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 preclinical studies, clinical trials and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel and infrastructure and extensive compliance and reporting. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be obtained or maintained, that any products developed will obtain necessary government regulatory approval or that any approved products will be commercially viable. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will generate significant revenue from product sales. The Company operates in an environment of rapid change in technology and substantial competition from other pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees, consultants and other third parties (including for clinical trials and some aspects of research and preclinical testing). The extent of the impact of the COVID-19 pandemic on the Company’s business will depend upon the duration and spread of the outbreak and the extent and severity of the impact on the Company’s clinical trial activities, research activities and suppliers, all of which are uncertain and cannot be predicted. The extent to which the coronavirus outbreak may materially impact the Company’s financial condition, liquidity or results of operations is uncertain. Fair Value Measurements The Company applies fair value accounting for all financial assets and liabilities and non-financial assets and liabilities that are recognized or disclosed at fair value in the financial statements on a recurring basis. Fair value is an exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or liability. As a basis for considering such assumptions, a three-level fair value hierarchy has been established, which prioritizes the inputs used in measuring fair value as follows: • Level 1 —Observable inputs, such as quoted prices in active markets for identical assets and liabilities. • Level 2 —Observable inputs other than Level 1 that are observable, either directly or indirectly, such as quoted prices for similar assets or liabilities, quoted prices in markets that are not active, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. • Level 3 —Unobservable inputs that are significant to the measurement of the fair value of the assets or liabilities that are supported by little or no market data. The Company accounts for transfers of financial instruments between levels of the fair value hierarchy on the date of the event or change in circumstance that caused the transfer. Accounts Receivable—Allowance for Doubtful Accounts The Company regularly reviews accounts receivable for collectability and establishes an allowance for probable credit losses and writes off uncollectible accounts as necessary. The Company has determined that no allowance was required at December 31, 2020 and 2019. The Company did not have any write-offs relating to uncollectible accounts receivable during the years ended December 31, 2020 and 2019. Property and Equipment, net Property and equipment are stated at cost less accumulated depreciation for acquired assets. Depreciation is computed using the straight-line method over the estimated useful lives of assets, ranging from three to five years. Leasehold improvements are amortized over the shorter of the useful life of the assets or the lease term. Upon sale or retirement of assets, the costs and related accumulated depreciation are removed from the balance sheet and the resulting gain or loss is reflected within operating expenses in the statements of operations and comprehensive loss. Maintenance and repairs are charged to expense as incurred. Impairment of Long-Lived Assets The Company reviews long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Recoverability is measured by comparison of the carrying amount to the future undiscounted net cash flows, which the assets are expected to generate. If such assets are considered to be impaired, the impairment to be recognized is typically measured by the amount by which the carrying amount of the assets exceeds the projected discounted future net cash flows arising from the asset. There have been no such impairments of long-lived assets in the years ended December 31, 2020 and 2019. Redeemable Convertible Preferred Stock The Company recorded all shares of redeemable convertible preferred stock at their respective fair values on the dates of issuance, net of issuance costs. The redeemable convertible preferred stock was recorded outside of permanent equity because while it was not mandatorily redeemable, in certain events considered not solely within the Company’s control, such as a merger or consolidation, sale, lease, or license of substantially all of the Company’s assets (each, a “deemed liquidation event”), the convertible preferred stock would become redeemable at the option of the holders of a majority of the outstanding series of redeemable convertible preferred stock. The Company did not adjust the carrying values of the redeemable convertible preferred stock to the liquidation preference of such shares because a deemed liquidation event obligating the Company to pay the liquidation preference did not occur. All outstanding shares of the redeemable convertible preferred stock converted into common stock upon the closing of the IPO in December 2020 . Common Stock Warrants The Company accounts for common stock warrants which meet the definition of a derivative as liabilities if the warrant requires net cash settlement or gives the holder the option of net cash settlement. The Company accounts for common stock warrants as equity if the contract requires physical settlement or net physical settlement or if the Company has the option of physical settlement or net physical settlement. Common stock warrants classified as liabilities are initially recorded at fair value and remeasured at fair value each balance sheet date with the offset adjustments recorded in other income (expense), net within the statements of operations and comprehensive loss. Common stock warrants classified as equity are initially measured at fair value on the grant date and are not subsequently remeasured. Revenue Recognition Effective January 1, 2019, the Company adopted Accounting Standards Update (“ASU”) 2014-09, Revenue from Contracts with Customers (Topic 606) The Company’s revenue is primarily derived through its license, research, development and commercialization agreements. The terms of these types of agreements may include (i) licenses to the Company’s technology, (ii) research and development services, and (iii) services or obligations in connection with participation in research or steering committees. Payments to the Company under these arrangements typically include one or more of the following: nonrefundable upfront and license fees, research funding, milestone and other contingent payments to the Company for the achievement of defined collaboration objectives and certain preclinical, clinical, regulatory and sales-based events, as well as royalties on sales of any commercialized products. Arrangements that include upfront payments are recorded as deferred revenue upon receipt or when due and are recognized as revenue as performance conditions are met. The event-based milestone payments, royalties and cost reimbursements represent variable consideration, and the Company uses the most likely amount method to estimate this variable consideration. Royalty payments are recognized when earned or as the sales occur. The Company records cost reimbursements as accounts receivable when right to consideration is unconditional. A performance obligation is a promise in a contract to transfer a distinct good or service to the customer and is the unit of account in ASC 606. The Company allocates the total transaction price to each performance obligation based on the estimated standalone selling price and recognizes revenue when, or as, the performance obligation is satisfied. The Company includes the unconstrained amount of estimated variable consideration in the transaction price. At the end of each reporting period, the Company re-evaluates the estimated variable consideration included in the transaction price and any related constraint, and if necessary, adjusts its estimate of the overall transaction price. Significant management judgment is required to determine the level of effort required under an arrangement and the period over which the Company expects to complete its performance obligations under the arrangement. Changes in these estimates can have a material effect on revenue recognized. Research and Development Expenses Costs related to research, design and development of programs are charged to research and development expense as incurred. Research and development costs include, but are not limited to, payroll and personnel expenses including stock-based compensation, materials, laboratory supplies, outside services and allocated overhead, including rent, insurance, repairs and maintenance, depreciation and utilities. The Company expenses all research and development costs in the period in which they are incurred. Costs incurred in obtaining technology licenses are charged to research and development expense as acquired in-process research and development if the technology licensed has not reached technological feasibility and has no alternative future use. Accrued Research and Development The Company has entered into various agreements with CROs and CMOs. The Company’s research and development accruals are estimated based on the level of services performed, progress of the studies, including the phase or completion of events, and contracted costs. The estimated costs of research and development provided, but not yet invoiced, are included in accrued and other current liabilities on the balance sheet. If the actual timing of the performance of services or the level of effort varies from the original estimates, the Company will adjust the accrual accordingly. Payments made to CROs or CMOs under these arrangements in advance of the performance of the related services are recorded as prepaid expenses and other current assets until the services are rendered. Stock-Based Compensation As of January 1, 2020, the Company adopted ASU 2018-07, Compensation - Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting Prior to the adoption of ASU 2018-07 on January 1, 2020, stock-based compensation expense related to stock options granted to nonemployees was recognized based on the vesting date fair value of options as the stock options were earned. The Company remeasured the stock-based compensation at each reporting period end with the resulting change in fair value being recognized in the statements of operations and comprehensive loss over the period the related services were rendered. The Company estimated the service period for the options based on the time that would be required to satisfy the service condition, assuming the service condition would be satisfied. Stock-based compensation expense was recognized over the estimated service period but was accelerated if the performance condition was achieved earlier than estimated. Income Taxes The Company accounts for income taxes under the asset and liability method, which requires, among other things, that deferred income taxes be provided for temporary differences between the tax basis of the Company’s assets and liabilities and their financial statement reported amounts. In addition, deferred tax assets are recorded for the future benefit of utilizing net operating losses and research and development credit carryforwards and are measured using the enacted tax rates and laws that will be in effect when such items are expected to reverse. A valuation allowance is provided against deferred tax assets unless it is more likely than not that they will be realized. The Company accounts for uncertain tax positions by assessing all material positions taken in any assessment or challenge by relevant taxing authorities. Assessing an uncertain tax position begins with the initial determination of the position’s sustainability and is measured at the largest amount of benefit that is greater than fifty percent likely of being realized upon ultimate settlement. The Company’s policy is to recognize interest and penalties related to the underpayment of income taxes as a component of income tax expense or benefit. To date, there have been no interest or penalties charged in relation to the unrecognized tax benefits. Embedded Derivative Embedded derivatives that are required to be bifurcated from the underlying host instrument are accounted for and valued as a separate financial instrument. An embedded derivative exists in the award agreement with the Cystic Fibrosis Foundation (“CFF”). As described in Note 15, the embedded derivative has been bifurcated and is classified as a liability on the balance sheet and separately accounted for at its fair value. The derivative liability is subject to remeasurement to fair value each reporting period. Changes in the fair value of the derivative liability are recognized as a component of other income (expense), net within the statements of operations and comprehensive loss. Deferred Offering Costs The Company capitalizes certain legal, accounting and other third-party fees that are directly related to the Company’s in-process financings, until such financings are consummated. After consummation of the financing, these costs are recorded as a reduction of the proceeds received as a result of the offering. In the event that a planned offering does not occur or is significantly delayed, all related deferred offering costs will be expensed immediately within the Company’s statements of operations and comprehensive loss. Net Loss Per Share Attributable to Common Stockholders The Company calculates basic and diluted net loss per share to common stockholders in conformity with the two-class method required for companies with participating securities. The Company considers all series of redeemable convertible preferred stock to be participating securities as the holders are entitled to receive non-cumulative dividends on a pari passu basis in the event the dividend is paid on common shares. Under the two-class method, the net loss attributable to common stockholders is not allocated to the redeemable convertible preferred stock as the holders of redeemable convertible preferred stock do not have a contractual obligation to share in losses. Basic net loss per share attributable to common stockholders is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of shares of common stock outstanding during the period, less shares subject to repurchase. Diluted net loss per share attributable to common stockholders is computed by giving effect to all potentially dilutive common shares outstanding for the period. For purposes of this calculation, redeemable convertible preferred shares, stock options to acquire shares of common stock, common stock warrants, and unvested common stock subject to repurchase, are considered potentially dilutive common shares, but have been excluded from the calculation of diluted net loss per share attributable to common stockholders as their effect is antidilutive. Recent Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (“FASB”) issued ASC 606. This standard outlines a single comprehensive model for entities to use in accounting for revenue arising from contracts with customers and supersedes most current revenue recognition guidance, including industry-specific guidance. Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. The Company adopted ASC 606 effective January 1, 2019 using the modified retrospective method only to contracts not completed as of this date. The Company recognized the cumulative effect of initially applying ASC 606 as an adjustment to the balance of accumulated deficit at January 1, 2019 with a cumulative effect adjustment of $ 0.3 million reflected as a decrease to the opening balance of accumulated deficit and a decrease to deferred revenue. In June 2018, FASB issued ASU 2018-07, Compensation—Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820), Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement In July 2017, the FASB issued ASU 2017-11, Earnings Per Share (Topic 260) Distinguishing Liabilities from Equity (Topic 480) Derivatives and Hedging (Topic 815) (Part I) Accounting for Certain Financial Instruments with Down Round Features, (Part II) Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Noncontrolling Interests with a Scope Exception New Accounting Pronouncements Not Yet Adopted As an “emerging growth company,” the Jumpstart Our Business Startups Act (“JOBS Act”) allows the Company to delay adoption of new or revised accounting pronouncements applicable to public companies until such pronouncements are made applicable to private companies. The Company has elected to use this extended transition period under the JOBS Act. The adoption dates discussed below reflects this election. In November 2018, the FASB issued ASU 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction Between Topic 808 and Topic 606 Revenue from Contracts with Customers Revenue from Contracts with Customers In August 2018, the FASB issued ASU 2018-15, Intangibles—Goodwill and Other—Internal-Use Software (Subtopic 350-40), Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) (“ASC 842”) Codification Improvements to Topic 842, Leases Leases Leases (Topic 842): Targeted Improvements,

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. Fair Value Measurements The following tables represent the Company’s fair value hierarchy for financial assets and financial liabilities measured at fair value on a recurring basis as of December 31, 2020 and 2019 (in thousands): Basis for Fair Value Measurements Fair Value as of (Level 1) (Level 2) (Level 3) December 31, 2020 Assets Money market funds $ 276,726 $ — $ — $ 276,726 Total $ 276,726 $ — $ — $ 276,726 Liabilities Derivative liability $ — $ — $ 122 $ 122 Total $ — $ — $ 122 $ 122 Basis for Fair Value Measurements Fair Value as of (Level 1) (Level 2) (Level 3) December 31, 2019 Assets Money market funds $ 15,876 $ — $ — $ 15,876 Total $ 15,876 $ — $ — $ 15,876 Liabilities Derivative liability $ — $ — $ 101 $ 101 Total $ — $ — $ 101 $ 101 Level 3 Inputs The fair value of the derivative liability is based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy. The fair value of the derivative liability was determined using a present value analysis with multiple scenarios. In determining the fair value of the derivative liability, the inputs impacting fair value include the change of control payment to CFF, the probability of a change of control event, the product status at time of a change of control event and the discount rate. See Note 15 for further discussion on embedded derivative. There were no transfers between Level 1, 2 and 3 during the periods presented. The following table sets forth a summary of the changes in the fair value of the Company’s Level 3 financial instruments (in thousands): Derivative Liability Balance as of December 31, 2018 $ 64 Change in fair value included in other income (expense), net 37 Balance as of December 31, 2019 $ 101 Change in fair value included in other income (expense), net 21 Balance as of December 31, 2020 $ 122

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2020
Property Plant And Equipment [Abstract]
Property and Equipment, Net	4. Property and Equipment, Net Property and equipment, net, consisted of the following (in thousands): December 31, 2020 December 31, 2019 Machinery and equipment $ 4,911 $ 3,761 Leasehold improvements 2,527 2,405 Furniture and fixtures 473 541 Office equipment 101 98 Computer equipment and software 366 306 Construction in progress 539 563 Total property and equipment 8,917 7,674 Less: Accumulated depreciation and amortization (3,844 ) (2,625 ) Property and equipment, net $ 5,073 $ 5,049 All property and equipment are maintained in the United States. Depreciation expense was $1.4 million and $1.0 million for the years ended December 31, 2020 and 2019, respectively.

Accrued and Other Liabilities,

Accrued and Other Liabilities, Current and Noncurrent	12 Months Ended
	Dec. 31, 2020
Payables And Accruals [Abstract]
Accrued and Other Liabilities, Current and Noncurrent	5. Accrued and Other Liabilities, Current and Noncurrent Accrued liabilities consisted of the following (in thousands): December 31, 2020 December 31, 2019 Payroll and related $ 3,437 $ 2,133 Accrued clinical and preclinical study costs 869 494 Consulting and professional 3,574 2,239 Other accrued expenses 491 481 Total accrued and other current liabilities $ 8,371 $ 5,347 Other liabilities, noncurrent consisted of the following (in thousands): December 31, 2020 December 31, 2019 Deferred rent, noncurrent $ 1,237 $ 1,023 Other payable, noncurrent 615 542 Total other liabilities, noncurrent $ 1,852 $ 1,565

Research and Collaboration Arra

Research and Collaboration Arrangements	12 Months Ended
	Dec. 31, 2020
Research And Collaboration Arrangements [Abstract]
Research and Collaboration Arrangements	6. Research and Collaboration Arrangements Collaboration and license revenue for each period was as follows (in thousands): Year Ended December 31, 2020 2019 uniQure $ 742 $ 26 Benitec — 7 CRF — — Roche 12,897 6,287 CFF (27 ) 118 AstraZeneca — 548 $ 13,612 $ 6,986 Deferred revenue for each period was as follows (in thousands): Deferred Revenue As of December 31, 2020 As of December 31, 2019 uniQure $ 4,396 $ 5,137 Benitec — — CRF — — Roche 14,318 13,640 CFF 1,098 690 AstraZeneca — — $ 19,812 $ 19,467 The total amount of revenue in the year ended December 31, 2020, which was included in deferred revenue at January 1, 2020, was $3.2 million. uniQure In January 2014, the Company and uniQure biopharma B.V. (“uniQure”) entered into a Collaboration and License Agreement (the “uniQure Agreement”) to collaborate on the discovery and non-clinical research activities related to the Company’s Therapeutic Vector Evolution platform in order to generate and validate vectors for gene delivery to treat diseases within the central nervous system and liver (together, the “uniQure Field”). The uniQure Agreement provided uniQure with a research license as well as an exclusive development and commercialization license for each project variant selected for further development. The initial research term is three years with an option for uniQure to extend the research term one time for an additional year. Once the Company’s research plan has concluded, uniQure is solely responsible for the continued development, manufacturing and commercialization of the project variants as potential product candidates. In October 2016, uniQure exercised its option to extend the research term for an additional year to January 2018. The Company was also required to work exclusively with uniQure in the uniQure Field (the “uniQure Exclusivity Clause”). Pursuant to the uniQure Agreement, the Company received upfront payments of $0.2 million, and was entitled to receive (i) contingent payments for the achievement of research and development milestones of up to $5.0 million for each licensed product selected under the arrangement, and (ii) royalties in the single digit range on future sales of the potential product candidates and sublicense consideration in the low teens to low thirties range on any future sublicensing arrangements. The Company also received capped research and development service fees based on contractual full-time employee rates per year. In connection with the performance obligations under the uniQure Agreement, the founders of 4D Molecular Therapeutics, LLC received equity options to purchase an aggregate of 609,744 of uniQure ordinary shares that vest over the initial three-year term of the agreement. The upfront payment of $0.2 million was recorded as deferred revenue and was recognized on a ratable basis over the estimated performance period of four years. Payments and reimbursements for research costs were recognized on an as-incurred basis. The options to purchase uniQure shares were deemed to be a noncash component of the arrangement consideration, as the vesting of options is linked to the uniQure Agreement and there is a requirement for the holders of the options to provide services under the agreement. The fair value of the uniQure options, which was estimated to be $10.6 million, was recognized ratably as revenue over the estimated performance period of four years and the associated compensation expense related to the stock options were recorded as research and development expense. In August 2019, the Company and uniQure entered into an Amended and Restated Collaboration and License Agreement (the “Amended uniQure Agreement”), which amended and restated the uniQure Agreement, and a separate Collaboration and License Agreement (the “Second uniQure Agreement”). Under these agreements, the Company agreed to transfer incremental rights and services to uniQure in exchange for uniQure eliminating the uniQure Exclusivity Clause and transferring other rights back to the Company. Under the Amended uniQure Agreement, uniQure continues to have an exclusive license to select AAV capsid variants (the “Selected Variants”) in the uniQure Field. uniQure continues to be solely responsible, at its cost, to develop and commercialize the compounds and products containing the Selected Variants. The amended uniQure Agreement eliminated the uniQure Exclusivity Clause in the uniQure Agreement. Furthermore, the contingent payments that the Company was entitled to from uniQure for the achievement of research and development milestones of up to $5.0 million for each licensed product selected under the uniQure Agreement were eliminated and sublicense consideration on any future sublicensing arrangements was reduced from the low teens to low thirties percentages to mid-single digit to mid-twenties percentages. Under the Second uniQure Agreement, the parties agreed to research and develop new AAV capsid variants (the “New Variants”) that are not Selected Variants that affect certain targets selected by uniQure (the “uniQure Targets”) in the uniQure Field. The Company is solely responsible, at its cost, for the research of the New Variants. The Company granted uniQure an exclusive license to a certain number of the New Variants (the “uniQure New Variants”) that affect the uniQure Targets. uniQure is solely responsible, at its cost, to develop and commercialize the compounds and products containing the uniQure New Variants that affect the uniQure Targets (the “Licensed Products”). The Company retains all rights to New Variants in the uniQure Field that affect targets other than the uniQure Targets. Under both the Amended uniQure Agreement and the Second uniQure Agreement, uniQure will be required to pay the Company royalties on worldwide annual net sales of Licensed Products at a mid-single digit percentage rate, subject to certain specified reductions. uniQure will also be required to pay the Company sublicensing consideration for sublicensing the Company’s intellectual property rights licensed under the Amended uniQure Agreement or the Second uniQure Agreement to third parties at a rate between the mid-single digit to mid-twenties. The Company has reciprocal obligations, at the same percentage rates as uniQure, to pay uniQure royalties and sublicensing consideration for sublicensing certain intellectual property rights licensed under the Amended uniQure Agreement or the Second uniQure Agreement to third parties. The Company concluded that the Amended uniQure Agreement and the Second uniQure Agreement should be accounted for as one combined contract that should be accounted for as a separate contract from the uniQure Agreement given that the incremental licensed intellectual property rights and research and development services are distinct from the rights and services previously transferred to uniQure under the uniQure Agreement and the transaction price increased by an amount that equals the standalone selling price of the incremental rights and services to be transferred to uniQure under the Amended uniQure Agreement and Second uniQure Agreement. Neither party was required to pay monetary consideration in connection with the execution of the Amended uniQure Agreement or the Second uniQure Agreement or for subsequent performance by the parties under those agreements, notwithstanding the potential future royalty and sublicense consideration described above. The fair value of the non-monetary consideration given by uniQure to the Company, for the intellectual property right is $5.1 million. This intellectual property right is considered to be an in-process research and development asset with no alternative future use and, accordingly, was written off as acquired in-process research and development expense in the year ended December 31, 2019. The incremental transaction price described in the paragraph above was recorded as deferred revenue given that the Company identified one single combined performance obligation under ASC 606, which includes the licenses to the New Variants, research services and participation in the joint steering committee (“JSC”). Revenue is being recognized using the input method based on actual costs incurred as a percentage of total budgeted costs as the Company completes its performance obligation. Based on the current estimated timelines, the deferred revenue is expected to be recognized as revenue over approximately two to three years from December 31, 2020. The Company determined the transaction price using the risk adjusted net present value analysis (“rNPV”) methodology to value the elimination of the uniQure exclusivity clause and other material rights received by the Company, including the potential royalties the Company would receive from uniQure. The rNPVs incorporate estimates and assumptions including the number of products the Company and uniQure would develop, the risk-adjusted probability of successfully developing a biopharmaceutical product, the probability that uniQure will develop a product, the research and development costs, the potential worldwide sales and associated commercialization costs, corporate tax rate, and discount rate. During the years ended December 31, 2020 and 2019 the Company recognized revenue of $0 and less than $0.1 million under the uniQure Agreement, respectively. During the years ended December 31, 2020 and 2019 the Company recognized revenue of $0.7 million and $0 under the Amended uniQure Agreement and the Second uniQure Agreement, respectively. As of December 31, 2020 and 2019, deferred revenue relating to uniQure was $4.4 million and $5.1 million, respectively. There were no amounts due from uniQure under the uniQure Agreement, Amended uniQure Agreement or Second uniQure Agreement as of December 31, 2020 and 2019. As of December 31, 2020 and 2019, the aggregate amount of the transaction price allocated to the remaining performance obligation was $4.4 million and $5.1 million, respectively. Benitec In November 2014, the Company and Benitec Biopharma Limited (“Benitec”) entered into a collaboration and license agreement to collaborate on the discovery and non-clinical research activities related to the Company’s Therapeutic Vector Evolution platform in order to generate and validate vectors for gene delivery to treat certain ophthalmic diseases (the “Benitec Agreement”). Benitec had the option of nominating up to three project variants as part of the Benitec Agreement. The Benitec Agreement provided Benitec with a temporary research license as well as an exclusive development and commercialization license for each project variant selected to further develop. The initial research term was two years and was automatically extended in six-month Pursuant to the Benitec Agreement, the Company received as consideration (i) an upfront payment of $0.5 million, (ii) capped research and development service fees based in part on prescribed full-time equivalent labor rates and (iii) reimbursements of pass-through and overhead costs incurred on behalf of Benitec . On January 24, 2017, the Benitec Agreement was amended to give Benitec sole responsibility for the performance of certain research work which would have generated research services revenue for the Company under the original agreement. Pursuant to the amendment, the Company received $0.5 million as consideration. This $0.5 million was recorded as deferred revenue and was recognized over the same period as the upfront payment. In March 2019, the Benitec Agreement was terminated based on mutual agreement between the Company and Benitec. The Company identified one combined performance obligation to provide the research license, exclusive development and commercialization licenses for each project variance selected to further develop, research services and participation in the JSC. The transaction price included the $1.0 million non-refundable upfront fees and $2.4 million reimbursement for costs incurred and the value of labor hours expended. The Company excluded any consideration related to sales-based milestones, including royalties, which are recognized when the related sales occur. For the year ended December 31, 2019, there was no change in the transaction price. During the years ended December 31, 2020 and 2019, the Company recognized revenue of $0 and less than $0.1 million, respectively. As of December 31, 2020 and 2019, deferred revenue relating to the Benitec Agreement was $0. There were no amounts due from Benitec under the Benitec Agreement as of December 31, 2020 and 2019. Upon termination of the Benitec Agreement in March 2019, the Company had no further obligations impacting revenue recognition. CRF In November 2015, the Company entered into a research funding and collaboration agreement (the “CRF Agreement”) with the Choroideremia Research Foundation (“CRF”), a non-profit organization dedicated to finding a cure for choroideremia, a rare inherited disorder that causes progressive vision loss, ultimately leading to complete blindness. The goal of the CRF Agreement is for CRF to contribute funding to help with the advancement of the Company’s choroideremia research program. The Company is responsible for all decision making and execution of any and all of the related activities to be completed in its sole discretion. The initial term of the CRF Research Plan is two years. The agreement includes contribution to CRF of up to $2.5 million upon certain development or approval milestones. The overall arrangement has automatic extensions of up to three additional years. As of December 31, 2020, no milestones have been achieved. Revenue was fully recognized for this agreement in the year ended December 31, 2017. There was no deferred revenue relating to the CRF Agreement as of December 31, 2020 and 2019. No amount was due from CRF under the CRF Agreement as of December 31, 2020 and 2019. Roche In November 2017, the Company entered into a collaboration and license agreement with F. Hoffmann-La Roche Ltd and Hoffmann-La Roche Inc. (together, “Roche”) to discover and develop products containing optimized next generation AAV Vectors focused on ophthalmological diseases and disorders excluding select criteria (the “2017 Roche Agreement”). The Company and Roche both have the ability to nominate products to discover, develop and commercialize. At the effective date, choroideremia was designated a Roche product. The Company is responsible for conducting research and development services prior to pivotal clinical studies, and Roche is responsible for conducting subsequent development and commercialization activities. In addition, Roche agreed to pay for research and development services at the agreed upon full-time employee rate for work performed for choroideremia under the 2017 Roche Agreement, except for the costs associated with the manufacturing work for choroideremia. For any product that the Company nominates and conducts research and development services under the 2017 Roche Agreement prior to pivotal clinical studies, including 4D-125 (for the treatment of XLRP) , Roche has an option to convert the status of the product to a Roche product during the 90-day option period. If Roche chooses to not exercise its option, the Company can continue subsequent development and commercialization activities and Roche will have no further rights with respect to such product. Pursuant to the 2017 Roche Agreement, the Company received an upfront payment of $21.0 million as consideration. In addition, the Company is entitled to contingent payments including (i) $1.0 million for each Roche nominated product beyond the first three, (ii) up to $30.0 million upon exercise of the option to convert a product the Company nominated and developed prior to pivotal clinical studies (iii) development milestone payments of up to $223.0 million, of which $86.0 million relates to choroideremia and the rest relate to other licensed products; and (iv) sales-based milestones of up to $123.0 million in connection with licensed products. The 2017 Roche Agreement also includes provisions that entitle the Company to receive royalty payments ranging from the mid-single digits to the mid-teens for the net sales of the licensed products, in each case subject to the reductions in accordance with the terms of the agreement. Under ASC 606, the Company identified a single combined performance obligation for the license, research services and participation in the JSC. Furthermore, the Company concluded that at the inception of the agreement, Roche’s option, exercisable prior to pivotal clinical study initiation, does not represent a material right and should be allocated to the single performance obligation and recognized as revenue upon Roche’s exercise of the option. The transaction price related to the agreement upon adoption of ASC 606 included the $21.0 million non-refundable upfront fee and $10.7 million for estimated reimbursements for research and development services at the agreed upon full-time employee rate and third party costs. The Company’s contract with Roche does not include a significant financing component. The Company concluded that the transaction price should not include the variable consideration related to development milestones as they were considered to be constrained as it is probable that the inclusion of such variable consideration could result in a significant reversal of cumulative revenue in the future. The Company excluded any consideration related to sales-based milestones, including royalties, which are recognized when the related sales occur. The transaction price and estimated period of performance will be re-evaluated at each reporting period. For the year ended December 31, 2020, an adjustment of $17.0 million was made to the transaction price to reflect an increase of $7.0 million in the scope of the project and expected reimbursable costs and the addition of $10.0 million of variable consideration as the uncertainty associated with two development milestones was resolved. The adjustments to the transaction price and total budgeted costs in 2020 resulted in a $4.6 million increase in revenue recognized in the year ended December 31, 2020 related to performance obligations partially satisfied in periods prior to January 1, 2020. For the year ended December 31, 2019, an adjustment of $4.5 million was made to the transaction price to reflect an increase in the scope of the project and expected reimbursable costs. The increase in the transaction price and total budgeted costs in 2019 resulted in a $1.6 million decrease in revenue recognized in the year ended December 31, 2019 related to performance obligations partially satisfied in periods prior to January 1, 2019. During the years ended December 31, 2020 and 2019, the Company recognized revenue of $12.9 million and $6.3 million, respectively. As of December 31, 2020 and 2019, deferred revenue relating to the Roche Agreement was $14.3 million and $13.6 million, respectively. Accounts receivable from Roche under this agreement as of December 31, 2020 and 2019 was $1.1 million and $0.6 million, respectively. As of December 31, 2020 and 2019, the aggregate amount of the transaction price allocated to the remaining performance obligation was $25.8 million and $21.6 million, respectively. Based on current timelines, the deferred revenue is expected to be recognized as revenue over the next three to four years as the Company continues to develop nominated products until the initiation of pivotal studies. CFF In September 2016, the Company entered into an award agreement for the Optimized Adeno-Associated Virus for Lung Epithelia Gene Delivery Development Program with CFF, a non-profit organization dedicated to finding a cure for cystic fibrosis, an inherited disorder that causes disease in the pulmonary airways leading to morbidity and mortality. Under this agreement, CFF contributes funding to help advance the Company’s CF research program. The agreement was subsequently amended in September 2017 and August 2018 (all three agreements are collectively referred to as the “CFF Agreement”). The total amount of the award under the CFF Agreement is $3.5 million. As of December 31, 2020 and 2019, the Company achieved milestones totaling $1.3 million and $0.9 million under the CFF Agreement, respectively. The remaining award amount will be paid by CFF based on achievement of certain development milestones by the Company. The Company expects to make payments to CFF equal to six times three times six times To date, the Company has not developed a commercial product in connection with the CFF Agreement, and it has not licensed, sold or otherwise transferred to another party the product developed under the CFF Agreement or the underlying technology. If at any time prior to the first commercial sale of a product developed as a result of the CFF Agreement, the Company ceases to use commercially reasonable efforts to develop or commercialize any product under the CFF Agreement for a continuous period of 180 consecutive days and fails to present a reasonable plan to resume commercially reasonable efforts, the Company will grant to CFF an irrevocable, exclusive worldwide interruption license under all of the Company’s interest in the research plan technology to exploit such product. Any third-party license granted by the Company shall be subject to such interruption license. Under ASC 606, the Company identified one performance obligation within the CFF grant agreement for research activities. The Company’s contract with CFF does not include a significant financing component. The Company concluded that the transaction price should not include the variable consideration related to future research milestones as they were considered to be constrained as it is probable that the inclusion of such variable consideration could result in a significant reversal of cumulative revenue in the future. The Company re-evaluates the transaction price and estimated period of performance at each reporting period. For the year ended December 31, 2020, an adjustment of $0.4 million was made to the transaction price to reflect the achievement of the third milestone related to in-vitro screenings under the CFF Agreement. During the years ended December 31, 2020 and 2019, the Company recognized revenue of less than $(0.1) million and $0.1 million, respectively. As of December 31, 2020 and 2019, deferred revenue relating to the CFF Agreement was $1.1 million and $0.7 million, respectively. Accounts receivable from CFF under the CFF Agreement as of both December 31, 2020 and 2019 was $0.4 million. The obligation to make payments to CFF upon a change of control meets the definition of an embedded derivative that is required to be bifurcated and separately accounted for as a derivative liability. The Company determined the estimated fair value of this derivative liability to be $0.1 AstraZeneca In December 2017, the Company entered into a collaboration and option agreement with MedImmune, Inc., the global biologics research and development arm of AstraZeneca (“AstraZeneca”) to discover and develop optimized AAV vectors to treat specific lung disease indications (the “AstraZeneca The initial research term was approximately twelve months with AstraZeneca’s Pursuant to the AstraZeneca Agreement, the Company received an upfront payment of $1.5 million as consideration. The Company identified a single combined performance obligation within the AstraZeneca agreement for the research program license, research and development activities and participation in the joint project team and JSC. The Company concluded that these activities are not distinct and, therefore, should be combined into a single combined performance obligation. Furthermore, the Company concluded that at the inception of the agreement, AstraZeneca’s license option, did not represent a material right and should be allocated to the single performance obligation and recognized as revenue upon AstraZeneca’s exercise of the option. The transaction price related to the agreement consists of the $1.5 million non-refundable upfront fee. The Company concluded that the transaction price should not include the variable consideration related to developmental milestones as they were considered to be constrained as it is probable that the inclusion of such variable consideration could result in a significant reversal of cumulative revenue in the future. The Company excluded any consideration related to sales-based milestones, including royalties, which are recognized when the related sales occur. The Company re-evaluates the transaction price and estimated period of performance at each reporting period. The Company’s contract with AstraZeneca does not include a significant financing component. Under ASC 606, the Company used the input method to measure progress toward completion of the performance obligation and concluded that revenue will be recognized based on actual resources consumed, labor hours expended and costs incurred as a percentage of total budgeted costs. In June 2019, the research phase concluded and the Company delivered its final report to AstraZeneca. In June 2020, AstraZeneca’s option to obtain the license of up to three project vector variants under the AstraZeneca Agreement expired unexercised. During the years ended December 31, 2020 and 2019, the Company recognized revenue of $0 and $0.5 million, respectively. As of December 31, 2020 and 2019, deferred revenue relating to the AstraZeneca Agreement was $0. No amount was due from AstraZeneca under this agreement as of December 31, 2020 and 2019.

License Arrangements

License Arrangements	12 Months Ended
	Dec. 31, 2020
License Arrangements [Abstract]
License Arrangements	7. License Arrangements The Company has exclusive, worldwide license agreements (the “UC Agreements”) with the Regents of the University of California (the “UC Regents”) relating to the use of certain patents and intellectual property surrounding its core technologies, including Therapeutic Vector Evolution. Pursuant to each of the UC Agreements executed prior to January 2019, the Company was obligated to pay a (i) non-refundable license fee of $5,000 upon execution, (ii) a non-refundable license fee of $5,000 each year thereafter, until sales of a licensed product are made and royalties are paid to the UC Regents, (iii) reimbursement of domestic and foreign patent filing, prosecution and maintenance fees, and (iv) either $50,000 or issuance of a 3% equity interest in the Company upon the closing of the first qualified financing at the option of the UC Regents. The Company’s first qualified financing occurred in 2015 and at the election of the UC Regents, the Company issued the UC Regents in January 2016 an amount of common stock equal to 6% of the equity interests in the Company pursuant to the applicable clause in each of the UC Agreements. Pursuant to an agreement with the UC Regents executed in January 2019 the Company paid a non-refundable license fee of $50,000 to the UC Regents upon execution of the agreement. The Company is obligated to pay a non-refundable license fee of $5,000 on the one-year anniversary of the contract effective date and each year thereafter, until sales of a licensed product are made and royalties are paid to the UC Regents. In addition, the Company is obligated to make certain contingent payments including (i) development milestones up to $3.1 million, (ii) low single digit royalties on the net sales of its developed products that consists of a minimum annual royalty of up to $0.1 million per year for the term of the Agreement beginning in the first calendar year after the year in which net sales first occurred, and (iii) sublicense consideration in the mid-teens to the mid-twenties-range on any future sublicensing arrangements the Company may enter into with third-party licensees. During the years ended December 31, 2020 and 2019, the Company incurred expenses of $0.2 million and $0.3 million, respectively, under the provisions of the UC Agreements.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2020
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	8. Commitments and Contingencies Operating Lease Commitments In May 2015, the Company executed a lease agreement for office and laboratory space in Emeryville, California. In January 2016, the Company executed the first amendment to the lease agreement for additional rentable office and laboratory space which extended the lease to March 31, 2023. In October 2018, the Company executed a second amendment to extend the lease to September 2026. Additionally, the second amendment provided a tenant improvement allowance of $0.2 million, which was paid to the Company in November 2018. The Company amortizes the tenant improvement allowance on a straight-line basis over the remaining term of the lease as a reduction of rent expense. In October 2018, the Company executed a second lease agreement for additional office and laboratory space in Emeryville, California. The new lease has an initial term of 87 months beginning on the rent commencement date with the option to renew the lease for one additional term of five years. The Company did not have to pay rent until October 2019. This lease agreement also provided for a tenant improvement allowance of $0.4 million, which was paid to the Company in December 2019. The Company amortizes the tenant improvement allowance on a straight-line basis over the remaining term of the lease as a reduction of rent expense. In May 2019, the Company amended the second lease agreement executed in October 2018 to add additional office and laboratory space. The amendment extended the term of the lease to December 31, 2029. The Company did not have to pay rent until December 2019. The annual rent for the additional space is $1.0 million per annum and escalates at 3% annually. This lease agreement also provides for a tenant improvement allowance of at least $1.6 million. The Company recognizes rent expense on a straight-line basis over the lease term with the difference between the rent payments and the straight-line rent expense recorded as deferred rent. Rent expense for the years ended December 31, 2020 and 2019 was $3.0 million and $1.3 million, respectively. Deferred rent as of December 31, 2020 and 2019 was $1.6 million and $1.2 million, respectively. In conjunction with the lease agreements and amendments, the Company paid total security deposits of $0.6 million, which is included in other assets within the balance sheets as of both December 31, 2020 and 2019. The following table summarizes the Company’s future minimum commitments under lease contracts (in thousands): As of December 31, 2020 2021 $ 2,956 2022 3,035 2023 3,115 2024 3,205 2025 3,258 2026 and beyond 11,820 Total $ 27,389 Indemnification Agreements In the ordinary course of business, the Company enters into agreements that may include indemnification provisions, such as with vendors and other parties. Pursuant to such agreements, the Company may indemnify, hold harmless and defend an indemnified party for losses suffered or incurred by the indemnified party. Some of the provisions will limit losses to those arising from third-party actions. In some cases, the indemnification will continue after the termination of the agreement. The maximum potential amount of future payments the Company could be required to make under these provisions is not determinable. The Company has never incurred material costs to defend lawsuits or settle claims related to these indemnification provisions. The Company has also entered into indemnification agreements with its directors and officers that require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers to the fullest extent permitted by Delaware corporate law. The Company currently maintains directors’ and officers’ liability insurance that would generally enable it to recover a portion of any future amounts paid. The Company believes the estimated fair value of its indemnification agreements in excess of applicable insurance coverage is not material. Legal Proceedings From time to time, the Company may become involved in legal proceedings arising from the ordinary course of its business. If applicable, the Company records a legal liability when it believes that it is both probable that a liability may be imputed, and the amount of the liability can be reasonably estimated. Significant judgment by the Company is required to determine both probability and the estimated amount. Included in Accrued and Other Current Liabilities within the balance sheet

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Income Taxes	9. Income Taxes The Company did not record any income tax expense during the years ended December 31, 2020 and 2019. The Company has a net operating loss and has provided a valuation allowance against net deferred tax assets due to uncertainties regarding the Company’s ability to realize these assets. All losses before income taxes arose in the United States. The effective tax rate of the Company’s income tax expense (benefit) differs from the federal statutory rate as follows: December 31, 2020 December 31, 2019 Federal statutory income tax rate 21.0 % 21.0 % Research tax credit 2.8 % 5.2 % Permanent differences (1.7 )% (0.7 )% Valuation allowance (22.1 )% (25.5 )% Provision for income taxes 0.0 % 0.0 % The tax effects of temporary differences that give rise to significant components of the deferred taxes are as follows (in thousands): December 31, 2020 December 31, 2019 Deferred Tax Assets Net operating loss carryforwards $ 21,132 $ 10,638 Other accrued liabilities 422 336 Deferred revenue 3,111 2,698 Research tax credits 6,018 4,457 Stock compensation 1,151 554 Intangible asset basis 977 1,591 Total deferred tax assets $ 32,811 $ 20,274 Less: valuation allowance (32,811 ) (20,274 ) Net deferred tax assets $ — $ — ASC 740, Income Taxes, The increase in the valuation allowance for each of the years ended December 31, 2020 and 2019 was primarily driven by losses and tax credits generated in the U.S. The Company had net operating loss (“NOL”) carryforwards of $100.6 million and $50.7 million as of December 31, 2020 and 2019, respectively, available to reduce future taxable income, if any, for federal income tax purposes. $9.5 million of the federal NOL carryforwards expire in 2037 and the remaining $91.1 million carryforward indefinitely. As of December 31, 2020 and 2019, the Company had federal research and development credit carryforwards of $5.7 million and $3.3 million, respectively, and state research and development credit carryforwards of $4.3 million and $3.3 million, respectively, available to reduce future taxable income, if any, for federal and California state income tax purposes, respectively. The federal credit carryforwards begin expiring in 2035 and the state credits carryforward indefinitely. Utilization of the NOL carryforwards and research credit carryforwards may be subject to an annual limitation due to the ownership percentage change limitations provided by the Internal Revenue Code and similar state provisions. Annual limitations may result in the expiration of the NOL and tax credit carryforwards before they are utilized. The reconciliation of the beginning and ending unrecognized tax benefits amounts is as follows (in thousands): Unrecognized Income Tax Benefits Balance as of December 31, 2018 $ 659 Additions for current year tax positions 907 Balance as of December 31, 2019 1,566 Additions for current year tax positions 1,729 Additions for tax positions of prior years 72 Balance as of December 31, 2020 $ 3,367 The entire amount of the unrecognized tax benefits would not impact the Company’s effective tax rate if recognized. During each of the years ended December 31, 2020 and 2019, the Company did not recognize accrued interest and penalties related to unrecognized tax benefits. The Company does not anticipate that the amount of existing unrecognized tax benefits will materially increase or decrease during the next 12 months. The Company files income tax returns in the U.S. federal and California tax jurisdictions. In general, the Company is no longer subject to tax examination by the Internal Revenue Service or state taxing authorities for years before 2016. Although the federal and state statutes are closed for purposes of assessing additional income tax in those prior years, the taxing authorities may still make adjustments to the NOL and credit carryforwards used in open years. Therefore, the tax statutes should be considered open as it relates to the NOL and credit carryforwards used in open years. The Company has no ongoing income tax examinations by tax authorities at this time. In March 2020, the Coronavirus Aid, Relief, and Economic Security Act (the “CARES Act”) was signed into law. The CARES Act changed certain provisions of the Tax Act. Under the CARES Act, NOLs arising in taxable years beginning after December 31, 2017 and before January 1, 2021 may be carried back to each of the five taxable years preceding the tax year of such loss, but NOLs arising in taxable years beginning after December 31, 2020 may not be carried back. In addition, the CARES Act eliminated the limitation on the deduction of NOLs to 80% of current year taxable income for taxable years beginning before January 1, 2021 and increased the amount of interest expense that may be deducted to 50% of adjusted taxable income for taxable years beginning in 2019 or 2020. The Company evaluated the impact of the CARES act on its tax provision and concluded that there was not a material impact. On December 21, 2020, the U.S. president signed into law the Consolidated Appropriations Act, 2021 which includes further COVID-19 economic relief and extension of certain expiring tax provisions. The relief package includes a tax provision clarifying that businesses with forgiven PPP loans can deduct regular business expenses that are paid for with the loan proceeds. Additional pandemic relief tax measures include an expansion of the employee retention credit, enhanced charitable contribution deductions, and a temporary full deduction for business expenses for food and beverages provided by a restaurant. The Company evaluated the impact of the Consolidated Appropriations Act, 2021 on its tax provision and concluded that there was not a material impact.

Redeemable Convertible Preferre

Redeemable Convertible Preferred Stock	12 Months Ended
	Dec. 31, 2020
Temporary Equity Disclosure [Abstract]
Redeemable Convertible Preferred Stock	10. Redeemable Convertible Preferred Stock As of December 31, 2020 and 2019, the Company’s certificate of incorporation authorized the Company to issue 0 and 7,375,638 shares of redeemable convertible preferred stock, respectively at the par value of $0.0001 per share. As of December 31, 2019, redeemable convertible preferred stock consisted of the following (in thousands, except per share and share amounts): Shares Authorized Original Issuance Price Shares Issued and Outstanding Liquidation Value Proceeds Net of Issuance Cost Series A 909,312 $ 7.70 909,312 $ 7,001 $ 6,960 Series A-1 1,311,687 $ 8.84 1,311,687 11,595 11,548 Series B 5,154,639 $ 17.46 5,154,632 90,000 84,472 Total 7,375,638 7,375,631 $ 108,596 $ 102,980 In April and June of 2020, the Company issued a total of 4,200,353 shares of Series C redeemable convertible preferred stock at $18.00 per share for net proceeds of $72.5 million. Prior to the closing of the Company’s IPO the holders of redeemable convertible preferred stock had various rights and preferences including the following: Liquidation Preference— In the event of a liquidation event, the holders of the shares of Series C and Series B redeemable convertible preferred stock were entitled to receive any distribution of any of the assets of the Company in preference to the holders of the Series A-1 redeemable convertible preferred stock, Series A redeemable convertible preferred stock or common stock, an amount per share equal to the greater of (i) the sum of the original Series C issue price plus all declared but unpaid dividends thereon or the original Series B issue price plus all declared but unpaid dividends and (ii) such amount per share payable had all shares of Series C and Series B redeemable convertible preferred stock been converted into common stock. If upon the occurrence of a liquidation event, the available assets were insufficient to pay the holders of Series C and Series B redeemable convertible preferred stock the full amount to which they were entitled, then the available assets would be distributed to the holders of the shares of Series C and Series B redeemable convertible preferred stock, in proportion to the full preferential amount each holder was otherwise entitled to receive. After full payment to holders of the Series C and Series B redeemable convertible preferred stock, payment would be made to the holders of Series A-1 redeemable convertible preferred stock, in preference to the holders of the Series A redeemable convertible preferred stock or common stock, in an amount equal to the greater of (i) the original Series A-1 issue price plus all declared but unpaid dividends and (ii) such amount per share payable had all shares of Series A-1 redeemable convertible preferred stock been converted into common stock. After full payment to holders of the Series A-1 redeemable convertible preferred stock, payment would be made to the holders of Series A redeemable convertible preferred stock, in preference to the holders of the common stock, in an amount equal to the greater of (i) the original Series A issue price plus all declared but unpaid dividends and (ii) such amount per share payable had all shares of Series A redeemable convertible preferred stock been converted into common stock. If upon the occurrence of a liquidation event, the available assets were insufficient to pay the holders of Series A-1 or Series A redeemable convertible preferred stock the full amount to which they are entitled, then the available assets would be distributed to the holders of such redeemable convertible preferred stock on a pro rata, on an equal priority, pari passu basis, in proportion to the full preferential amount such holder was otherwise entitled to receive. Notwithstanding the above, for purposes of determining the amount each holder of shares of redeemable convertible preferred stock would be entitled to receive with respect to a Liquidation Event, each such holder of shares of a series of redeemable convertible preferred stock would be deemed to have converted such holder’s shares of such series into shares of common stock immediately prior to the Liquidation Event if, as a result of an actual conversion, such holder would receive, in the aggregate, an amount greater than the amount that would be distributed to such holder if such holder did not convert such series of redeemable convertible preferred stock into shares of common stock. If any such holder was deemed to have converted shares of redeemable convertible preferred stock into common stock pursuant to this paragraph, then such holder would not be entitled to receive any distribution that would otherwise be made to holders of redeemable convertible preferred stock that had not converted into shares of common stock. Conversion— Shares of any series of redeemable convertible preferred stock could be converted, at the option of the holder, into such number of fully paid and non-assessable shares of common stock using a conversion rate determined by dividing the applicable original issue price by the applicable conversion price, as adjusted for any anti-dilution adjustments. The Company’s redeemable convertible preferred stock was convertible into the Company’s shares of common stock on a one-for-one basis. Prior to the issuance of Series C redeemable convertible preferred stock in April 2020, shares of redeemable convertible preferred stock would automatically be converted into shares of common stock at the then effective conversion price for such share, immediately prior to either: (i) the completion of an underwritten public offering of the Company’s common stock at a price of at least 1.5 times the original Series B issuance price for any initial public offering consummated at any time prior to the first anniversary of the Series B original issuance date, or 1.25 times the original Series B issuance price for any such IPO thereafter and that provides at least $30.0 million of gross proceeds to the Company or (ii) the conversion by the holders of redeemable convertible preferred stock, which requires the vote of the holders of a majority of the then outstanding shares of redeemable convertible preferred stock, voting together as a single class on an as-converted to common stock basis. After the issuance of the Series C redeemable convertible preferred stock in April 2020, shares of redeemable convertible preferred stock would automatically be converted into shares of common stock at the then effective conversion price for such share, immediately prior to either: (i) the completion of an underwritten public offering of the Company’s common stock at a price of at least 1.25 times the original Series C issuance price, as adjusted for any stock splits, stock dividends, combinations, subdivisions, recapitalizations or the like that provides at least $50.0 million of gross proceeds to the Company or (ii) the conversion by the holders of redeemable convertible preferred stock, which requires the vote of the holders of a majority of the then outstanding shares of redeemable convertible preferred stock voting together as a single class on an as-converted to common stock basis; provided, however that any automatic conversion of the Series C and Series B redeemable convertible preferred stock under (ii) shall require the consent of the holders of a majority of Series C and Series B redeemable convertible preferred stock then outstanding voting together as a single class on an as-converted to common stock basis. Dividends— The redeemable convertible preferred stock dividends were not cumulative and were payable only when declared by the board of directors. No such dividends were declared. Such dividends were in preference to any dividends to holders of common stock. Voting Rights— Each holder of redeemable convertible preferred stock would be entitled to the number of votes equal to the number of shares of common stock into which the shares of redeemable convertible preferred stock held by such holder could be converted as of the record date. Redemption and Balance Sheet Classification— The redeemable convertible preferred stock was recorded in mezzanine equity because while it was not mandatorily redeemable, it would become redeemable at the option of the stockholders upon the occurrence of a deemed liquidation event that was considered not solely within the Company’s control. Funding Agreement with CFF — In April 2020, CFF made a $10.0 million investment in the Company’s Series C redeemable convertible preferred stock financing. In return for the investment, CFF received shares of Series C redeemable convertible preferred stock, and the Company and CFF entered into a Funding Agreement (“the Funding Agreement”). Pursuant to the terms of the Funding Agreement, except in the event of a technical failure, the $10.0 million received from CFF will be used to advance the development program for 4D-710, the Company’s lead product in cystic fibrosis, or any other therapeutic approved by the Program Advisory Group (“PAG”) to alleviate pulmonary complications of cystic fibrosis (“the Funding Agreement Product”). CFF is committed to provide an additional $4.0 million of funding upon acceptance of an Investigational New Drug application or its equivalent to allow for human testing of the Funding Agreement Product (“Acceptance”), except in the event of a change of control transaction occurring prior to Acceptance or Acceptance occurring after April 29, 2026. At the time of Acceptance, CFF will receive shares of common stock priced at the 10-day average reported closing price of the Company’s common stock on the date of Acceptance. Except in the event of a technical failure, the Company is committed to providing an amount equal to the funding provided by CFF to be used solely to advance the Funding Agreement Product. A technical failure is defined as a determination by the Company, after consultation with and approval of the PAG that (i) the Funding Agreement Product has failed to reach its intended endpoints due to safety issues, lack of sufficient transgene expression and/or efficacy, each despite commercially reasonable efforts and (ii) the exercise of further commercially reasonable efforts is unlikely to correct such failure. Under the terms of the Funding Agreement, neither the $10.0 million investment in the Series C redeemable convertible preferred stock, which converted to common stock as of December 31, 2020, nor the $4.0 million of funding upon Acceptance are restricted as to withdrawal or usage.

Common Stock

Common Stock	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Common Stock	11. Common Stock As of each of December 31, 2020 and 2019, the Company’s certificate of incorporation authorized the Company to issue 300,000,000 and 50,000,000 shares of common stock, respectively, at the par value of $0.0001 per share. The holder of each share of common stock is entitled to one vote per share. Common stockholders are entitled to dividends if and when declared by the board of directors, subject to the prior rights of the redeemable preferred stockholders. As of December 31, 2020 and 2019, no dividends on common stock had been declared by the board of directors. As of December 31, 2020 and 2019, the Company has reserved common stock, on an as-converted basis, for future issuance as follows: December 31, 2020 December 31, 2019 Conversion of redeemable convertible preferred stock — 7,375,631 Issuance of common stock under the 2015 Equity Incentive Plan — 125,353 Issuance of common stock under the 2020 Equity Incentive Plan 2,606,546 — Issuance of common stock under the Employee Stock Share Purchase Plan 252,337 — Exercise of options issued and outstanding 3,194,113 2,474,152 Exercise of common stock warrants 98,669 68,669 Total common stock reserved 6,151,665 10,043,805 Restricted Common Stock During 2015, the Company issued common stock to the Company founders of 4,710,060 shares, of which 4,473,374 were fully vested upon issuance. The remainder were deemed to be restricted based on their vesting conditions. The stock agreement contains certain provisions that allow the Company to repurchase unvested portions of stock from such founders in the event they depart from the Company. The repurchase rights on the restricted common stock lapsed over time and fully expired in March 2019. As of December 31, 2020 and 2019, no shares of restricted common stock remained subject to repurchase.

Stock-based Compensation

Stock-based Compensation	12 Months Ended
	Dec. 31, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-based Compensation	12. Stock-based Compensation 2020 Incentive Award Plan In December 2020, the Company adopted the 2020 Incentive Award Plan (“2020 Plan”), which became effective on December 10, 2020. The 2020 Plan initially reserved 2,606,546 shares of common stock for the issuance of stock options, stock appreciation rights, restricted stock awards, restricted stock units, performance bonus awards, performance stock units, dividend equivalents or other stock or cash based award granted to employees, directors and consultants of the Company. All options are exercisable over a period not to exceed the contractual term of ten years from the date the stock options were issued. As of December 31, 2020, there were 2,606,546 shares available for grant under the 2020 Plan. Following the effectiveness of the 2020 Plan, the Company will not make any further grants under the 2015 Equity Incentive Plan (the “2015 Plan”). However, the 2015 plan continues to govern the terms of options that remain outstanding under the 2015 Plan. 2015 Equity Incentive Plan The 2015 Plan No additional grants will be made under the 2015 Plan, and all outstanding grants under the 2015 Plan that are repurchased, forfeited, expire or are cancelled will become available for grant under the 2020 Plan in accordance with its terms. Employee Stock Purchase Plan In December 2020, the Company adopted the 2020 Employee Stock Purchase Plan (the “2020 ESPP”). Under the 2020 ESPP, 252,337 shares of our common stock were initially reserved for employee purchases of our common stock under terms and provisions established by the Board of Directors and approved by our stockholders. Under the 2020 ESPP our employees may purchase common stock through payroll deductions at a price equal to 85% of the lower of the fair market value of the stock at the beginning of the offering period or at the end of each applicable purchase period. The 2020 ESPP provides for a series of overlapping 24-month offering periods comprising of four six-month Stock Options The following table summarizes the stock options activity: Options Outstanding Number of Shares Available for Grant Number of Shares Underlying Outstanding Options Weighted- Average Exercise Price Weighted- Average Remaining Contractual Term (in years) Aggregate Intrinsic Value (in thousands) Balances at December 31, 2019 125,353 2,474,152 $ 6.77 8.46 $ 19,974 Options authorized 3,468,198 — — Options granted (1,321,743 ) 1,321,743 16.83 Options exercised — (267,044 ) 3.21 10,212 Options expired 103,637 (103,637 ) 6.24 Options forfeited 231,101 (231,101 ) 9.51 Balances at December 31, 2020 2,606,546 3,194,113 $ 11.05 $ 8.46 $ 97,105 Shares exercisable, December 31, 2020 1,207,394 $ 6.24 $ 7.36 $ 42,515 Shares vested and expected to vest, December 31, 2020 3,194,113 $ 11.05 $ 8.46 $ 97,105 The following table is a summary of stock compensation expense for employees and nonemployees by function (in thousands): Year Ended December 31, 2020 Year Ended December 31, 2019 Research and development $ 2,670 $ 2,191 General and administrative 2,314 1,350 Total stock-based compensation $ 4,984 $ 3,541 During the years ended December 31, 2020 and 2019, the Company granted 1,267,743 and 1,101,840 stock options to employees with a weighted-average grant date fair value of $11.84 and $8.46 per share, respectively, and 54,000 and 35,000 stock options to nonemployees with a weighted-average grant date fair value of $11.05 and $8.43 per share, respectively. The total fair value of options vested during the years ended December 31, 2020 and 2019 was $5.0 million and $2.2 million, respectively. As of December 31, 2020, the unrecognized stock-based compensation of unvested options was $19.4 million and is expected to be recognized over a weighted-average period of 3.1 years. Stock-based compensation expense recorded for employee options was $ million and $ 2.8 million for the years ended December 31, 2020 and 2019, respectively. Stock-based compensation expense recorded for nonemployee consultants was $ 0.4 million and $ 0.7 million for the years ended December 31, 2020 and 2019, respectively. Prior to the Company’s IPO, the fair value of the shares of common stock underlying the stock options was estimated by the board of directors at various dates considering the Company’s most recently available third-party valuations of common stock as well as a number of objective and subjective factors including valuation of comparable companies, sales of redeemable convertible preferred stock, operating and financial performance and general and industry specific economic outlook, amongst other factors. The fair value was determined in accordance with the guidance provided by the American Institute of Certified Public Accountants’ Practice Guide, Valuation of Privately-Held-Company Equity Securities Issued as Compensation. Subsequent to the Company’s IPO, the fair value of the Company’s common stock is determined based on its closing market price. The Company estimates the fair value of employee and nonemployee stock options using the Black-Scholes valuation model. The fair value of employee and nonemployee stock options is recognized on a straight-line basis over the requisite service period of the awards. The fair value of the Company’s stock options was estimated using the following assumptions for the years ended December 31, 2020 and 2019. Year Ended December 31, 2020 Year Ended December 31, 2019 Employee Nonemployee Employee Nonemployee Expected term 5.5 –6.3 years 6.0 years 5.5 –6.3 years 6.1 –10.0 years Expected volatility 82.1% – 83.8% 82.1% – 83.1% 81.9% – 83.0% 81.3% – 83.7% Risk-free interest rate 0.4% – 0.7% 0.4% – 0.7% 1.5% – 2.3% 1.4% – 2.4% Expected dividend yield 0 % 0 % 0 % 0 % Expected Term . The expected term for employee options is calculated using the simplified method as the Company does not have sufficient historical information to provide a basis for estimate. The simplified method is based on the average of the vesting tranches and the contractual life of each grant. The expected term for nonemployee options is the contractual term of the options. Expected Volatility . The expected volatility was estimated based on a study of publicly traded peer companies as the Company did not have sufficient trading history for its common stock. The Company selected the peer group based on similarities in industry, stage of development, size and financial leverage with the Company’s principal business operations. For each grant, the Company measured historical volatility over a period equivalent to the expected term. Risk-free Interest Rate . The risk-free interest rate is based on the yield available on U.S. Treasury zero-coupon issues whose term is similar in duration to the expected term of the respective stock option. Expected Dividend Yield . The Company has not paid and does not anticipate paying any dividends on its common stock in the future. Accordingly, the Company has estimated the dividend yield to be zero.

Common Stock Warrants

Common Stock Warrants	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Common Stock Warrants	13. Common Stock Warrants In 2016, the Company issued a warrant for 45,000 shares of the Company’s common stock to a service provider with an exercise price of $1.14 per share, of which 15,000 warrant shares become exercisable upon completion of an offering of securities in a private placement by the Company with net proceeds in excess of $25.0 million and 30,000 warrant shares become exercisable upon completion of an IPO by the Company. The warrant expires in 2023. As the services had been completed at the date the warrant had been issued, the fair value of the warrant was determined at the issuance date. 15,000 of these warrant shares became exercisable upon the completion of the Series B financing in 2018 and 30,000 of these warrants became exercisable upon completion of the IPO in 2020. The Company recorded less than $0.1 million fair value of these warrant shares, as determined under the Black-Scholes Model, within operating expenses in the statements of operations and comprehensive loss and within additional paid-in-capital in the balance sheets during the year ended December 31, 2018 and December 31, 2020. In May 2018, the Company issued a warrant for 23,669 shares of the Company’s common stock to a service provider with an exercise price of $3.19 per share and recorded $0.1 million within in additional paid-in-capital upon issuance of the warrant. The warrant expires in 2025. In 2020, the Company issued a warrant for 30,000 shares of the Company’s common stock to a service provider with an exercise price of $18.00 per share. This warrant vests over a period of four years and expires in 2027. The fair value of the warrant was determined at the issuance date using the Black-Scholes Model. In 2020, an expense of less than $0.1 million was recorded within operating expenses in the statements of operations and comprehensive loss and within additional paid-in-capital in the balance sheets in connection with the vested portion of warrant shares.

Net Loss Per Share Attributable

Net Loss Per Share Attributable to Common Stockholders	12 Months Ended
	Dec. 31, 2020
Earnings Per Share [Abstract]
Net Loss Per Share Attributable to Common Stockholders	14. Net Loss Per Share Attributable to Common Stockholders The following table sets forth the computation of basic and diluted net loss per share attributable to common stockholders (in thousands, except share and per share data): Year Ended December 31, 2020 Year Ended December 31, 2019 Numerator Net loss attributable to common stockholders $ (56,693 ) $ (49,306 ) Denominator Weighted-average shares outstanding 6,430,555 5,143,776 Less: Weighted-average shares subject to repurchase used in computing net loss per share attributable to common stockholders, basic and diluted — (1,216 ) Weighted-average shares outstanding used in computing net loss per share attributable to common stockholders, basic and diluted 6,430,555 5,142,560 Net loss per share attributable to common stockholders—basic and diluted $ (8.82 ) $ (9.59 ) The following outstanding shares of potentially dilutive securities were excluded from the computation of diluted net loss per share attributable to common stockholders for the period presented because including them would have been antidilutive: December 31, 2020 December 31, 2019 Redeemable convertible preferred stock — 7,375,631 Options to purchase common stock 3,194,113 2,474,152 Common stock warrants 98,669 68,669 Total 3,292,782 9,918,452

Derivative Liability

Derivative Liability	12 Months Ended
	Dec. 31, 2020
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Derivative Liability	15. Derivative Liability The Company identified an embedded derivative resulting from the change of control provision in the CFF Agreement. Embedded derivatives that are required to be bifurcated from the underlying host instrument are accounted for and valued as separate financial instruments. At the inception of the derivative in 2017, the Company recognized this derivative as a liability and revenue was reduced by the initial fair value of the derivative liability. The Company remeasures the derivative liability to fair value at each reporting period and records the change in fair value of the derivative liability as other income (expense), net. The Company uses a present value analysis with multiple scenarios, which incorporates assumptions and estimates to value the derivative instrument. The Company assesses these assumptions and estimates on a periodic basis as additional information impacting the assumptions is obtained. Estimates and assumptions impacting the fair value measurement include the change of control payment to CFF (range of $0 to $10.6 million at December 31, 2020), the probability of a change of control event, the probability of the product achieving development or commercial status at time of change of control (range of 3.4% to 12.3% at December 31, 2020) and the discount rate (14% at December 31, 2020). The Company determined the estimated fair value of this liability as of the inception date of the CFF Agreement and concluded that the amount was immaterial. The Company determined the fair value of this derivative liability was $0.1 million as of December 31, 2020 and 2019.

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2020
Related Party Transactions [Abstract]
Related Party Transactions	16. Related Party Transactions In August 2019, the Company and uniQure entered into the amended uniQure Agreement and the Second uniQure Agreement. Under these agreements, the Company agreed to transfer incremental rights and services to uniQure in exchange for uniQure eliminating the exclusivity clause in the uniQure Agreement and transferring other rights back to the Company. Further details and the accounting for these agreements is discussed in Note 6. As of December 31, 2019, Pfizer owned 1,474,992 shares of the Company’s redeemable convertible preferred stock and had a representative director on the Company’s board of directors. In April 2020, as part of Series C Preferred Stock Purchase Agreement, Pfizer acquired 166,666 shares of the Company’s redeemable convertible preferred stock. On December 11, 2020 upon completion of the IPO, Pfizer’s 1,641,658 shares of redeemable convertible preferred stock were converted into 1,641,658 shares of common stock. Pfizer also acquired additional 25,000 shares of common stock at $23.00 per share during the IPO. As of December 31, 2020, Pfizer owns 1,666,658 shares of common stock and has a representative director on the Company’s board of directors. In the years ended December 31, 2020 and 2019, the Company paid $93,000 and $52,000, respectively, to David Schaffer, PhD, the co-founder and Chief Scientific Advisor of the Company for consulting services. In April 2019, the Company entered into two sponsored research agreements (“SRAs”) with the UC Regents to conduct research in a research facility on the Berkeley campus, under the direction of Dr. Schaffer. The SRAs have a three year term ending in May 2022. Under the SRAs, the Company has an option to license (on a royalty-bearing basis) all intellectual property generated under the SRAs. The total amount the Company is committed to pay to the UC Regents under the SRAs is $ 1.5 million, of which $ 1.0 million and $ 0.4 million was paid as of December 31, 2020 and 2019, respectively . In the years ended December 31, 2020 and 2019, the Company recorded $ 0.5 million and $ 0.3 million, respectively, of expense related to SRAs. As of December 31, 2020 and 2019, accounts payable related to the SRAs was $ 0.2 million and $ 0.3 million, respectively. Any patent prosecution costs incurred under the SRAs will also be borne by the Company. The Company can terminate the SRAs for convenience and without cause with 60 days’ notice. In 2016, the Chief Executive Officer and several other employees of the Company founded Ignite Immunotherapy (“Ignite”). From 2016 through October 2019, the Company’s Chief Executive Officer served as the Chief Executive Officer and Executive Chairman of Ignite and certain executives of the Company held ownership interests in Ignite and were members of the board of directors of Ignite. Additionally, during this time period, Pfizer, which is a related party of the Company, held a significant equity stake in Ignite.

401(K) Plan

401(K) Plan	12 Months Ended
	Dec. 31, 2020
Compensation And Retirement Disclosure [Abstract]
401(K) Plan	17. 401(k) Plan In 2014, the Company adopted a 401(k) plan for all employees who have met certain eligibility requirements. The 401(k) plan allows employees to make pre-tax and post-tax contributions up to the maximum allowable amount set by the Internal Revenue Service. The Company made contributions to the Plan for eligible participants, and recorded contribution expenses of $0.4 million and $0.3 million for the years ended December 31, 2020 and 2019, respectively.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying financial statements have been prepared in accordance with United States Generally Accepted Accounting Principles (“U.S. GAAP”).
Use of Estimates and Judgements	Use of Estimates and Judgements The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and judgements that affect the reported amounts of assets, liabilities, revenue and expenses; and disclosure of contingent assets and liabilities as of the date of the financial statements. Such estimates include the determination of useful lives for property and equipment, the contract term, transaction price and costs of collaboration agreements, as well as estimates of the fair value of common stock (prior to the IPO) , stock options and derivative instruments and income tax uncertainties. Actual results could differ from those estimates. Due to the coronavirus (“COVID-19”) pandemic, there has been uncertainty and disruption in the global economy and financial markets. The Company is not aware of any specific event or circumstance that would require an update to its estimates or judgments or a revision of the carrying value of its assets or liabilities as of December 31, 2020. While there was not a material impact to the Company’s financial statements as of December 31, 2020, these estimates may change, as new events occur and additional information is obtained, as well as other factors related to the COVID-19 pandemic that could result in material impacts to the financial statements in future reporting periods.
Segment Information	Segment Information The Company operates and manages its business as one reportable and operating segment. The Company’s chief executive officer, who is the chief operating decision maker, reviews financial information on a company-wide basis for purposes of allocating resources and assessing financial performance. As of and for the years ended December 31, 2020 and 2019, all of the Company’s long-lived assets were located in the United States and all revenue was earned in the United States.
Concentration of Credit Risk	Concentration of Credit Risk Financial instruments that potentially subject the Company to a concentration of credit risk consist of cash and cash equivalents and accounts receivable. The Company’s cash is held at two financial institutions in the United States of America. The Company’s cash equivalents are invested in money market funds. The Company has not experienced any losses on its deposits of cash and cash equivalents. Such deposits may, at times, exceed federally insured limits. The Company’s partners in collaboration and license agreements who represent 10% or more of the Company’s total revenue are as follows: Year Ended December 31, 2020 Year Ended December 31, 2019 Customer A 95% 90% Customer B * * Total 95% 90% * Less than 10% The Company’s partners in collaboration and license agreements who represent 10% or more of the Company’s total accounts receivable are as follows: December 31, 2020 December 31, 2019 Customer A 74% 64% Customer B 26% 36% Total 100% 100% The Company’s total revenues by geographic region, based on the location of the customer, are as follows (in thousands): Year Ended December 31, 2020 Year Ended December 31, 2019 Australia $ — $ 7 Netherlands 742 26 Switzerland 12,897 6,287 United States (27 ) 666 $ 13,612 $ 6,986
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers all highly liquid investments with original maturities of three months or less at the date of purchase to be cash equivalents. Cash equivalents consist of money market funds.
Other Risks and Uncertainties	Other Risks and Uncertainties The Company is subject to risks and uncertainties common to early-stage companies in the biotechnology industry, including, but not limited to, development by competitors of new technological innovations, protection of proprietary technology, dependence on key personnel, suppliers for key raw materials, contract manufacturing organizations (“CMOs”) and contract research organizations (“CROs”), compliance with government regulations 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 preclinical studies, clinical trials and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel and infrastructure and extensive compliance and reporting. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be obtained or maintained, that any products developed will obtain necessary government regulatory approval or that any approved products will be commercially viable. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will generate significant revenue from product sales. The Company operates in an environment of rapid change in technology and substantial competition from other pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees, consultants and other third parties (including for clinical trials and some aspects of research and preclinical testing). The extent of the impact of the COVID-19 pandemic on the Company’s business will depend upon the duration and spread of the outbreak and the extent and severity of the impact on the Company’s clinical trial activities, research activities and suppliers, all of which are uncertain and cannot be predicted. The extent to which the coronavirus outbreak may materially impact the Company’s financial condition, liquidity or results of operations is uncertain.
Fair Value Measurements	Fair Value Measurements The Company applies fair value accounting for all financial assets and liabilities and non-financial assets and liabilities that are recognized or disclosed at fair value in the financial statements on a recurring basis. Fair value is an exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or liability. As a basis for considering such assumptions, a three-level fair value hierarchy has been established, which prioritizes the inputs used in measuring fair value as follows: • Level 1 —Observable inputs, such as quoted prices in active markets for identical assets and liabilities. • Level 2 —Observable inputs other than Level 1 that are observable, either directly or indirectly, such as quoted prices for similar assets or liabilities, quoted prices in markets that are not active, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. • Level 3 —Unobservable inputs that are significant to the measurement of the fair value of the assets or liabilities that are supported by little or no market data. The Company accounts for transfers of financial instruments between levels of the fair value hierarchy on the date of the event or change in circumstance that caused the transfer.
Accounts Receivable—Allowance for Doubtful Accounts	Accounts Receivable—Allowance for Doubtful Accounts The Company regularly reviews accounts receivable for collectability and establishes an allowance for probable credit losses and writes off uncollectible accounts as necessary. The Company has determined that no allowance was required at December 31, 2020 and 2019. The Company did not have any write-offs relating to uncollectible accounts receivable during the years ended December 31, 2020 and 2019.
Property and Equipment, Net	Property and Equipment, net Property and equipment are stated at cost less accumulated depreciation for acquired assets. Depreciation is computed using the straight-line method over the estimated useful lives of assets, ranging from three to five years. Leasehold improvements are amortized over the shorter of the useful life of the assets or the lease term. Upon sale or retirement of assets, the costs and related accumulated depreciation are removed from the balance sheet and the resulting gain or loss is reflected within operating expenses in the statements of operations and comprehensive loss. Maintenance and repairs are charged to expense as incurred.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets
Redeemable Convertible Preferred Stock	Redeemable Convertible Preferred Stock The Company recorded all shares of redeemable convertible preferred stock at their respective fair values on the dates of issuance, net of issuance costs. The redeemable convertible preferred stock was recorded outside of permanent equity because while it was not mandatorily redeemable, in certain events considered not solely within the Company’s control, such as a merger or consolidation, sale, lease, or license of substantially all of the Company’s assets (each, a “deemed liquidation event”), the convertible preferred stock would become redeemable at the option of the holders of a majority of the outstanding series of redeemable convertible preferred stock. The Company did not adjust the carrying values of the redeemable convertible preferred stock to the liquidation preference of such shares because a deemed liquidation event obligating the Company to pay the liquidation preference did not occur. All outstanding shares of the redeemable convertible preferred stock converted into common stock upon the closing of the IPO in December 2020 .
Common Stock Warrants	Common Stock Warrants The Company accounts for common stock warrants which meet the definition of a derivative as liabilities if the warrant requires net cash settlement or gives the holder the option of net cash settlement. The Company accounts for common stock warrants as equity if the contract requires physical settlement or net physical settlement or if the Company has the option of physical settlement or net physical settlement. Common stock warrants classified as liabilities are initially recorded at fair value and remeasured at fair value each balance sheet date with the offset adjustments recorded in other income (expense), net within the statements of operations and comprehensive loss. Common stock warrants classified as equity are initially measured at fair value on the grant date and are not subsequently remeasured.
Revenue Recognition	Revenue Recognition Effective January 1, 2019, the Company adopted Accounting Standards Update (“ASU”) 2014-09, Revenue from Contracts with Customers (Topic 606) The Company’s revenue is primarily derived through its license, research, development and commercialization agreements. The terms of these types of agreements may include (i) licenses to the Company’s technology, (ii) research and development services, and (iii) services or obligations in connection with participation in research or steering committees. Payments to the Company under these arrangements typically include one or more of the following: nonrefundable upfront and license fees, research funding, milestone and other contingent payments to the Company for the achievement of defined collaboration objectives and certain preclinical, clinical, regulatory and sales-based events, as well as royalties on sales of any commercialized products. Arrangements that include upfront payments are recorded as deferred revenue upon receipt or when due and are recognized as revenue as performance conditions are met. The event-based milestone payments, royalties and cost reimbursements represent variable consideration, and the Company uses the most likely amount method to estimate this variable consideration. Royalty payments are recognized when earned or as the sales occur. The Company records cost reimbursements as accounts receivable when right to consideration is unconditional. A performance obligation is a promise in a contract to transfer a distinct good or service to the customer and is the unit of account in ASC 606. The Company allocates the total transaction price to each performance obligation based on the estimated standalone selling price and recognizes revenue when, or as, the performance obligation is satisfied. The Company includes the unconstrained amount of estimated variable consideration in the transaction price. At the end of each reporting period, the Company re-evaluates the estimated variable consideration included in the transaction price and any related constraint, and if necessary, adjusts its estimate of the overall transaction price. Significant management judgment is required to determine the level of effort required under an arrangement and the period over which the Company expects to complete its performance obligations under the arrangement. Changes in these estimates can have a material effect on revenue recognized.
Research and Development Expenses	Research and Development Expenses Costs related to research, design and development of programs are charged to research and development expense as incurred. Research and development costs include, but are not limited to, payroll and personnel expenses including stock-based compensation, materials, laboratory supplies, outside services and allocated overhead, including rent, insurance, repairs and maintenance, depreciation and utilities. The Company expenses all research and development costs in the period in which they are incurred. Costs incurred in obtaining technology licenses are charged to research and development expense as acquired in-process research and development if the technology licensed has not reached technological feasibility and has no alternative future use.
Accrued Research and Development	Accrued Research and Development The Company has entered into various agreements with CROs and CMOs. The Company’s research and development accruals are estimated based on the level of services performed, progress of the studies, including the phase or completion of events, and contracted costs. The estimated costs of research and development provided, but not yet invoiced, are included in accrued and other current liabilities on the balance sheet. If the actual timing of the performance of services or the level of effort varies from the original estimates, the Company will adjust the accrual accordingly. Payments made to CROs or CMOs under these arrangements in advance of the performance of the related services are recorded as prepaid expenses and other current assets until the services are rendered.
Stock-Based Compensation	Stock-Based Compensation As of January 1, 2020, the Company adopted ASU 2018-07, Compensation - Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting Prior to the adoption of ASU 2018-07 on January 1, 2020, stock-based compensation expense related to stock options granted to nonemployees was recognized based on the vesting date fair value of options as the stock options were earned. The Company remeasured the stock-based compensation at each reporting period end with the resulting change in fair value being recognized in the statements of operations and comprehensive loss over the period the related services were rendered. The Company estimated the service period for the options based on the time that would be required to satisfy the service condition, assuming the service condition would be satisfied. Stock-based compensation expense was recognized over the estimated service period but was accelerated if the performance condition was achieved earlier than estimated.
Income Taxes	Income Taxes The Company accounts for income taxes under the asset and liability method, which requires, among other things, that deferred income taxes be provided for temporary differences between the tax basis of the Company’s assets and liabilities and their financial statement reported amounts. In addition, deferred tax assets are recorded for the future benefit of utilizing net operating losses and research and development credit carryforwards and are measured using the enacted tax rates and laws that will be in effect when such items are expected to reverse. A valuation allowance is provided against deferred tax assets unless it is more likely than not that they will be realized. The Company accounts for uncertain tax positions by assessing all material positions taken in any assessment or challenge by relevant taxing authorities. Assessing an uncertain tax position begins with the initial determination of the position’s sustainability and is measured at the largest amount of benefit that is greater than fifty percent likely of being realized upon ultimate settlement. The Company’s policy is to recognize interest and penalties related to the underpayment of income taxes as a component of income tax expense or benefit. To date, there have been no interest or penalties charged in relation to the unrecognized tax benefits.
Embedded Derivative	Embedded Derivative Embedded derivatives that are required to be bifurcated from the underlying host instrument are accounted for and valued as a separate financial instrument. An embedded derivative exists in the award agreement with the Cystic Fibrosis Foundation (“CFF”). As described in Note 15, the embedded derivative has been bifurcated and is classified as a liability on the balance sheet and separately accounted for at its fair value. The derivative liability is subject to remeasurement to fair value each reporting period. Changes in the fair value of the derivative liability are recognized as a component of other income (expense), net within the statements of operations and comprehensive loss.
Deferred Offering Costs	Deferred Offering Costs The Company capitalizes certain legal, accounting and other third-party fees that are directly related to the Company’s in-process financings, until such financings are consummated. After consummation of the financing, these costs are recorded as a reduction of the proceeds received as a result of the offering. In the event that a planned offering does not occur or is significantly delayed, all related deferred offering costs will be expensed immediately within the Company’s statements of operations and comprehensive loss.
Net Loss Per Share Attributable to Common Stockholders	Net Loss Per Share Attributable to Common Stockholders The Company calculates basic and diluted net loss per share to common stockholders in conformity with the two-class method required for companies with participating securities. The Company considers all series of redeemable convertible preferred stock to be participating securities as the holders are entitled to receive non-cumulative dividends on a pari passu basis in the event the dividend is paid on common shares. Under the two-class method, the net loss attributable to common stockholders is not allocated to the redeemable convertible preferred stock as the holders of redeemable convertible preferred stock do not have a contractual obligation to share in losses. Basic net loss per share attributable to common stockholders is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of shares of common stock outstanding during the period, less shares subject to repurchase. Diluted net loss per share attributable to common stockholders is computed by giving effect to all potentially dilutive common shares outstanding for the period. For purposes of this calculation, redeemable convertible preferred shares, stock options to acquire shares of common stock, common stock warrants, and unvested common stock subject to repurchase, are considered potentially dilutive common shares, but have been excluded from the calculation of diluted net loss per share attributable to common stockholders as their effect is antidilutive.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (“FASB”) issued ASC 606. This standard outlines a single comprehensive model for entities to use in accounting for revenue arising from contracts with customers and supersedes most current revenue recognition guidance, including industry-specific guidance. Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. The Company adopted ASC 606 effective January 1, 2019 using the modified retrospective method only to contracts not completed as of this date. The Company recognized the cumulative effect of initially applying ASC 606 as an adjustment to the balance of accumulated deficit at January 1, 2019 with a cumulative effect adjustment of $ 0.3 million reflected as a decrease to the opening balance of accumulated deficit and a decrease to deferred revenue. In June 2018, FASB issued ASU 2018-07, Compensation—Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820), Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement In July 2017, the FASB issued ASU 2017-11, Earnings Per Share (Topic 260) Distinguishing Liabilities from Equity (Topic 480) Derivatives and Hedging (Topic 815) (Part I) Accounting for Certain Financial Instruments with Down Round Features, (Part II) Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Noncontrolling Interests with a Scope Exception New Accounting Pronouncements Not Yet Adopted As an “emerging growth company,” the Jumpstart Our Business Startups Act (“JOBS Act”) allows the Company to delay adoption of new or revised accounting pronouncements applicable to public companies until such pronouncements are made applicable to private companies. The Company has elected to use this extended transition period under the JOBS Act. The adoption dates discussed below reflects this election. In November 2018, the FASB issued ASU 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction Between Topic 808 and Topic 606 Revenue from Contracts with Customers Revenue from Contracts with Customers In August 2018, the FASB issued ASU 2018-15, Intangibles—Goodwill and Other—Internal-Use Software (Subtopic 350-40), Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) (“ASC 842”) Codification Improvements to Topic 842, Leases Leases Leases (Topic 842): Targeted Improvements,

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Schedule of Concentration of Risk	The Company’s partners in collaboration and license agreements who represent 10% or more of the Company’s total revenue are as follows: Year Ended December 31, 2020 Year Ended December 31, 2019 Customer A 95% 90% Customer B * * Total 95% 90% * Less than 10% The Company’s partners in collaboration and license agreements who represent 10% or more of the Company’s total accounts receivable are as follows: December 31, 2020 December 31, 2019 Customer A 74% 64% Customer B 26% 36% Total 100% 100%
Schedule of Revenues by Geographic Region	The Company’s total revenues by geographic region, based on the location of the customer, are as follows (in thousands): Year Ended December 31, 2020 Year Ended December 31, 2019 Australia $ — $ 7 Netherlands 742 26 Switzerland 12,897 6,287 United States (27 ) 666 $ 13,612 $ 6,986

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Fair Value Hierarchy for Financial Assets and Financial Liabilities Measured at Fair Value on Recurring Basis	The following tables represent the Company’s fair value hierarchy for financial assets and financial liabilities measured at fair value on a recurring basis as of December 31, 2020 and 2019 (in thousands): Basis for Fair Value Measurements Fair Value as of (Level 1) (Level 2) (Level 3) December 31, 2020 Assets Money market funds $ 276,726 $ — $ — $ 276,726 Total $ 276,726 $ — $ — $ 276,726 Liabilities Derivative liability $ — $ — $ 122 $ 122 Total $ — $ — $ 122 $ 122 Basis for Fair Value Measurements Fair Value as of (Level 1) (Level 2) (Level 3) December 31, 2019 Assets Money market funds $ 15,876 $ — $ — $ 15,876 Total $ 15,876 $ — $ — $ 15,876 Liabilities Derivative liability $ — $ — $ 101 $ 101 Total $ — $ — $ 101 $ 101
Summary of Changes in Fair Value of Level 3 Financial Instruments	The following table sets forth a summary of the changes in the fair value of the Company’s Level 3 financial instruments (in thousands): Derivative Liability Balance as of December 31, 2018 $ 64 Change in fair value included in other income (expense), net 37 Balance as of December 31, 2019 $ 101 Change in fair value included in other income (expense), net 21 Balance as of December 31, 2020 $ 122

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2020
Property Plant And Equipment [Abstract]
Schedule of Property and Equipment, Net	Property and equipment, net, consisted of the following (in thousands): December 31, 2020 December 31, 2019 Machinery and equipment $ 4,911 $ 3,761 Leasehold improvements 2,527 2,405 Furniture and fixtures 473 541 Office equipment 101 98 Computer equipment and software 366 306 Construction in progress 539 563 Total property and equipment 8,917 7,674 Less: Accumulated depreciation and amortization (3,844 ) (2,625 ) Property and equipment, net $ 5,073 $ 5,049

Accrued and Other Liabilities_2

Accrued and Other Liabilities, Current and Noncurrent (Tables)	12 Months Ended
	Dec. 31, 2020
Payables And Accruals [Abstract]
Summary of Accrued Liabilities	Accrued liabilities consisted of the following (in thousands): December 31, 2020 December 31, 2019 Payroll and related $ 3,437 $ 2,133 Accrued clinical and preclinical study costs 869 494 Consulting and professional 3,574 2,239 Other accrued expenses 491 481 Total accrued and other current liabilities $ 8,371 $ 5,347
Summary of Other Liabilities, Noncurrent	Other liabilities, noncurrent consisted of the following (in thousands): December 31, 2020 December 31, 2019 Deferred rent, noncurrent $ 1,237 $ 1,023 Other payable, noncurrent 615 542 Total other liabilities, noncurrent $ 1,852 $ 1,565

Research and Collaboration Ar_2

Research and Collaboration Arrangements (Tables)	12 Months Ended
	Dec. 31, 2020
Research And Collaboration Arrangements [Abstract]
Summary of Collaboration and License Revenue	Collaboration and license revenue for each period was as follows (in thousands): Year Ended December 31, 2020 2019 uniQure $ 742 $ 26 Benitec — 7 CRF — — Roche 12,897 6,287 CFF (27 ) 118 AstraZeneca — 548 $ 13,612 $ 6,986
Summary of Deferred Revenue	Deferred revenue for each period was as follows (in thousands): Deferred Revenue As of December 31, 2020 As of December 31, 2019 uniQure $ 4,396 $ 5,137 Benitec — — CRF — — Roche 14,318 13,640 CFF 1,098 690 AstraZeneca — — $ 19,812 $ 19,467

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2020
Commitments And Contingencies Disclosure [Abstract]
Summary of Future Minimum Commitments Under Lease Contracts	The following table summarizes the Company’s future minimum commitments under lease contracts (in thousands): As of December 31, 2020 2021 $ 2,956 2022 3,035 2023 3,115 2024 3,205 2025 3,258 2026 and beyond 11,820 Total $ 27,389

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Schedule of Effective Tax Rate of Income Tax Expense (Benefit) Differs from Federal Statutory Rate	The effective tax rate of the Company’s income tax expense (benefit) differs from the federal statutory rate as follows: December 31, 2020 December 31, 2019 Federal statutory income tax rate 21.0 % 21.0 % Research tax credit 2.8 % 5.2 % Permanent differences (1.7 )% (0.7 )% Valuation allowance (22.1 )% (25.5 )% Provision for income taxes 0.0 % 0.0 %
Schedule of Tax Effects of Temporary Differences to Significant Components of Deferred Taxes	The tax effects of temporary differences that give rise to significant components of the deferred taxes are as follows (in thousands): December 31, 2020 December 31, 2019 Deferred Tax Assets Net operating loss carryforwards $ 21,132 $ 10,638 Other accrued liabilities 422 336 Deferred revenue 3,111 2,698 Research tax credits 6,018 4,457 Stock compensation 1,151 554 Intangible asset basis 977 1,591 Total deferred tax assets $ 32,811 $ 20,274 Less: valuation allowance (32,811 ) (20,274 ) Net deferred tax assets $ — $ —
Reconciliation of Beginning and Ending Unrecognized Tax Benefits	The reconciliation of the beginning and ending unrecognized tax benefits amounts is as follows (in thousands): Unrecognized Income Tax Benefits Balance as of December 31, 2018 $ 659 Additions for current year tax positions 907 Balance as of December 31, 2019 1,566 Additions for current year tax positions 1,729 Additions for tax positions of prior years 72 Balance as of December 31, 2020 $ 3,367

Redeemable Convertible Prefer_2

Redeemable Convertible Preferred Stock (Tables)	12 Months Ended
	Dec. 31, 2020
Temporary Equity Disclosure [Abstract]
Summary of Redeemable Convertible Preferred Stock	As of December 31, 2019, redeemable convertible preferred stock consisted of the following (in thousands, except per share and share amounts): Shares Authorized Original Issuance Price Shares Issued and Outstanding Liquidation Value Proceeds Net of Issuance Cost Series A 909,312 $ 7.70 909,312 $ 7,001 $ 6,960 Series A-1 1,311,687 $ 8.84 1,311,687 11,595 11,548 Series B 5,154,639 $ 17.46 5,154,632 90,000 84,472 Total 7,375,638 7,375,631 $ 108,596 $ 102,980

Common Stock (Tables)

Common Stock (Tables)	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Schedule of Common Share Reserved for Future Issuance	As of December 31, 2020 and 2019, the Company has reserved common stock, on an as-converted basis, for future issuance as follows: December 31, 2020 December 31, 2019 Conversion of redeemable convertible preferred stock — 7,375,631 Issuance of common stock under the 2015 Equity Incentive Plan — 125,353 Issuance of common stock under the 2020 Equity Incentive Plan 2,606,546 — Issuance of common stock under the Employee Stock Share Purchase Plan 252,337 — Exercise of options issued and outstanding 3,194,113 2,474,152 Exercise of common stock warrants 98,669 68,669 Total common stock reserved 6,151,665 10,043,805

Stock-based Compensation (Table

Stock-based Compensation (Tables)	12 Months Ended
	Dec. 31, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of Stock Options Activity	The following table summarizes the stock options activity: Options Outstanding Number of Shares Available for Grant Number of Shares Underlying Outstanding Options Weighted- Average Exercise Price Weighted- Average Remaining Contractual Term (in years) Aggregate Intrinsic Value (in thousands) Balances at December 31, 2019 125,353 2,474,152 $ 6.77 8.46 $ 19,974 Options authorized 3,468,198 — — Options granted (1,321,743 ) 1,321,743 16.83 Options exercised — (267,044 ) 3.21 10,212 Options expired 103,637 (103,637 ) 6.24 Options forfeited 231,101 (231,101 ) 9.51 Balances at December 31, 2020 2,606,546 3,194,113 $ 11.05 $ 8.46 $ 97,105 Shares exercisable, December 31, 2020 1,207,394 $ 6.24 $ 7.36 $ 42,515 Shares vested and expected to vest, December 31, 2020 3,194,113 $ 11.05 $ 8.46 $ 97,105
Summary of Stock Compensation Expense for Employees and Nonemployees by Function	The following table is a summary of stock compensation expense for employees and nonemployees by function (in thousands): Year Ended December 31, 2020 Year Ended December 31, 2019 Research and development $ 2,670 $ 2,191 General and administrative 2,314 1,350 Total stock-based compensation $ 4,984 $ 3,541
Schedule of Assumptions Used in Black-Scholes Valuation Model to Estimate Fair Value of Stock Options	The Company estimates the fair value of employee and nonemployee stock options using the Black-Scholes valuation model. The fair value of employee and nonemployee stock options is recognized on a straight-line basis over the requisite service period of the awards. The fair value of the Company’s stock options was estimated using the following assumptions for the years ended December 31, 2020 and 2019. Year Ended December 31, 2020 Year Ended December 31, 2019 Employee Nonemployee Employee Nonemployee Expected term 5.5 –6.3 years 6.0 years 5.5 –6.3 years 6.1 –10.0 years Expected volatility 82.1% – 83.8% 82.1% – 83.1% 81.9% – 83.0% 81.3% – 83.7% Risk-free interest rate 0.4% – 0.7% 0.4% – 0.7% 1.5% – 2.3% 1.4% – 2.4% Expected dividend yield 0 % 0 % 0 % 0 %

Net Loss Per Share Attributab_2

Net Loss Per Share Attributable to Common Stockholders (Tables)	12 Months Ended
	Dec. 31, 2020
Earnings Per Share [Abstract]
Computation of Basic and Diluted Net Loss Per Share	The following table sets forth the computation of basic and diluted net loss per share attributable to common stockholders (in thousands, except share and per share data): Year Ended December 31, 2020 Year Ended December 31, 2019 Numerator Net loss attributable to common stockholders $ (56,693 ) $ (49,306 ) Denominator Weighted-average shares outstanding 6,430,555 5,143,776 Less: Weighted-average shares subject to repurchase used in computing net loss per share attributable to common stockholders, basic and diluted — (1,216 ) Weighted-average shares outstanding used in computing net loss per share attributable to common stockholders, basic and diluted 6,430,555 5,142,560 Net loss per share attributable to common stockholders—basic and diluted $ (8.82 ) $ (9.59 )
Outstanding Shares of Potentially Dilutive Securities Excluded from Computation of Diluted Net Loss Per Share	The following outstanding shares of potentially dilutive securities were excluded from the computation of diluted net loss per share attributable to common stockholders for the period presented because including them would have been antidilutive: December 31, 2020 December 31, 2019 Redeemable convertible preferred stock — 7,375,631 Options to purchase common stock 3,194,113 2,474,152 Common stock warrants 98,669 68,669 Total 3,292,782 9,918,452

The Company - Additional Inform

The Company - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2020	Jan. 01, 2021	Dec. 31, 2019	Dec. 31, 2018
Organization And Nature Of Business [Line Items]
Outstanding shares of redeemable convertible preferred stock	0	0		7,375,631	7,375,631
Accumulated deficit	$ 135,679	$ 135,679		$ 79,025
Convertible Preferred Stock | Subsequent Event
Organization And Nature Of Business [Line Items]
Outstanding shares of redeemable convertible preferred stock			0
Common Stock
Organization And Nature Of Business [Line Items]
Number of shares sold and issued		9,660,000
Common Stock | Initial Public Offering
Organization And Nature Of Business [Line Items]
Number of shares sold and issued	9,660,000
Stock sold and issued price per share	$ 23	$ 23
Common stock issued upon underwriters exercise of overallotment option	1,260,000
Aggregate net proceeds, after deducting underwriting discounts and commissions and offering costs	$ 204,700
Number shares issued upon conversion of redeemable convertible preferred shares	11,575,984	11,575,984

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Details)	12 Months Ended
	Dec. 31, 2020USD ($)SegmentFinancial_Institution	Dec. 31, 2019USD ($)	Jan. 01, 2019USD ($)	Dec. 31, 2018USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of reportable segments | Segment	1
Number of operating segments | Segment	1
Number of financial institutions cash is held | Financial_Institution	2
Allowance for doubtful accounts	$ 0	$ 0
Write-offs relating to uncollectible accounts receivable	0	0
Impairments of long-lived assets	0	0
Stockholders' equity attributable to parent	256,387,000	(72,970,000)		$ (27,587,000)
Additional Paid-in Capital
Summary Of Significant Accounting Policies [Line Items]
Stockholders' equity attributable to parent	$ 392,063,000	6,054,000		2,438,000
Cumulative Effect, Period of Adoption, Adjustment
Summary Of Significant Accounting Policies [Line Items]
Stockholders' equity attributable to parent				$ 307,000
Cumulative Effect, Period of Adoption, Adjustment | Additional Paid-in Capital
Summary Of Significant Accounting Policies [Line Items]
Stockholders' equity attributable to parent		$ (39,000)
ASU 2014-09
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted	true
Change in accounting principle, accounting standards update, adoption date	Jan. 1, 2019
ASU 2014-09 | Cumulative Effect, Period of Adoption, Adjustment | Deferred Revenue
Summary Of Significant Accounting Policies [Line Items]
Stockholders' equity attributable to parent			$ 300,000
ASU 2018-07
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted	true
Change in accounting principle, accounting standards update, adoption date	Jan. 1, 2020
Change in accounting principle, accounting standards update, immaterial effect	false
ASU 2018-13
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted	true
Change in accounting principle, accounting standards update, adoption date	Jan. 1, 2020
ASU 2017-11
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted	true
Change in accounting principle, accounting standards update, adoption date	Jan. 1, 2020
Change in accounting principle, accounting standards update, immaterial effect	true
Minimum
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life	3 years
Maximum
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life	5 years
Maximum | ASU 2018-07 | Cumulative Effect, Period of Adoption, Adjustment | Additional Paid-in Capital
Summary Of Significant Accounting Policies [Line Items]
Stockholders' equity attributable to parent	$ 100,000

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Schedule of Concentration of Risk (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Revenue | Customer Concentration Risk
Concentration Risk [Line Items]
Concentration risk, percentage	95.00%	90.00%
Revenue | Customer Concentration Risk | Customer A
Concentration Risk [Line Items]
Concentration risk, percentage	95.00%	90.00%
Accounts Receivable | Credit Concentration Risk
Concentration Risk [Line Items]
Concentration risk, percentage	100.00%	100.00%
Accounts Receivable | Credit Concentration Risk | Customer A
Concentration Risk [Line Items]
Concentration risk, percentage	74.00%	64.00%
Accounts Receivable | Credit Concentration Risk | Customer B
Concentration Risk [Line Items]
Concentration risk, percentage	26.00%	36.00%

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Schedule of Concentration of Risk (Details) (Parenthetical) - Revenue - Customer Concentration Risk	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Concentration Risk [Line Items]
Concentration risk, percentage	95.00%	90.00%
Customer B | Maximum
Concentration Risk [Line Items]
Concentration risk, percentage	10.00%	10.00%

Summary of Significant Accoun_7

Summary of Significant Accounting Policies - Schedule of Revenues by Geographic Region (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Revenues From External Customers And Long Lived Assets [Line Items]
Revenues	$ 13,612	$ 6,986
Australia
Revenues From External Customers And Long Lived Assets [Line Items]
Revenues	0	7
Netherlands
Revenues From External Customers And Long Lived Assets [Line Items]
Revenues	742	26
Switzerland
Revenues From External Customers And Long Lived Assets [Line Items]
Revenues	12,897	6,287
United States
Revenues From External Customers And Long Lived Assets [Line Items]
Revenues	$ (27)	$ 666

Fair Value Measurements - Fair

Fair Value Measurements - Fair Value Hierarchy for Financial Assets and Financial Liabilities Measured at Fair Value on Recurring Basis (Details) - Fair Value on Recurring Basis - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	$ 276,726	$ 15,876
Liabilities	122	101
Derivative Liability
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liabilities	122	101
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	276,726	15,876
Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	276,726	15,876
Level 1 | Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	276,726	15,876
Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liabilities	122	101
Level 3 | Derivative Liability
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liabilities	$ 122	$ 101

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Assets, Level 1 to Level 2 transfers	$ 0	$ 0
Assets, Level 2 to Level 1 Transfers	0	0
Assets, Transfers into Level 3	0	0
Assets, Transfers out of Level 3	0	0
Liabilities, Level 1 to Level 2 transfers	0	0
Liabilities, Level 2 to Level 1 transfers	0	0
Liabilities, Transfers into Level 3	0	0
Liabilities, Transfers out of Level 3	$ 0	$ 0

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Changes in Fair Value of Level 3 Financial Instruments (Details) - Derivative Liability - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 101	$ 64
Change in fair value included in other income (expense), net	$ 21	$ 37
Fair Value, Recurring Basis, Unobservable Input Reconciliation, Liability, Gain (Loss), Statement of Income [Extensible List]	Other income (expense), net	Other income (expense), net
Ending Balance	$ 122	$ 101

Property and Equipment, Net - S

Property and Equipment, Net - Schedule of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Property Plant And Equipment [Line Items]
Total property and equipment	$ 8,917	$ 7,674
Less: Accumulated depreciation and amortization	(3,844)	(2,625)
Property and equipment, net	5,073	5,049
Machinery and Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	4,911	3,761
Leasehold Improvements
Property Plant And Equipment [Line Items]
Total property and equipment	2,527	2,405
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Total property and equipment	473	541
Office Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	101	98
Computer Equipment and Software
Property Plant And Equipment [Line Items]
Total property and equipment	366	306
Construction in Progress
Property Plant And Equipment [Line Items]
Total property and equipment	$ 539	$ 563

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Property Plant And Equipment [Abstract]
Depreciation expense	$ 1.4	$ 1

Accrued and Other Liabilities_3

Accrued and Other Liabilities, Current and Noncurrent - Summary of Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Payables And Accruals [Abstract]
Payroll and related	$ 3,437	$ 2,133
Accrued clinical and preclinical study costs	869	494
Consulting and professional	3,574	2,239
Other accrued expenses	491	481
Total accrued and other current liabilities	$ 8,371	$ 5,347

Accrued and Other Liabilities_4

Accrued and Other Liabilities, Current and Noncurrent - Summary of Other Liabilities, Noncurrent (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Payables And Accruals [Abstract]
Deferred rent, noncurrent	$ 1,237	$ 1,023
Other payable, noncurrent	615	542
Total other liabilities, noncurrent	$ 1,852	$ 1,565

Research and Collaboration Ar_3

Research and Collaboration Arrangements - Summary of Collaboration and License Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration and license revenue	$ 13,612	$ 6,986
Collaboration and License Revenue
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration and license revenue	13,612	6,986
Collaboration and License Revenue | uniQure
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration and license revenue	742	26
Collaboration and License Revenue | Benitec
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration and license revenue	0	7
Collaboration and License Revenue | CRF
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration and license revenue	0	0
Collaboration and License Revenue | Roche
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration and license revenue	12,897	6,287
Collaboration and License Revenue | CFF
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration and license revenue	(27)	118
Collaboration and License Revenue | AstraZeneca
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration and license revenue	$ 0	$ 548

Research and Collaboration Ar_4

Research and Collaboration Arrangements - Summary of Deferred Revenue (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Jan. 01, 2019
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	$ 19,812	$ 3,200	$ 19,467	$ 3,600
uniQure
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	4,396		5,137
Benitec
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	0		0
CRF
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	0		0
Roche
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	14,318		13,640
CFF
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	1,098		690
AstraZeneca
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	$ 0		$ 0

Research and Collaboration Ar_5

Research and Collaboration Arrangements - Additional Information (Details) - USD ($)	Aug. 19, 2019	Jan. 31, 2014	Dec. 31, 2020	Dec. 31, 2019	Jan. 01, 2020	Jan. 01, 2019
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue			$ 19,812,000	$ 19,467,000	$ 3,200,000	$ 3,600,000
uniQure
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue			4,396,000	5,137,000
Due from related parties			0	0
uniQure | uniQure Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaborative arrangement, initial research term		3 years
Upfront payments received		$ 200,000
Options to purchase ordinary shares		609,744
Ordinary shares vesting period		3 years
Fair value of options		$ 10,600,000
Revenue recognized			$ 0
uniQure | uniQure Agreement | Minimum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue, estimated performance period			2 years
uniQure | uniQure Agreement | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent payments entitled to receive for achievement of research and development milestones		$ 5,000,000
Deferred revenue, estimated performance period			3 years
Revenue recognized				100,000
uniQure | Amended uniQure Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue			$ 4,400,000	5,100,000
Fair value of non monetary consideration of intellectual property rights	$ 5,100,000
Revenue recognized			700,000	0
Aggregate amount of the transaction price remaining performance obligation			$ 4,400,000	$ 5,100,000
uniQure | Amended uniQure Agreement | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent payments entitled to receive for achievement of research and development milestones	$ 5,000,000

Research and Collaboration Ar_6

Research and Collaboration Arrangements - Additional Information (Details 1)	Jan. 31, 2014
uniQure | uniQure Agreement | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2021-01-01
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue, estimated performance period	4 years

Research and Collaboration Ar_7

Research and Collaboration Arrangements - Benitec Agreement - Additional Information (Details)	Jan. 24, 2017USD ($)	Mar. 31, 2019USD ($)	Nov. 30, 2014USD ($)Project	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration and license revenue				$ 13,363,000	$ 6,960,000
Benitec | Benitec Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of project variants | Project			3
Collaborative arrangement, initial research term			2 years
Collaborative arrangement extended increments term			6 months
Upfront payments received		$ 1,000,000	$ 500,000
Consideration received upon amendment of agreement	$ 500,000
Deferred revenue recognized as upfront payment	$ 500,000
Collaborative arrangement reimbursements for costs incurred		2,400,000
Change in collaborative arrangement transaction price					0
Collaboration and license revenue				0
Deferred revenue				0	0
Due from related parties				$ 0	0
Remaining performance obligations		$ 0
Benitec | Benitec Agreement | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaborative arrangement additional research term			5 years
Collaboration and license revenue					$ 100,000

Research and Collaboration Ar_8

Research and Collaboration Arrangements - CRF Agreement - Additional Information (Details) - CRF - CRF Agreement - USD ($)	1 Months Ended
	Nov. 30, 2015	Dec. 31, 2020	Dec. 31, 2019
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaborative arrangement, initial research term	2 years
Collaborative arrangement contribution upon development milestone	$ 2,500,000
Collaborative arrangement additional research term	3 years
Deferred revenue		$ 0	$ 0
Due from related parties		$ 0	$ 0

Research and Collaboration Ar_9

Research and Collaboration Arrangements - Roche Agreement - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Nov. 30, 2017	Dec. 31, 2020	Dec. 31, 2019
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized		$ 13,363	$ 6,960
Roche | 2017 Roche Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payments received	$ 21,000
Contingent payment for each nominated product beyond the first three	1,000
Maximum contingent payment upon exercise of the option to convert a product	30,000
Development milestone payments	223,000
Development milestone payments relating to Choroideremia	86,000
Sales-based milestone payments	123,000
Estimated reimbursements for research and development services	$ 10,700
Collaborative arrangement adjustment to transaction price		17,000	4,500
Collaborative arrangement increase of project scope and expected reimbursable costs		7,000
Collaborative arrangement addition of variable consideration		10,000
Increase (decrease) in revenue recognized		4,600	(1,600)
Revenue recognized		12,900	6,300
Deferred revenue		14,300	13,600
Accounts receivable		1,100	600
Remaining performance obligations		$ 25,800	$ 21,600

Research and Collaboration A_10

Research and Collaboration Arrangements - Roche Agreement - Additional Information (Details 1) - Roche - 2017 Roche Agreement - Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2021-01-01	Dec. 31, 2020
Minimum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue, estimated performance period	3 years
Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue, estimated performance period	4 years

Research and Collaboration A_11

Research and Collaboration Arrangements - CFF - Additional Information (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Sep. 30, 2016USD ($)Installment	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized		$ 13,363	$ 6,960
Estimated fair value of derivative liability		100	100
CFF
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaborative arrangement total amount of award	$ 3,500
Collaborative arrangement achieved milestones		1,300	900
Expected payments percentage to equal actual award received	600.00%
Number of installments to pay actual award | Installment	3
Actual award payment period upon first commercial sale of product	4 years
Continuous period where the Company ceases to develop or commercialize any product	180 days
Collaborative arrangement transaction price adjustment		400
Revenue recognized			100
Deferred revenue		1,100	700
Accounts receivable		400	400
Remaining performance obligations		1,100	700
Estimated fair value of derivative liability		100	$ 100
CFF | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Future sales-based milestone payments percentage	300.00%
Royalties percentage on actual award received	600.00%
Revenue recognized		$ (100)

Research and Collaboration A_12

Research and Collaboration Arrangements - CFF - Additional Information (Details 1) - CFF - Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2021-01-01	Dec. 31, 2020
Minimum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue, estimated performance period	2 years
Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue, estimated performance period	3 years

Research and Collaboration A_13

Research and Collaboration Arrangements - AstraZeneca - Additional Information (Details) - AstraZeneca	1 Months Ended	12 Months Ended
	Dec. 31, 2017USD ($)Project	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of projects vector variants granted option to license to development and commercialization | Project	3
Collaborative arrangement, initial research term	12 months
Collaborative arrangement, extend additional research term	6 months
Collaborative arrangement, projects expires on conclusion of research phase	12 months
Upfront payments received	$ 1,500,000
Revenue recognized		$ 0	$ 500,000
Deferred revenue		0	0
Due from related parties		$ 0	$ 0

License Arrangements - Addition

License Arrangements - Additional Information (Details) - UC Agreements - USD ($)	1 Months Ended		12 Months Ended
	Jan. 31, 2019	Jan. 31, 2016	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Research And Development Arrangement Contract To Perform For Others [Line Items]
Non-refundable license fee payable upon execution					$ 5,000
Non-refundable license fee payable upon execution every year thereafter until sales of a licensed product and payment of royalties					5,000
Amount obligated to pay upon closing first qualified financing at option of UC Regents					$ 50,000
Percentage of equity interest upon closing first qualified financing at option of UC Regents					3.00%
Percentage of equity interests issued as common stock		6.00%
Amount paid non-refundable license fee	$ 50,000
Non-refundable license fee payable annually	$ 5,000
Incurred expenses			$ 200,000	$ 300,000
Maximum
Research And Development Arrangement Contract To Perform For Others [Line Items]
Development milestones			3,100,000
Minimum
Research And Development Arrangement Contract To Perform For Others [Line Items]
Annual royalty			$ 100,000

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details) - USD ($)	1 Months Ended					12 Months Ended
	Dec. 31, 2019	May 31, 2019	Nov. 30, 2018	Oct. 31, 2018	Jan. 31, 2016	Dec. 31, 2020	Dec. 31, 2019
Commitment And Contingencies [Line Items]
Lease extended term date		Dec. 31, 2029			Mar. 31, 2023
Tenant improvement allowance	$ 400,000		$ 200,000
Lease extended term period				2026-09
Lease initial term				87 months
Lease option to renew term description						The new lease has an initial term of 87 months beginning on the rent commencement date with the option to renew the lease for one additional term of five years.
Lease existence of option to renew term				true
Lease renewal term				5 years
Annual rent		$ 1,000,000				$ 3,000,000	$ 1,300,000
Percentage of annual rent escalation		3.00%
Deferred rent	1,200,000					1,600,000	1,200,000
Other Assets [Member]
Commitment And Contingencies [Line Items]
Security deposit	$ 600,000					600,000	$ 600,000
Accrued And Other Current Liabilities
Commitment And Contingencies [Line Items]
Accrual related to estimated settlement of claim						$ 1,400,000
Minimum
Commitment And Contingencies [Line Items]
Tenant improvement allowance		$ 1,600,000

Commitments and Contingencies_2

Commitments and Contingencies - Summary of Future Minimum Commitments Under Lease Contracts (Details) $ in Thousands	Dec. 31, 2020USD ($)
Commitments And Contingencies Disclosure [Abstract]
2021	$ 2,956
2022	3,035
2023	3,115
2024	3,205
2025	3,258
2026 and beyond	11,820
Total	$ 27,389

Income Taxes - Schedule of Effe

Income Taxes - Schedule of Effective Tax Rate of Income Tax Expense (Benefit) Differs from Federal Statutory Rate (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	21.00%	21.00%
Research tax credit	2.80%	5.20%
Permanent differences	(1.70%)	(0.70%)
Valuation allowance	(22.10%)	(25.50%)
Provision for income taxes	0.00%	0.00%

Income Taxes - Schedule of Tax

Income Taxes - Schedule of Tax Effects of Temporary Differences to Significant Components of Deferred Taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred Tax Assets
Net operating loss carryforwards	$ 21,132	$ 10,638
Other accrued liabilities	422	336
Deferred revenue	3,111	2,698
Research tax credits	6,018	4,457
Stock compensation	1,151	554
Intangible asset basis	977	1,591
Total deferred tax assets	32,811	20,274
Less: valuation allowance	(32,811)	(20,274)
Net deferred tax assets	$ 0	$ 0

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Operating Loss Carryforwards [Line Items]
Increase in valuation allowance, deferred tax asset	$ 12,500,000	$ 12,500,000
Net operating loss carryforwards	100,600,000	50,700,000
NOLS expected to expire unutilized	900,000
Accrued interest and penalties related to unrecognized tax benefits	$ 0	0
CARES Act, relief description	The CARES Act changed certain provisions of the Tax Act. Under the CARES Act, NOLs arising in taxable years beginning after December 31, 2017 and before January 1, 2021 may be carried back to each of the five taxable years preceding the tax year of such loss, but NOLs arising in taxable years beginning after December 31, 2020 may not be carried back. In addition, the CARES Act eliminated the limitation on the deduction of NOLs to 80% of current year taxable income for taxable years beginning before January 1, 2021 and increased the amount of interest expense that may be deducted to 50% of adjusted taxable income for taxable years beginning in 2019 or 2020.
Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 9,500,000
Net operating loss carryforwards, expiration year	2037
Net operating loss carryforward indefinitely	$ 91,100,000
Research and development credit carryforward	$ 5,700,000	3,300,000
Tax credit carryforward beginning expiration year	2035
State
Operating Loss Carryforwards [Line Items]
Research and development credit carryforward	$ 4,300,000	$ 3,300,000
Tax credit carryforward, expiration description	indefinitely

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Beginning and Ending Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Balance at beginning of year	$ 1,566	$ 659
Additions for current year tax positions	1,729	907
Additions for tax positions of prior years	72	0
Balance at end of year	$ 3,367	$ 1,566

Redeemable Convertible Prefer_3

Redeemable Convertible Preferred Stock - Additional Information (Details) - USD ($)	1 Months Ended		12 Months Ended
	Jun. 30, 2020	Apr. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Temporary Equity [Line Items]
Preferred stock, authorized			0	7,375,638
Preferred stock, par value per share			$ 0.0001	$ 0.0001
Temporary equity, shares issued			0	7,375,631
Net proceeds from issuance of convertible preferred stock			$ 72,468,000	$ (10,000)
Outstanding shares of redeemable convertible preferred stock			0	7,375,631	7,375,631
Redeemable convertible preferred stock, conversion basis			one-for-one basis
Redeemable convertible preferred stock, terms of conversion			Prior to the issuance of Series C redeemable convertible preferred stock in April 2020, shares of redeemable convertible preferred stock would automatically be converted into shares of common stock at the then effective conversion price for such share, immediately prior to either: (i) the completion of an underwritten public offering of the Company’s common stock at a price of at least 1.5 times the original Series B issuance price for any initial public offering consummated at any time prior to the first anniversary of the Series B original issuance date, or 1.25 times the original Series B issuance price for any such IPO thereafter and that provides at least $30.0 million of gross proceeds to the Company or (ii) the conversion by the holders of redeemable convertible preferred stock, which requires the vote of the holders of a majority of the then outstanding shares of redeemable convertible preferred stock, voting together as a single class on an as-converted to common stock basis. After the issuance of the Series C redeemable convertible preferred stock in April 2020, shares of redeemable convertible preferred stock would automatically be converted into shares of common stock at the then effective conversion price for such share, immediately prior to either: (i) the completion of an underwritten public offering of the Company’s common stock at a price of at least 1.25 times the original Series C issuance price, as adjusted for any stock splits, stock dividends, combinations, subdivisions, recapitalizations or the like that provides at least $50.0 million of gross proceeds to the Company or (ii) the conversion by the holders of redeemable convertible preferred stock, which requires the vote of the holders of a majority of the then outstanding shares of redeemable convertible preferred stock voting together as a single class on an as-converted to common stock basis; provided, however that any automatic conversion of the Series C and Series B redeemable convertible preferred stock under (ii) shall require the consent of the holders of a majority of Series C and Series B redeemable convertible preferred stock then outstanding voting together as a single class on an as-converted to common stock basis.
Funding Agreement Product | CFF
Temporary Equity [Line Items]
Additional funding upon acceptance of investigational New Drug application		$ 4,000,000
Series C Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Temporary equity, shares issued	4,200,353	4,200,353
Temporary equity, shares issued price per share	$ 18	$ 18
Net proceeds from issuance of convertible preferred stock	$ 72,500,000	$ 72,500,000
Minimum gross proceeds from IPO for automatic conversion to common stock			$ 50,000,000
Series C Redeemable Convertible Preferred Stock | Funding Agreement Product | CFF
Temporary Equity [Line Items]
Investment in redeemable convertible preferred stock financing		$ 10,000,000
Series B Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Preferred stock, authorized				5,154,639
Temporary equity, shares issued				5,154,632
Temporary equity, shares issued price per share				$ 17.46
Outstanding shares of redeemable convertible preferred stock				5,154,632
Minimum gross proceeds from IPO for automatic conversion to common stock			30,000,000
Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Redeemable convertible preferred stock dividend declared			$ 0

Redeemable Convertible Prefer_4

Redeemable Convertible Preferred Stock - Summary of Redeemable Convertible Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Temporary Equity [Line Items]
Shares Authorized	0	7,375,638
Shares Issued	0	7,375,631
Shares Outstanding	0	7,375,631	7,375,631
Liquidation Value	$ 0	$ 108,596
Proceeds Net of Issuance Cost	$ 0	$ 102,980	$ 102,980
Series A Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Shares Authorized		909,312
Original Issuance Price		$ 7.70
Shares Issued		909,312
Shares Outstanding		909,312
Liquidation Value		$ 7,001
Proceeds Net of Issuance Cost		$ 6,960
Series A-1 Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Shares Authorized		1,311,687
Original Issuance Price		$ 8.84
Shares Issued		1,311,687
Shares Outstanding		1,311,687
Liquidation Value		$ 11,595
Proceeds Net of Issuance Cost		$ 11,548
Series B Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Shares Authorized		5,154,639
Original Issuance Price		$ 17.46
Shares Issued		5,154,632
Shares Outstanding		5,154,632
Liquidation Value		$ 90,000
Proceeds Net of Issuance Cost		$ 84,472

Common Stock - Additional Infor

Common Stock - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2015
Class Of Stock [Line Items]
Common stock, shares authorized	300,000,000	50,000,000
Common stock, par value	$ 0.0001	$ 0.0001
Common stock voting rights	one
Common stock dividend	$ 0	$ 0
Restricted Common Stock
Class Of Stock [Line Items]
Shares vested upon issuance			4,473,374
Common stock remained subject to repurchase	0	0
Founder
Class Of Stock [Line Items]
Stock issued			4,710,060

Common Stock - Schedule of Comm

Common Stock - Schedule of Common Share Reserved for Future Issuance (Details) - shares	Dec. 31, 2020	Dec. 31, 2019
Class Of Stock [Line Items]
Common stock reserved	6,151,665	10,043,805
Redeemable Convertible Preferred Stock
Class Of Stock [Line Items]
Common stock reserved	0	7,375,631
2015 Equity Incentive Plan
Class Of Stock [Line Items]
Common stock reserved	0	125,353
2020 Equity Incentive Plan
Class Of Stock [Line Items]
Common stock reserved	2,606,546	0
Employee Stock Share Purchase Plan
Class Of Stock [Line Items]
Common stock reserved	252,337	0
Options Issued and Outstanding
Class Of Stock [Line Items]
Common stock reserved	3,194,113	2,474,152
Common Stock Warrants
Class Of Stock [Line Items]
Common stock reserved	98,669	68,669

Stock-based Compensation - Addi

Stock-based Compensation - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares reserved for issuance	6,151,665	6,151,665	10,043,805
Number of shares available for grant	2,606,546	2,606,546	125,353
Number of options outstanding	3,194,113	3,194,113	2,474,152
Number of stock options granted		1,321,743
Fair value of options vested		$ 5,000	$ 2,200
Unrecognized stock-based compensation of unvested options	$ 19,400	$ 19,400
Unrecognized stock-based compensation, expected to be recognized over weighted-average period		3 years 1 month 6 days
Stock-based compensation expense		$ 4,984	$ 3,541
Expected dividend yield		0.00%	0.00%
Employee
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of stock options granted		1,267,743	1,101,840
Weighted-average grant date fair value		$ 11.84	$ 8.46
Stock-based compensation expense		$ 4,600	$ 2,800
Expected dividend yield		0.00%	0.00%
Nonemployee
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of stock options granted		54,000	35,000
Weighted-average grant date fair value		$ 11.05	$ 8.43
Stock-based compensation expense		$ 400	$ 700
Expected dividend yield		0.00%	0.00%
2020 Incentive Award Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares reserved for issuance	2,606,546	2,606,546
Number of shares available for grant	2,606,546	2,606,546
2020 Incentive Award Plan | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options exercisable period		10 years
2015 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares reserved for issuance	0	0	125,353
Number of shares available for grant	0	0
Number of options outstanding	3,194,113	3,194,113
Minimum percentage of outstanding shares held by individual		10.00%
Exercise price as percentage of fair market value, minimum		110.00%
2015 Equity Incentive Plan | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options exercisable period		10 years
2020 ESPP
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares authorized and reserved for issuance	252,337	252,337
Employee purchase price of common stock as percentage of fair market value	85.00%
Common stock overlapping offering period	24 months
Duration of purchase period	6 months
Percentage of maximum contributions of employee's eligible compensation	15.00%	15.00%

Stock-based Compensation - Summ

Stock-based Compensation - Summary of Stock Options Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Shares Available for Grant, Beginning Balance	125,353
Number of Shares Available for Grant, Options authorized	3,468,198
Number of Shares Available for Grant, Options granted	(1,321,743)
Number of Shares Available for Grant, Options expired	103,637
Number of Shares Available for Grant, Options forfeited	231,101
Number of Shares Available for Grant, Ending Balance	2,606,546	125,353
Number of Shares Underlying Outstanding Options, Beginning Balance	2,474,152
Number of Shares Underlying Outstanding Options, granted	1,321,743
Number of Shares Underlying Outstanding Options, exercised	(267,044)
Number of Shares Underlying Outstanding Options, expired	(103,637)
Number of Shares Underlying Outstanding Options, forfeited	(231,101)
Number of Shares Underlying Outstanding Options, Ending Balance	3,194,113	2,474,152
Number of Shares Underlying Outstanding Options, Shares exercisable, December 31, 2020	1,207,394
Number of Shares Underlying Outstanding Options, Shares vested and expected to vest, December 31, 2020	3,194,113
Weighted-Average Exercise Price, Beginning Balance	$ 6.77
Options granted	16.83
Options exercised	3.21
Options expired	6.24
Options forfeited	9.51
Weighted-Average Exercise Price, Ending Balance	11.05	$ 6.77
Weighted-Average Exercise Price, Shares exercisable, December 31,2020	6.24
Weighted-Average Exercise Price, Shares vested and expected to vest, December 31,2020	$ 11.05
Weighted-Average Remaining Contractual Term (in years), Outstanding	8 years 5 months 15 days	8 years 5 months 15 days
Weighted-Average Remaining Contractual Term (in years), Shares exercisable, December 31,2020	7 years 4 months 9 days
Weighted-Average Remaining Contractual Term (in years), Shares vested and expected to vest, December 31,2020	8 years 5 months 15 days
Aggregate Intrinsic Value, Outstanding Balance	$ 97,105	$ 19,974
Aggregate Intrinsic Value, Options exercised	10,212
Aggregate Intrinsic Value, Shares exercisable, December 31,2020	42,515
Aggregate Intrinsic Value, Shares vested and expected to vest, December 31,2020	$ 97,105

Stock-based Compensation - Su_2

Stock-based Compensation - Summary of Stock Compensation Expense for Employees and Nonemployees by Function (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	$ 4,984	$ 3,541
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	2,670	2,191
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	$ 2,314	$ 1,350

Stock-based Compensation - Sche

Stock-based Compensation - Schedule of Assumptions Used in Black-Scholes Valuation Model to Estimate Fair Value of Stock Options (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected dividend yield	0.00%	0.00%
Employee
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility, minimum	82.10%	81.90%
Expected volatility, maximum	83.80%	83.00%
Risk-free interest rate, minimum	0.40%	1.50%
Risk-free interest rate, maximum	0.70%	2.30%
Expected dividend yield	0.00%	0.00%
Nonemployee
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term	6 years
Expected volatility, minimum	82.10%	81.30%
Expected volatility, maximum	83.80%	83.70%
Risk-free interest rate, minimum	0.40%	1.40%
Risk-free interest rate, maximum	0.70%	2.40%
Expected dividend yield	0.00%	0.00%
Minimum | Employee
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term	5 years 6 months	5 years 6 months
Minimum | Nonemployee
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term		6 years 1 month 6 days
Maximum | Employee
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term	6 years 3 months 18 days	6 years 3 months 18 days
Maximum | Nonemployee
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term		10 years

Common Stock Warrants - Additio

Common Stock Warrants - Additional Information (Details) - USD ($)	1 Months Ended	12 Months Ended
	May 31, 2018	Dec. 31, 2020	Dec. 31, 2018	Dec. 31, 2016
Class Of Warrant Or Right [Line Items]
Number of warrants issued	23,669	30,000		45,000
Exercise price	$ 3.19	$ 18		$ 1.14
Additional paid-in-capital upon issuance of the warrant	$ 100,000
Proceeds from offering of securities in a private placement				$ 25,000,000
Warrant expiration year	2025	2027		2023
Warrant vesting period		4 years
Maximum
Class Of Warrant Or Right [Line Items]
Fair value of warrant shares		$ 100,000	$ 100,000
Series B Financing
Class Of Warrant Or Right [Line Items]
Warrant shares exercisable			15,000
Private Placement
Class Of Warrant Or Right [Line Items]
Warrant shares exercisable				15,000
Initial Public Offering
Class Of Warrant Or Right [Line Items]
Warrant shares exercisable		30,000		30,000

Net Loss Per Share Attributab_3

Net Loss Per Share Attributable to Common Stockholders - Computation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Numerator
Net loss attributable to common stockholders	$ (56,693)	$ (49,306)
Denominator
Weighted-average shares outstanding	6,430,555	5,143,776
Less: Weighted-average shares subject to repurchase used in computing net loss per share attributable to common stockholders, basic and diluted	0	(1,216)
Weighted-average shares outstanding used in computing net loss per share attributable to common stockholders, basic and diluted	6,430,555	5,142,560
Net loss per share attributable to common stockholders, basic and diluted	$ (8.82)	$ (9.59)

Net Loss Per Share Attributab_4

Net Loss Per Share Attributable to Common Stockholders - Outstanding Potentially Dilutive Securities Excluded From Computation of Diluted Net Loss Per Share (Details) - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities	3,292,782	9,918,452
Redeemable Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities	0	7,375,631
Options to Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities	3,194,113	2,474,152
Common Stock Warrants
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities	98,669	68,669

Derivative Liability - Addition

Derivative Liability - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Derivative [Line Items]
Change in control discount rate	14.00%
Estimated fair value of derivative liability	$ 100,000	$ 100,000
Minimum
Derivative [Line Items]
Change of control payment	$ 0
Change in control event percentage	3.40%
Maximum
Derivative [Line Items]
Change of control payment	$ 10,600,000
Change in control event percentage	12.30%

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Details)	Dec. 11, 2020$ / sharesshares	Apr. 30, 2020shares	Apr. 30, 2019USD ($)agreement	Dec. 31, 2020USD ($)shares	Dec. 31, 2019USD ($)shares	Dec. 31, 2018shares
Related Party Transaction [Line Items]
Temporary equity, shares outstanding				0	7,375,631	7,375,631
Shares sold, price per share | $ / shares	$ 23
Common stock, shares, outstanding				26,681,983	5,178,955
uniQure
Related Party Transaction [Line Items]
Related party closing date				Jun. 17, 2020
Pfizer
Related Party Transaction [Line Items]
Common stock, shares, outstanding				1,666,658
Pfizer | Common Stock
Related Party Transaction [Line Items]
Number of shares acquired	25,000
Shares converted, number of common stock issued	1,641,658
Pfizer | Redeemable Convertible Preferred Stock
Related Party Transaction [Line Items]
Temporary equity, shares outstanding					1,474,992
Convertible preferred stock, shares converted	1,641,658
Number of shares acquired		166,666
David Schaffer, PhD, | Consulting Services
Related Party Transaction [Line Items]
Related party expenses | $				$ 93,000	$ 52,000
UC Regents | SRAs
Related Party Transaction [Line Items]
Related party expenses | $				500,000	300,000
Number of sponsored research agreements entered | agreement			2
Agreement term			3 years
Amount due to related party | $			$ 1,500,000
Payment to related party | $				1,000,000	400,000
Accounts payable, related party | $				$ 200,000	$ 300,000
Ignite
Related Party Transaction [Line Items]
Related party closing date				Oct. 18, 2019

401(K) Plan - Additional Inform

401(K) Plan - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
2014 Plan
Defined Contribution Plan Disclosure [Line Items]
Employer contribution	$ 0.4	$ 0.3