Akouos (AKUS) 10-K 29 Mar 21 2020 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2020	Mar. 22, 2021	Jun. 30, 2020
Cover [Abstract]
Document Type	10-K
Entity Registrant Name	AKOUOS, INC.
Document Period End Date	Dec. 31, 2020
Entity Central Index Key	0001722271
Title of 12(b) Security	Common Stock, $0.0001 Par Value per Share
Trading Symbol	AKUS
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Current Fiscal Year End Date	--12-31
Entity Well Known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Interactive Data Current	Yes
ICFR Auditor Attestation Flag	false
Entity Common Stock, Shares Outstanding		34,402,023
Entity Public Float			$ 519,288,728
Amendment Flag	false
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Entity Shell Company	false
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 68,932	$ 25,078
Marketable securities	239,078
Prepaid expenses and other current assets	2,626	1,061
Total current assets	310,636	26,139
Property and equipment, net	15,388	11,492
Operating lease right-of-use assets	5,964	6,214
Restricted cash	1,317	1,317
Other assets	45
Total assets	333,350	45,162
Current liabilities:
Accounts payable	895	339
Accrued expenses and other current liabilities	8,699	4,962
Operating lease liabilities	1,115	641
Total current liabilities	10,709	5,942
Operating lease liabilities, net of current portion	12,027	13,143
Other long‑term liabilities		188
Total liabilities	22,736	19,273
Commitments and contingencies (See Note 13)
Convertible preferred stock (Series Seed, Seed 1, A, and B), $0.0001 par value; no shares authorized at December 31, 2020 and 178,349,005 shares authorized at December 31, 2019; no shares issued and outstanding at December 31, 2020 and 178,349,005 shares issued and outstanding at December 31, 2019		58,690
Stockholders’ equity (deficit):
Common stock, $0.0001 par value; 200,000,000 shares authorized at December 31, 2020 and 235,000,000 shares authorized at December 31, 2019; 34,383,719 shares issued and outstanding at December 31, 2020 and 997,165 shares issued and outstanding at December 31, 2019	3
Additional paid-in capital	392,322	323
Accumulated other comprehensive income	13
Accumulated deficit	(81,724)	(33,124)
Total stockholders’ equity (deficit)	310,614	(32,801)
Total liabilities, convertible preferred stock and stockholders’ equity	$ 333,350	$ 45,162

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
CONSOLIDATED BALANCE SHEETS
Convertible preferred stock, Par value	$ 0.0001	$ 0.0001
Convertible preferred stock, Shares authorized	0	178,349,005
Convertible preferred stock, Shares issued	0	178,349,005
Convertible preferred stock, Shares outstanding	0	178,349,005
Convertible preferred stock, Liquidation preference		$ 57,685
Preferred stock, Par value	$ 0.0001	$ 0.0001
Preferred stock, Shares authorized	5,000,000	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	200,000,000	235,000,000
Common stock, Shares issued	34,383,719	997,165
Common stock, Shares outstanding	34,383,719	997,165

CONSOLIDATED STATEMENT OF OPERA

CONSOLIDATED STATEMENT OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Operating expenses:
Research and development	$ 34,297	$ 20,473
General and administrative	14,583	3,410
Total operating expenses	48,880	23,883
Loss from operations	(48,880)	(23,883)
Other income (expense):
Change in fair value of preferred stock tranche liability		(2,260)
Interest income	567	413
Other income (expense), net	(287)	(11)
Total other income (expense), net	280	(1,858)
Net loss	$ (48,600)	$ (25,741)
Net loss per share attributable to common stockholders, basic and diluted	$ (2.77)	$ (42.49)
Weighted-average common shares outstanding, basic and diluted	17,550,847	605,824
Unrealized gain on marketable securities	$ 13
Total other comprehensive income	13
Total comprehensive loss	$ (48,587)	$ (25,741)

CONSOLIDATED STATEMENTS OF CONV

CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY/(DEFICIT) - USD ($) $ in Thousands	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income	Accumulated Deficit	Total
Balances at beginning of period at Dec. 31, 2018					$ 27,647
Convertible preferred stock, Shares outstanding, beginning of period at Dec. 31, 2018					103,015,190
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of convertible preferred stock, net					$ 25,016
Issuance of convertible preferred stock, net (in shares)					75,333,815
Reclassification of preferred stock tranche liability upon settlement					$ 6,027
Balances at end of period at Dec. 31, 2019					$ 58,690
Convertible preferred stock, Shares outstanding, end of period at Dec. 31, 2019					178,349,005
Balances at beginning of period at Dec. 31, 2018		$ 154		$ (7,383)	$ (7,229)
Balances at beginning of period (in shares) at Dec. 31, 2018	935,887
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options		8			8
Issuance of common stock upon exercise of stock options (in shares)	9,365
Issuance of restricted common stock upon early exercise of stock options (in shares)	51,913
Vesting of restricted common stock from early-exercised stock options and sales of restricted common stock		23			23
Stock-based compensation expense		138			138
Net loss				(25,741)	(25,741)
Balances at end of period at Dec. 31, 2019		323		(33,124)	(32,801)
Balances at end of period (in shares) at Dec. 31, 2019	997,165
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of convertible preferred stock, net					$ 104,837
Issuance of convertible preferred stock, net (in shares)					221,399,223
Conversion of convertible preferred stock to common stock					$ (163,527)
Conversion of convertible preferred stock, shares					(399,748,228)
Convertible preferred stock, Shares outstanding, end of period at Dec. 31, 2020					0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options		66			$ 66
Issuance of common stock upon exercise of stock options (in shares)	41,882
Vesting of restricted common stock from early-exercised stock options and sales of restricted common stock		79			79
Stock-based compensation expense		4,495			4,495
Issuance of common stock upon completion of initial public offering, net of commissions, underwriting discounts and offering costs	$ 1	223,849			223,850
Issuance of common stock upon completion of initial public offering, net of commissions, underwriting discounts and offering costs, shares	14,375,000
Conversion of Stock, Amount Issued	$ 2	163,510			163,512
Conversion of Stock, Shares Issued	18,969,672
Net loss				(48,600)	(48,600)
Unrealized gain on marketable securities			$ 13		13
Balances at end of period at Dec. 31, 2020	$ 3	$ 392,322	$ 13	$ (81,724)	$ 310,614
Balances at end of period (in shares) at Dec. 31, 2020	34,383,719

CONSOLIDATED STATEMENTS OF CO_2

CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS EQUITY/DEFICIT (Parenthetical) - USD ($) $ in Thousands	1 Months Ended		12 Months Ended
	Feb. 29, 2020	Sep. 30, 2019	Dec. 31, 2020	Dec. 31, 2019
Issuance cost			$ 3,419
Series A Preferred Stock [Member]
Issuance cost		$ 100		$ 34
Series B Preferred Stock [Member]
Issuance cost	$ 200		$ 228

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Cash flows from operating activities:
Net loss	$ (48,600)	$ (25,741)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	1,925	361
Net amortization of premiums and accretion of discounts on marketable securities	281
Amortization of operating lease right-of-use assets	250	336
Change in fair value of preferred stock tranche liability		2,260
Stock-based compensation expense	4,495	138
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,565)	(742)
Accounts payable	583	(76)
Other assets	(45)
Operating lease liabilities	(642)	671
Accrued expenses and other current liabilities	2,739	3,283
Other long-term liabilities	(188)
Net cash used in operating activities	(40,767)	(19,510)
Cash flows from investing activities:
Purchases of property and equipment	(4,533)	(3,440)
Purchases of marketable securities	(268,846)
Maturities of marketable securities	29,500
Net cash used in investing activities	(243,879)	(3,440)
Cash flows from financing activities:
Proceeds from issuance of convertible preferred stock, net of issuance costs paid	104,837	25,016
Proceeds from initial public offering of common stock	227,269
Payments of finance lease obligations	(253)	(99)
Proceeds from exercise of stock options and sale of restricted common stock	66	88
Payments of initial public offering costs	(3,419)
Net cash provided by financing activities	328,500	25,005
Net increase in cash, cash equivalents and restricted cash	43,854	2,055
Cash, cash equivalents and restricted cash at beginning of period	26,395	24,340
Cash, cash equivalents and restricted cash at end of period	70,249	26,395
Supplemental cash flow information:
Cash paid for interest	6	10
Supplemental disclosure of non‑cash investing and financing information:
Purchases of property and equipment included in accounts payable and accrued expenses	2,071	783
Settlement of preferred stock tranche liability		6,027
Operating lease liabilities arising from obtaining right-of-use assets	6,600	6,550
Leasehold improvements paid by lessor		6,563
Reconciliation of cash, cash equivalents and restricted cash:
Total cash, cash equivalents and restricted cash shown in the statement of cash flows	$ 70,249	$ 26,395

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	12 Months Ended
	Dec. 31, 2020
Nature of the Business and Basis of Presentation
Nature of the Business and Basis of Presentation	1. Nature of the Business and Basis of Presentation Akouos, Inc., together with its consolidated subsidiary (the “Company” or “Akouos”), is a precision genetic medicine company dedicated to its mission of developing gene therapies with the potential to restore, improve, and preserve high‑acuity physiologic hearing for individuals who live with disabling hearing loss worldwide. The Company was formed as a limited liability corporation under the laws of the Commonwealth of Massachusetts in March 2016 under the name Akouos, LLC and converted into a corporation under the laws of the State of Delaware in November 2016 under the name Akouos, Inc. 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, dependence on key personnel, protection of proprietary technology, compliance with government regulations, the impact of the COVID‑19 pandemic, and the ability to secure additional capital to fund operations. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel and infrastructure, and extensive compliance‑reporting capabilities. Even if the Company’s development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales. On June 18, 2020, the Company effected a one-for-21.073 reverse stock split of its issued and outstanding shares of common stock and a proportional adjustment to the existing conversion ratios for each series of the Company’s convertible preferred stock. Accordingly, all share and per share amounts for all periods presented in the accompanying consolidated financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect this reverse stock split and adjustment of the preferred stock conversion ratios. On June 30, 2020, the Company completed its initial public offering of its common stock and issued and sold 14,375,000 shares of its common stock, at a public offering price of $17.00 per share, for gross proceeds of $244.4 million, or net proceeds of $223.8 million after deducting underwriting discounts, commissions, and offering expenses. Upon the closing of the Company’s initial public offering on June 30, 2020, all shares of convertible preferred stock automatically converted into 18,969,672 shares of common stock. The accompanying consolidated financial statements have been prepared on the basis of continuity of operations, realization of assets, and the satisfaction of liabilities and commitments in the ordinary course of business. The Company has primarily funded its operations with proceeds from sales of convertible preferred stock and proceeds from the Company’s initial public offering of common stock. The Company has incurred losses and negative cash flows from operations since its inception. As of December 31, 2020, the Company had an accumulated deficit of $81.7 million. The Company expects that its operating losses and negative cash flows will continue for the foreseeable future. The Company expects that its existing cash, cash equivalents and marketable securities will be sufficient to fund its operating expenses and capital expenditure requirements for at least 12 months from the issuance date of the consolidated financial statements. The Company expects to seek additional funding through public and private equity financings, debt financings, collaborations, licensing arrangements, and/or strategic alliances. The Company may not be able to obtain financing on acceptable terms, or at all, and the Company may not be able to enter into collaborations or other such arrangements. The terms of any financing may adversely affect the holdings or the rights of the Company’s stockholders. If the Company is unable to obtain funding, the Company could be forced to delay, reduce, or eliminate some or all of its research and development programs, product portfolio expansion, or commercialization efforts, which could adversely affect its business prospects. Although management continues to pursue these plans, there is no assurance that the Company will be successful in obtaining sufficient funding on terms acceptable to the Company to fund continuing operations, if at all. Impact of the COVID‑19 Pandemic The COVID-19 pandemic, which began in December 2019 and has spread worldwide, has caused many governments to implement measures to slow the spread of the pandemic. The pandemic and government measures taken in response have had and will continue to have a significant impact, both direct and indirect, on businesses and commerce, as worker shortages have occurred, supply chains have been disrupted, and facilities and production have been suspended. The Company has experienced manufacturing delays at its third-party manufacturer, including delays related to the COVID-19 pandemic, and may experience additional delays in the future, which could further delay product development timelines. The future progression of the pandemic and its continued effects on the Company’s business and operations are uncertain. The COVID-19 pandemic may affect the Company’s ability to initiate and complete nonclinical studies, delay the initiation of its planned clinical trial or future clinical trials, disrupt regulatory activities, or have other adverse effects on its business and operations. In particular, the Company and its third-party manufacturers and contract research organizations may face additional disruptions that may affect the Company’s ability to initiate and complete nonclinical studies, obtain nonclinical and clinical supplies, and initiate clinical trial sites. The pandemic may cause significant disruptions in the financial markets, which could impact the Company’s ability to raise additional funds to support its operations. Moreover, the pandemic has significantly impacted economies worldwide and could result in adverse effects on the Company’s business and operations. As described above, to date, the Company has experienced a business disruption at its third-party manufacturer, including delays related to the COVID-19 pandemic. The Company is continuing to monitor the impact of the COVID-19 pandemic on its business and financial statements. To date, the Company has not incurred impairment losses in the carrying values of its assets as a result of the pandemic and it is not aware of any specific related event or circumstance that would require it to revise its estimates reflected in these consolidated financial statements. The extent to which the COVID-19 pandemic will continue to directly or indirectly impact the Company’s business, results of operations and financial condition, including planned and future clinical trials and research and development costs, will depend on future developments that are highly uncertain, including as a result of new information that may emerge concerning COVID-19, the actions taken to contain or treat it, and the duration and intensity of the related effects. Basis of Presentation The accompanying consolidated financial statements reflect the operations of the Company and its wholly owned, domestic subsidiary. Intercompany balances and transactions have been eliminated in consolidation. The accompanying consolidated financial statements have been prepared in conformity with generally accepted accounting principles in the United States of America (“GAAP”). Any reference in these notes to applicable guidance is meant to refer to the authoritative GAAP as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Update (“ASU”) of the Financial Accounting Standards Board (“FASB”).

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2020
Summary of Significant Accounting Policies
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Use of Estimates The preparation of the Company’s consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements, and the reported amounts of revenue and expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual of research and development expenses. The Company bases its estimates on historical experience, known trends, and other market‑specific or other relevant factors that it believes to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates, as there are changes in circumstances, facts, and experience. Changes in estimates are recorded in the period in which they become known. Actual results may differ from those estimates or assumptions. Concentrations of Credit Risk and of Significant Suppliers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash, cash equivalents and marketable securities. The Company maintains its cash, cash equivalents and marketable securities at one accredited financial institution. The Company’s cash equivalents as of December 31, 2020 consisted of U.S. government money market funds. Marketable securities as of December 31, 2020 consisted of U.S. Treasury notes with maturities of less than 12 months. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. The Company is dependent on third‑party vendors for its product candidates. In particular, the Company relies, and expects to continue to rely, on a small number of vendors to manufacture materials and components required for the production of its product candidates. These programs could be adversely affected by a significant interruption in the manufacturing process. Cash Equivalents The Company considers all highly liquid investments with a remaining maturity when purchased of three months or less to be cash equivalents. Restricted Cash In connection with the Company’s lease agreement entered into in December 2018 (see Note 12), the Company is required to maintain a letter of credit of $1.3 million for the benefit of the landlord. As of the years ended December 31, 2020 and 2019, this amount was classified as restricted cash (non‑current) in the consolidated balance sheets. The Company is required to maintain a separate cash balance of less than $0.1 million pledged as collateral for a credit card account. As of December 31, 2020 and 2019, this amount was classified as restricted cash (non‑current) on the consolidated balance sheets. Marketable Securities Marketable securities represent holdings of available-for-sale marketable debt securities in accordance with the Company’s investment policy. The Company has classified its investments with maturities beyond one year as current, based on their highly liquid nature and because such marketable securities represent the investment of cash that is available for current operations. Investments in marketable securities are recorded at fair value, with any unrealized gains and losses reported within accumulated other comprehensive income (loss) as a separate component of stockholders’ equity (deficit) until realized or until a determination is made that an other-than-temporary decline in market value has occurred. The amortized cost of debt securities is adjusted for amortization of premiums and accretion of discounts to maturity. Such amortization and accretion, together with interest on securities sold, is determined based on the specific identification method and any realized gains or losses on the sale of investments are reflected as a component of other income (expense). Fair Value Measurements Certain assets and liabilities of the Company are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and financial liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: · Level 1—Quoted prices in active markets for identical assets or liabilities. · Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. · Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies, and similar techniques. The Company’s cash equivalents and marketable securities are carried at fair value, determined according to the fair value hierarchy described above (see Note 3). The carrying values of the Company’s accounts payable and accrued expenses approximate their fair values due to the short‑term nature of these liabilities. Property and Equipment Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization expense is recognized using the straight‑line method over the estimated useful life of each asset as follows: Estimated Useful Life Laboratory equipment 5 years Furniture and fixtures 4 years Leasehold improvements Shorter of term of lease or 15 years Costs for capital assets not yet placed into service are capitalized as construction‑in‑progress and depreciated once placed into service. Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation and amortization are removed from the accounts and any resulting gain or loss is included in loss from operations. Expenditures for repairs and maintenance that do not improve or extend the life of the respective assets are charged to expense as incurred. Classification and Accretion of Convertible Preferred Stock The Company’s convertible preferred stock was classified outside of stockholders’ equity (deficit) on the consolidated balance sheets because the holders of such shares had liquidation rights in the event of a deemed liquidation that, in certain situations, was not solely within the control of the Company and would require the redemption of the then‑outstanding convertible preferred stock. The Series Seed convertible preferred stock (the “Series Seed preferred stock”), Series Seed 1 convertible preferred stock (the “Series Seed 1 preferred stock”), Series A convertible preferred stock (the “Series A preferred stock”), and Series B convertible preferred stock (the “Series B preferred stock,” and collectively with the Series Seed preferred stock, the Series Seed 1 preferred stock, and the Series A preferred stock, the “Preferred Stock”) of the Company were not redeemable, except in the event of a deemed liquidation. Because the occurrence of a deemed liquidation event was not currently probable, the carrying values of the convertible preferred stock were not being accreted to their redemption values. Subsequent adjustments to the carrying values to the convertible preferred stock would be made only when a deemed liquidation event becomes probable. Upon the closing of the Company’s initial public offering on June 30, 2020, all outstanding shares of convertible preferred stock automatically converted into 18,969,672 shares of common stock. Impairment of Long‑Lived Assets Long‑lived assets consist of property and equipment. Long‑lived assets to be held and used are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends, and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long‑lived asset group for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long‑lived asset group to its carrying value. An impairment loss would be recognized in loss from operations when estimated undiscounted future cash flows expected to result from the use of an asset group are less than its carrying amount. Impairment is measured based on the excess of the carrying value of the related assets over the fair value of such assets. The Company did not record any impairment losses on long‑lived assets during the years ended December 31, 2020 and 2019. Segment Information The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. The Company’s focus is on developing gene therapies with the potential to restore, improve, and preserve high-acuity physiologic hearing for individuals with hearing loss. All of the Company’s long‑lived assets are located in the United States. Research and Development Costs Costs for research and development activities are expensed in the period in which they are incurred. Research and development expenses consist of costs incurred in performing research and development activities, including salaries and bonuses, stock‑based compensation, employee benefits, facilities costs, laboratory supplies, depreciation and amortization, manufacturing expenses, and external costs of vendors engaged to conduct research and preclinical development activities as well as the cost of licensing technology. Upfront payments under license agreements are expensed upon receipt of the license, and annual maintenance fees under license agreements are expensed in the period in which they are incurred. Milestone payments under license agreements are accrued, with a corresponding expense being recognized, in the period in which the milestone is determined to be probable of achievement and the related amount is reasonably estimable. Nonrefundable advance payments for goods or services to be received in the future for use in research and development activities are recorded as prepaid expenses. The prepaid amounts are expensed as the related goods are delivered or the services are performed. Research, Development, and Manufacturing Contract Costs and Accruals The Company has entered into various research, development, and manufacturing contracts with research institutions and other companies. These agreements are generally cancelable, and related costs are recorded as research and development expenses as incurred. The Company records accruals for estimated ongoing research and development costs. When billing terms under these contracts do not coincide with the timing of when the work is performed, the Company is required to make estimates of outstanding obligations to those third parties as of period end. Any accrual estimates are based on a number of factors, including the Company’s knowledge of the progress towards completion of the research, development, and manufacturing activities, invoicing to date under the contracts, communication from the research institutions and other companies of any actual costs incurred during the period that have not yet been invoiced, and the costs included in the contracts. Significant judgments and estimates may be made in determining the accrued balances at the end of any reporting period. Actual results could differ from the estimates made by the Company. The historical accrual estimates made by the Company have not been materially different from the actual costs. Patent Costs All patent‑related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses. Stock‑Based Compensation The Company measures stock options with service‑based vesting granted to employees, non‑employees, and directors based on the fair value on the date of grant using the Black‑Scholes option‑pricing model. The Company measures restricted common stock awards using the difference, if any, between the purchase price per share of the award and the fair value of the Company’s common stock at the date of grant. Compensation expense for employee awards is recognized over the requisite service period, which is generally the vesting period of the award. Compensation expense for non‑employee awards is recognized in the same manner as if the Company had paid cash in exchange for the goods or services, which is generally the vesting period of the award. The Company uses the straight‑line method to record the expense of awards with only service‑based vesting conditions. The Company has not granted awards with performance‑ or market‑based vesting conditions. The Company accounts for forfeitures of share‑based awards as they occur. The Company classifies stock‑based compensation expense in its consolidated statements of operations and comprehensive loss in the same manner in which the award recipient’s payroll costs are classified or in which the award recipient’s service payments are classified. Comprehensive Income (Loss) Comprehensive income (loss) includes net loss as well as other changes in stockholders’ equity (deficit) that result from transactions and economic events other than those with stockholders. For the year ended December 31, 2020, comprehensive income (loss) included less than $0.1 million of unrealized gains on marketable securities. For the year ended December 31, 2019, there was no difference between net loss and comprehensive loss. Income Taxes The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the consolidated financial statements or in the Company’s tax returns. Deferred tax assets and liabilities are determined on the basis of the differences between the financial statements and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of the deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies. The Company accounts for uncertainty in income taxes recognized in the consolidated financial statements by applying a two‑step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more‑likely‑than‑not to be sustained, the tax position is then assessed to determine the amount of benefit to recognize in the consolidated financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties. The Company’s policy is to recognize interest and/or penalties related to income tax matters in income tax expense. The Company had no amounts accrued for interest and penalties on its consolidated balance sheets at December 31, 2020 and 2019. Net Loss per Share The Company follows the two‑class method when computing net income (loss) per share as the Company has issued shares that meet the definition of participating securities. The two‑class method determines net income (loss) per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two‑class method requires income (loss) available to common stockholders for the period to be allocated between common and participating securities based upon their respective rights to share in undistributed earnings as if all income (loss) for the period had been distributed. Basic net income (loss) per share attributable to common stockholders is computed by dividing net income (loss) attributable to common stockholders by the weighted‑average number of common shares outstanding for the period. Diluted net income (loss) per share attributable to common stockholders is computed by adjusting net income (loss) attributable to common stockholders to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net income (loss) per share attributable to common stockholders is computed by dividing the diluted net income (loss) attributable to common stockholders by the weighted‑average number of common shares outstanding for the period, including potential dilutive common shares. For purposes of this calculation, the Company’s outstanding stock options, unvested restricted common stock, and convertible preferred stock are considered potential dilutive common shares. The Company’s participating securities contractually entitle the holders of such securities to participate in dividends but do not contractually require the holders of such securities to participate in losses of the Company. Accordingly, in periods in which the Company reports a net loss, such losses are not allocated to such participating securities. In periods in which the Company reports a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive common shares are not assumed to have been issued if their effect is anti‑dilutive. The Company reported a net loss attributable to common stockholders for the years ended December 31, 2020 and 2019. Leases The Company determines if an arrangement is a lease at inception. A contract contains a lease if the contract conveys the right to control the use of an identified asset for a period of time in exchange for consideration. The Company classifies leases at the lease commencement date as operating or finance leases and records a right‑of‑use asset and a lease liability on the consolidated balance sheet for all leases with an initial lease term of greater than 12 months. Leases with an initial term of 12 months or less are not recorded on the balance sheet, but payments are recognized as expense on a straight‑line basis over the lease term. The Company often enters into contracts that contain both lease and non‑lease components. Non‑lease components may include maintenance, utilities, and other operating costs. The Company combines the lease and non‑lease components of fixed costs in its lease arrangements as a single lease component. Variable costs, such as utilities or maintenance costs, are not included in the measurement of right‑of‑use assets and lease liabilities, but rather are expensed when the event determining the amount of variable consideration to be paid occurs. Lease liabilities and their corresponding right‑of‑use assets are recorded based on the present value of future lease payments over the expected lease term. The present value of future lease payments is determined by using the interest rate implicit in the lease if that rate is readily determinable; otherwise, the Company uses its estimated secured incremental borrowing rate for that lease term. The Company estimates its secured incremental borrowing rate for each lease based on the rate of interest that the Company would have to pay to borrow an amount equal to the lease payments on a collateralized basis over a similar term. Certain of the Company’s leases include options to extend or terminate the lease. The amounts determined for the Company’s right‑of‑use assets and lease liabilities generally do not assume that renewal options or early‑termination provisions, if any, are exercised, unless it is reasonably certain that the Company will exercise such options. Recently Adopted Accounting Pronouncements In August 2018, the FASB issued ASU No. 2018‑13, Fair Value Measurement (Topic 820): Disclosure Requirements for Fair Value Measurement (“ASU 2018‑13”), which modifies the existing disclosure requirements for fair value measurements in Topic 820. The new disclosure requirements include disclosure related to changes in unrealized gains or losses included in other comprehensive income (loss) for recurring Level 3 fair value measurements held at the end of each reporting period and the explicit requirement to disclose the range and weighted‑average of significant unobservable inputs used for Level 3 fair value measurements. The other provisions of ASU 2018‑13 include eliminated and modified disclosure requirements. An entity is permitted to early adopt any removed or modified disclosures upon issuance of ASU 2018‑13 and delay adoption of the additional disclosures until their effective date. For all entities, this guidance is required to be adopted for annual periods beginning after December 15, 2019, including interim periods within those fiscal years. Early adoption is permitted. The Company adopted ASU 2018‑13 as of the required effective date of January 1, 2020, and the adoption did not have a material impact on the Company’s consolidated financial statements. Recently Issued Accounting Pronouncements From time to time, new accounting pronouncements are issued by the FASB or other standard‑setting bodies that the Company adopts as of the specified effective date. The Company qualifies as an “emerging growth company” as defined in the Jumpstart Our Business Startups Act of 2012 and has elected not to “opt out” of the extended transition related to complying with new or revised accounting standards, which means that when a standard is issued or revised and it has different application dates for public and non‑public companies, the Company can adopt the new or revised standard at the time non‑public companies adopt the new or revised standard and can do so until such time that the Company either (i) irrevocably elects to “opt out” of such extended transition period or (ii) no longer qualifies as an emerging growth company. The Company may choose to early adopt any new or revised accounting standards whenever such early adoption is permitted for non‑public companies. In June 2016, the FASB issued ASU No. 2016‑13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments (“ASU 2016‑13”), which requires the measurement and recognition of expected credit losses for financial assets held at amortized cost. ASU 2016‑13 replaces the existing incurred loss impairment model with an expected loss model. It also eliminates the concept of other‑than‑temporary impairment and requires credit losses related to available‑for‑sale debt securities to be recorded through an allowance for credit losses rather than as a reduction in the amortized cost basis of the securities. These changes may result in earlier recognition of credit losses. In November 2018, the FASB issued ASU No. 2018‑19, Codification Improvements to Topic 326, Financial Instruments—Credit Losses , which narrowed the scope and changed the effective date for non‑public entities for ASU 2016‑13. The FASB subsequently issued supplemental guidance within ASU No. 2019‑05, Financial Instruments—Credit Losses (Topic 326): Targeted Transition Relief , which provides an option to irrevocably elect the fair value option for certain financial assets previously measured at amortized cost basis. For public entities that are Securities and Exchange Commission filers, excluding entities eligible to be smaller reporting companies, ASU 2016‑13 is effective for annual periods beginning after December 15, 2019, including interim periods within those fiscal years. For all other entities, ASU 2016‑13 is effective for annual periods beginning after December 15, 2022, including interim periods within those fiscal years. Early adoption is permitted. The Company is currently evaluating the impact that the adoption of ASU 2016‑13 will have on its consolidated financial statements.

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2020
Fair Value Measurements
Fair Value Measurements	3. Fair Value Measurements The following tables present the Company’s fair value hierarchy for its assets and liabilities that are measured at fair value on a recurring basis and indicate the level within the fair value hierarchy of the valuation techniques the Company utilized to determine such fair value (in thousands): Fair Value Measurements at December 31, 2020 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 68,016 $ — $ — $ 68,016 Restricted cash: Money market funds 1,292 — — 1,292 Marketable securities: U.S. Treasury notes — 239,078 — 239,078 $ 69,308 $ 239,078 $ — $ 308,386 Fair Value Measurements at December 31, 2019 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 24,281 $ — $ — $ 24,281 Restricted cash: Money market funds 1,292 — — 1,292 $ 25,573 $ — $ — $ 25,573 U.S. government money market funds were valued by the Company based on quoted market prices, which represent a Level 1 measurement within the fair value hierarchy. As of December 31, 2020, the Company’s marketable securities consisted of U.S. Treasury notes, which were valued based on Level 2 inputs. In determining the fair value of its U.S. Treasury notes, the Company relied on quoted prices for similar securities in active markets or other inputs that are observable or can be corroborated by observable market data. During the years ended December 31, 2020 and 2019, there were no transfers between Level 1, Level 2 and Level 3.

Marketable Securities

Marketable Securities	12 Months Ended
	Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Marketable securities	4. Marketable Securities As of December 31, 2020, the fair value of available-for-sale marketable debt securities by type of security was as follows (in thousands): December 31, 2020 Gross Gross Amortized Unrealized Unrealized Fair Cost Gain Loss Value Assets Marketable securities: U.S. Treasury notes $ 239,065 $ 16 $ (3) $ 239,078 $ 239,065 $ 16 $ (3) $ 239,078 At December 31, 2020, all available-for-sale marketable securities have contractual maturities of less than one year. At December 31, 2019, the Company did not hold any marketable securities.

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2020
Property and Equipment, Net
Property and Equipment, Net	5. Property and Equipment, Net Property and equipment, net consisted of the following (in thousands): 2020 2019 Laboratory equipment $ 4,576 $ 2,536 Furniture and fixtures 461 358 Leasehold improvements 8,915 8,915 Construction in progress 3,755 77 17,707 11,886 Less: Accumulated depreciation and amortization (2,319) (394) $ 15,388 $ 11,492 Depreciation and amortization expense for the years ended December 31, 2020 and 2019 was $1.9 million and $0.4 million, respectively. As of December 31, 2020, the gross amount of assets under the finance leases was $0.5 million and the related accumulated amortization was $0.2 million. As of December 31, 2019, the gross amount of assets under finance leases was $0.5 million and the related accumulated amortization was $0.1 million.

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	12 Months Ended
	Dec. 31, 2020
Accrued Expenses and Other Current Liabilities
Accrued Expenses and Other Current Liabilities	6. Accrued Expenses and Other Current Liabilities Accrued expenses and other current liabilities consisted of the following (in thousands): 2020 2019 Accrued external research, development, and manufacturing expenses $ 2,442 $ 1,157 Accrued employee compensation and benefits 2,438 2,605 Accrued professional fees 1,175 67 Payments due for leasehold improvements 2,071 756 Other 573 377 $ 8,699 $ 4,962

Convertible Preferred Stock

Convertible Preferred Stock	12 Months Ended
	Dec. 31, 2020
Convertible Preferred Stock
Convertible Preferred Stock	7. Convertible Preferred Stock The Company has issued Series Seed preferred stock, Series Seed 1 preferred stock, Series A preferred stock, and Series B preferred stock. Upon the closing of the Company’s initial public offering on June 30, 2020, all outstanding shares of Preferred Stock automatically converted into 18,969,672 shares of common stock. The Company’s Series A preferred stock purchase agreement obligated the Series A investors to participate in a subsequent offering of Series A preferred stock upon the achievement of specified development milestones by the Company or upon the vote of at least 55% of the holders of the Series A preferred stock. The Company determined that the preferred stock tranche right was required to be recorded as a liability because it was a freestanding instrument that would require the Company to transfer assets upon exercise of the right. The preferred stock tranche right met the definition of a freestanding financial instrument because it was legally detachable and separately exercisable from the Series A preferred stock. The preferred stock tranche right was recorded as a liability at its fair value of $4.8 million upon issuance, with a corresponding adjustment being recorded to reduce the carrying value of the Series A preferred stock. The estimated fair value of the preferred stock tranche right was determined using a Black-Scholes option-pricing model. The liability was subsequently remeasured to fair value at each reporting date until settled, and the changes in the fair value of the preferred stock tranche liability were recognized as a component of other income (expense) in the consolidated statements of operations and comprehensive loss. In September 2019, pursuant to a vote of the holders of Series A preferred stock, the preferred stock tranche right was exercised, upon which the Company issued and sold the additional 75,333,815 shares of Series A preferred stock that were subject to the tranche right at a price of $0.33252 per share for gross proceeds of $25.1 million. The Company incurred issuance costs in connection with this transaction of less than $0.1 million. Immediately prior to the issuance of such shares, the preferred stock tranche right was remeasured to fair value for the last time with the change in fair value recognized as a component of other income (expense). Upon the issuance of the additional shares of preferred stock in September 2019, the preferred stock tranche liability was settled, resulting in a reclassification of the $6.0 million fair value of the preferred stock tranche liability at that time to the carrying value of the Series A preferred stock. In February 2020, the Company issued and sold 221,399,223 shares of Series B preferred stock, at a price of $0.47455 per share, for gross proceeds of $105.1 million. The Company incurred issuance costs in connection with this transaction of $0.2 million. Upon issuance of each series of Preferred Stock, the Company assessed the embedded conversion and liquidation features of the shares and determined that such features did not require the Company to separately account for these features. The Company also concluded that no beneficial conversion features existed on the issuance date of each series of Preferred Stock. As of December 31, 2020, there were no shares of redeemable convertible preferred stock outstanding as a result of the conversion into common stock in connection with the Company’s initial public offering. As of December 31, 2019, Preferred Stock consisted of the following (in thousands, except share amounts): December 31, 2019 Preferred Stock Common Stock Preferred Stock Issued and Carrying Liquidation Issuable Upon Authorized Outstanding Value Preference Conversion Series Seed preferred stock 25,622,520 25,622,520 $ 6,989 $ 7,100 1,215,893 Series Seed 1 preferred stock 2,058,855 2,058,855 570 485 97,701 Series A preferred stock 150,667,630 150,667,630 51,131 50,100 7,149,794 178,349,005 178,349,005 $ 58,690 $ 57,685 8,463,388 The holders of Preferred Stock have the following rights and preferences: Voting Prior to the Company’s initial public offering, and except as otherwise provided by law or by the other provisions of the Company’s certificate of incorporation, the holders of Preferred Stock were entitled to vote, together with the holders of common stock, on matters submitted to stockholders for a vote. The holders of Preferred Stock were entitled to the number of votes equal to the number of shares of common stock into which the shares of Preferred Stock could convert on the record date for determination of stockholders entitled to vote. In addition, the holders of Series A preferred stock, voting exclusively and as a separate class, were entitled to elect two directors of the Company, and the holders of Series B preferred stock, voting exclusively and as a separate class, were entitled to elect one director of the Company. The holders of Preferred Stock and common stock, voting together as a single class, were entitled to elect the remaining directors of the Company. Conversion Upon the closing of a qualified public offering of common stock, as defined in the Company’s certificate of incorporation, or approval of the holders of at least seventy-one percent (71%) of the shares of Preferred Stock then outstanding (voting together as a single class and not as separate series, and on an as-converted to common stock basis), all outstanding shares of redeemable convertible preferred stock would have automatically converted into common stock at the then-applicable conversion rate for such shares. At December 31, 2020, there were no redeemable convertible preferred shares outstanding as a result of the conversion into common shares at a 21.073-for-one conversion ratio, in connection with the initial public offering. Dividends The holders of Preferred Stock were entitled to receive non-cumulative dividends if and when declared by the Company’s board of directors at the applicable dividend rate of each series of Preferred Stock (subject to appropriate adjustment in the event of any stock split, stock dividend, combination, or other similar recapitalization with respect to such shares) per annum. Dividends on Preferred Stock were payable in preference and priority to any payment of any dividend on common stock. No dividends were declared or paid during the years ended December 31, 2020 or 2019. Liquidation In the event of any voluntary or involuntary liquidation, dissolution, or winding-up of the Company or deemed liquidation event, the holders of shares of Series A preferred stock and Series B preferred stock were entitled to receive, in preference to holders of the Series Seed preferred stock and Series Seed 1 preferred stock and common stockholders, an amount per share equal to the original issue price of the Series A preferred stock or Series B preferred stock, respectively, plus any dividends declared but unpaid on such shares. In the event that the assets available for distribution to the Company's stockholders were not sufficient to permit payment to the holders of Series A preferred stock and Series B preferred stock in the full amount to which they were entitled, the assets available for distribution would have been distributed on a pro rata basis among the holders of the Series A preferred stock and Series B preferred stock. After the payment in full to the holders of Series A preferred stock and Series B preferred stock, the holders of shares of Series Seed preferred stock and Series Seed 1 preferred stock were entitled to receive, in preference to common stockholders, an amount per share equal to the original issue price of the Series Seed preferred stock and Series Seed 1 preferred stock, as applicable, plus any dividends declared but unpaid on such shares. In the event that the assets available for distribution to the Company's stockholders were not sufficient to permit payment to the holders of Series Seed preferred stock and Series Seed 1 preferred stock in the full amount to which they were entitled, the assets available for distribution would have been distributed on a pro rata basis among the holders of the Series Seed preferred stock and Series Seed 1 preferred stock. After the payment of all preferential amounts to the holders of Preferred Stock, to the extent available, the remaining assets available for distribution were to be distributed among the holders of the Preferred Stock and common stock pro rata based on an as-converted basis; provided, that if the aggregate proceeds that the holders of any series of Preferred Stock would have been entitled to receive pursuant to the preceding provisions exceeds three times the original issue price of such series of Preferred Stock (the “Maximum Participation Amount”), then the holder of such series of Preferred Stock would have been entitled to receive the greater of the Maximum Participation Amount or the amount such holder would have received if the Preferred Stock had been converted to common stock immediately prior to such event.

Stockholders' Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2020
Stockholders' Equity
Stockholders' Equity	8. Stockholders’ Equity Common Stock As of December 31, 2020 and 2019, the Company’s certificate of incorporation, as amended and restated, authorized the Company to issue 200,000,000 and 235,000,000 shares, respectively, of common stock $0.0001 par value per share. On June 18, 2020, the Company effected a one-for-21.073 reverse stock split of its issued and outstanding shares of common stock and a proportional adjustment to the existing conversion ratios for each series of the Company’s convertible preferred stock. Accordingly, all share and per share amounts for all periods presented in the accompanying consolidated financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect this reverse stock split adjustment of the preferred stock conversion ratios. On June 30, 2020, the Company completed its initial public offering of its common stock and issued and sold 14,375,000 shares of common stock, at a public offering price of $17.00 per share, for gross proceeds of $244.4 million, or net proceeds of $223.8 million after deducting underwriting discounts, commissions, and offering expenses. Preferred Stock As of December 31, 2020, the Company’s certificate of incorporation, as amended and restated, authorized the Company to issue 5,000,000 shares of preferred stock, $0.0001 par value per share. As of December 31, 2020, no shares of preferred stock were issued or outstanding.

Stock_Based Compensation

Stock‑Based Compensation	12 Months Ended
	Dec. 31, 2020
Stock‑Based Compensation
Stock-Based Compensation	9. Stock‑Based Compensation 2016 Stock Plan The Company’s 2016 Stock Plan (the “2016 Plan”) provides for the Company to grant incentive stock options or non‑qualified stock options, restricted stock, restricted stock units, and other equity awards to employees, directors, and consultants of the Company. The 2016 Plan is administered by the board of directors or, at the discretion of the board of directors, by a committee of the board of directors. The board of directors may also delegate to one or more officers of the Company the power to grant awards to employees and certain officers of the Company. The exercise prices, vesting, and other restrictions are determined at the discretion of the board of directors, or its committee or any such officer if so delegated. Stock options granted under the 2016 Plan with service‑based vesting conditions generally vest over four years and expire after ten years. As of December 31, 2019, the total number of shares of common stock that were issuable under the 2016 Plan was 1,508,669 shares, of which 363,350 shares remained available for future issuance as of December 31, 2019. During the year ended December 31, 2020, the Company increased the number of shares of common stock authorized for issuance under the 2016 plan from 1,508,669 to 3,722,685 shares. As of December 31, 2020, no shares remained available for future issuance under the 2016 Plan. 2020 Stock Plan On May 28, 2020, the Company’s board of directors adopted, and on June 17, 2020 its stockholders approved, the 2020 Stock Plan (the “2020 Plan”). The 2020 Plan provides for the grant of incentive stock options, non-statutory stock options, stock appreciation rights, restricted stock awards, restricted stock units, and other stock-based awards. The number of shares initially reserved for issuance under the 2020 Plan is the sum of 4,294,594, plus the number of shares (up to 3,622,691 shares) equal to the sum of (i) the number of shares remaining available for issuance under the 2016 Plan upon the effectiveness of the 2020 Plan and (ii) the number of shares of common stock subject to outstanding awards granted under the 2016 Plan that expire, terminate or are otherwise surrendered, cancelled, forfeited or repurchased by the Company at their original issuance price pursuant to a contractual repurchase right. The number of shares of common stock that may be issued under the 2020 Plan will automatically increase on the first day of each fiscal year, beginning with the fiscal year ending December 31, 2021 and continuing for each fiscal year until, and including, the fiscal year ending December 31, 2030, equal to the lowest of (i) 2,728,610 shares, (ii) 4% of the number of shares of common stock outstanding on such date, and (iii) an amount determined by the Company’s board of directors. The shares of common stock underlying any awards that are forfeited, cancelled, held back upon exercise or settlement of an award to satisfy the exercise price or tax withholding, repurchased or are otherwise terminated by the Company under the 2020 Plan will be added back to the shares of common stock available for issuance under the 2020 Plan. As of December 31, 2020, the total number of shares of common stock available for issuance under the 2020 Plan is 4,212,479 shares. 2020 Employee Stock Purchase Plan On May 28, 2020, the Company’s board of directors adopted, and on June 17, 2020 its stockholders approved, the 2020 Employee Stock Purchase Plan (the “2020 ESPP”). A total of 360,651 shares of common stock were initially reserved for issuance under this plan. The number of shares of common stock that may be issued under the 2020 ESPP will automatically increase on the first day of each fiscal year, beginning with the fiscal year commencing on January 1, 2021 and continuing for each fiscal year until, and including, the fiscal year commencing on January 1, 2031, equal to the lowest of (i) 640,630 shares, (ii) 1% of the number of shares of common stock outstanding on such date, and (iii) an amount determined by the Company’s board of directors. As of December 31, 2020, the total number of shares of common stock for issuance under the 2020 Employee Stock Purchase Plan is 360,651. Stock Option Valuation The fair value of stock option grants is estimated using the Black‑Scholes option‑pricing model. The Company historically has been a private company and lacks company‑specific historical and implied volatility information. Therefore, it estimates its expected stock volatility based on the historical volatility of a publicly traded set of peer companies and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded stock price. For options with service‑based vesting conditions, the expected term of the Company’s stock options has been determined utilizing the “simplified” method for awards that qualify as “plain‑vanilla” options. The risk‑free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield is based on the fact that the Company has never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future. The following table presents, on a weighted‑average basis, the assumptions used in the Black‑Scholes option‑pricing model to determine the fair value of stock options granted: Year Ended December 31, 2020 2019 Risk‑free interest rate 0.51 % 1.61 % Expected volatility 89.4 % 86.1 % Expected dividend yield — — Expected term (in years) 6.0 6.0 Stock Options The following table summarizes the Company’s stock option activity since December 31, 2019: Weighted‑ Weighted‑ Average Average Aggregate Number of Exercise Contractual Intrinsic Shares Price Term Value (in years) (in thousands) Outstanding as of December 31, 2019 961,751 $ 1.69 9.5 $ 524 Granted 2,812,584 12.44 Exercised (41,882) 1.58 Forfeited (57,766) 3.32 Outstanding as of December 31, 2020 3,674,687 $ 9.92 $ 38,170 Vested and expected to vest as of December 31, 2020 3,674,687 $ 9.92 $ 38,170 Options exercisable as of December 31, 2020 588,000 $ 4.94 $ 8,797 The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common stock for those stock options that had strike prices lower than the fair value of the Company’s common stock. The aggregate intrinsic value of stock options exercised during the years ended December 31, 2020 and 2019 was $0.8 million and less than $0.1 million, respectively. The weighted‑average grant‑date fair value of stock options granted during the years ended December 31, 2020 and 2019 was $9.93 per share and $1.57 per share, respectively. Early Exercise of Stock Options into Restricted Stock Certain option grants permit option holders to elect to exercise unvested options in exchange for unvested common stock. The options that are exercised prior to vesting will continue to vest according to the respective option agreement, and such unvested shares are subject to repurchase by the Company at the optionee’s original exercise price in the event the optionee’s service with the Company voluntarily or involuntarily terminates. A summary of the Company’s unvested common stock from option early exercises that is subject to repurchase by the Company is as follows: Shares Unvested restricted common stock as of December 31, 2019 102,896 Vested (55,165) Unvested restricted common stock as of December 31, 2020 47,731 Proceeds from the early exercise of options are recorded as a liability within accrued expenses and other current liabilities on the consolidated balance sheet. The liability for unvested common stock subject to repurchase is then reclassified to additional paid‑in capital as the Company’s repurchase right lapses. The shares purchased by the employees and directors pursuant to the early exercise of stock options are not deemed, for accounting purposes, to be outstanding until those shares have vested. As of December 31, 2020 and 2019, the liability related to the payments for unvested shares from early‑exercised options was less than $0.1 million and $0.1 million, respectively. Restricted Common Stock Awards The Company has both (i) granted restricted stock awards, with the recipient not paying for the shares of common stock, and (ii) issued and sold restricted stock, with the recipient purchasing the common stock at its fair value per share. In both circumstances, the restricted shares of common stock have service‑based vesting conditions and unvested shares are either subject to forfeiture by the employee or subject to repurchase by the Company, at the lesser of holder’s original purchase price or fair value, in the event the holder’s service with the Company voluntarily or involuntarily terminates. Service‑based restricted stock awards generally vest over four years. Proceeds from the issuance and sale of restricted common stock are recorded as a liability within accrued expenses and other current liabilities on the consolidated balance sheet. The liability for unvested common stock subject to repurchase is then reclassified to additional paid‑in capital as the Company’s repurchase right lapses. Shares of restricted common stock granted or sold to employees and directors are not deemed, for accounting purposes, to be outstanding until those shares have vested. The Company did not grant or sell restricted common stock awards during 2020 or 2019. As of December 31, 2020 and December 31, 2019, the liability related to the payments received for shares of unvested restricted stock was less than $0.1 million at each date. The following table summarizes the Company’s restricted common stock award activity for the year ended December 31, 2020: Weighted‑ Average Grant‑Date Shares Fair Value Unvested restricted common stock as of December 31, 2019 209,742 $ 0.0443 Vested (115,834) 0.0427 Unvested restricted common stock as of December 31, 2020 93,908 $ 0.0529 The total fair value of restricted common stock vested during the years ended December 31, 2020 and 2019 was less than $0.1 million and $0.2 milliofn, respectively. Stock‑Based Compensation The Company records compensation cost for all share‑based payment arrangements, including employee, director, and consultant stock options and restricted stock. The Company recorded stock‑based compensation expense in the following expense categories of its consolidated statements of operations and comprehensive loss (in thousands): December 31, 2020 2019 Research and development expenses $ 1,486 $ 76 General and administrative expenses 3,009 62 $ 4,495 $ 138 In connection with the termination of an employee in July 2020, the Company accelerated the vesting of 1,541 shares of restricted stock and 60,948 unvested options, of which 22,684 shares were held by such employee. The modification of these awards resulted in the recognition of $1.1 million of stock-based compensation expense during the year ended December 31, 2020, which was classified as general and administrative expenses. As of December 31, 2020, there was $25.5 million of unrecognized compensation cost related to unvested stock options, which is expected to be recognized over a weighted‑average period of 2.3 years.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2020
Income Taxes
Income Taxes	10. Income Taxes For the years ended December 31, 2020 and 2019 the Company recorded no income tax benefits for the net operating losses incurred or for the research and development tax credits generated in each period, due to its uncertainty of realizing a benefit from those items. All of the Company’s operating losses since inception have been generated in the United States. A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate is as follows: Year Ended December 31, 2020 2019 Federal statutory income tax rate (21.0) % (21.0) % State income taxes, net of federal benefit (6.0) (5.7) Federal and state research and development tax credits (2.5) (3.8) Non‑deductible items 1.0 2.0 Other — 0.9 Change in deferred tax asset valuation allowance 28.5 27.6 Effective income tax rate 0.0 % 0.0 % Net deferred tax assets as of December 31, 2020 and 2019 consisted of the following (in thousands): December 31, 2020 2019 Deferred tax assets: Net operating loss carryforwards $ 19,558 $ 7,676 Research and development tax credit carryforwards 2,843 1,615 Stock-based compensation 626 4 Operating lease liabilities 3,642 3,884 Other 792 516 Total deferred tax assets 27,461 13,695 Deferred tax liabilities: Property and equipment (1,934) (1,923) Operating lease right‑of‑use assets (1,711) (1,809) Total deferred tax liabilities (3,645) (3,732) Valuation allowance (23,816) (9,963) Net deferred tax assets $ — $ — As of December 31, 2020, the Company had federal net operating loss carryforwards of $71.7 million, which may be available to offset future taxable income, of which $0.4 million of the total net operating loss carryforwards expire at various dates beginning in 2036, while the remaining $71.3 million do not expire but are limited in their usage to an annual deduction equal to 80% of annual taxable income. In addition, as of December 31, 2020, the Company had state net operating loss carryforwards of $71.3 million, which may be available to offset future taxable income and expire at various dates beginning in 2036. As of December 31, 2020, the Company also had federal and state research and development tax credit carryforwards of $2.2 million and $0.8 million, respectively, which may be available to reduce future tax liabilities and expire at various dates beginning in 2036 and 2033, respectively. Utilization of the U.S. federal and state net operating loss carryforwards and research and development tax credit carryforwards may be subject to a substantial annual limitation under Section 382 and Section 383 of the Internal Revenue Code of 1986 (the “Code”), and corresponding provisions of state law, due to ownership changes that have occurred previously or that could occur in the future. These ownership changes may limit the amount of carryforwards that can be utilized annually to offset future taxable income and tax liabilities. In general, an ownership change, as defined by Section 382 of the Code, results from transactions increasing the ownership of certain stockholders or public groups in the stock of a corporation by more than 50% over a three‑year period. The Company has not conducted a study to assess whether a change of control has occurred or whether there have been multiple changes of control since inception due to the significant complexity and cost associated with such a study. If the Company has experienced a change of control, as defined by Section 382 of the Code, at any time since inception, utilization of the net operating loss carryforwards or research and development tax credit carryforwards may be subject to an annual limitation, which is determined by first multiplying the value of the Company’s stock at the time of the ownership change by the applicable long‑term tax‑exempt rate, and then could be subject to additional adjustments. Any limitation may result in expiration of a portion of the net operating loss carryforwards or research and development tax credit carryforwards before their utilization. Further, until a study is completed by the Company and any limitation is known, no amounts are being presented as an uncertain tax position. The Company has evaluated the positive and negative evidence bearing upon its ability to realize the deferred tax assets, which consist primarily of net operating loss carryforwards and research and development tax credit carryforwards. Management has considered the Company’s history of cumulative net losses incurred since inception, estimated future taxable income, and prudent and feasible tax planning strategies and has concluded that it is more likely than not that the Company will not realize the benefits of federal and state net deferred tax assets. Accordingly, a full valuation allowance has been established against the net deferred tax assets as of December 31, 2020 and 2019. The Company reevaluates the positive and negative evidence at each reporting period. The changes in the valuation allowance for deferred tax assets during the years ended December 31, 2020 and 2019 related primarily to the increases in net operating loss carryforwards and research and development tax credit carryforwards. The changes in the valuation allowance for 2020 and 2019 were as follows (in thousands): Year Ended December 31, 2020 2019 Valuation allowance at beginning of year $ 9,963 $ 2,869 Increases recorded to income tax provision 13,853 7,094 Valuation allowance at end of year $ 23,816 $ 9,963 The Company assesses the uncertainty in its income tax positions to determine whether a tax position of the Company is more likely than not to be sustained upon examination, including resolution of any related appeals of litigation processes, based on the technical merits of the position. For tax positions meeting the more‑likely‑than‑not threshold, the tax amount recognized in the financial statements is reduced by the largest benefit that has a greater than 50% likelihood of being realized upon the ultimate settlement with the relevant taxing authority. As of December 31, 2020 and 2019, the Company had not recorded any reserves for uncertain tax positions or related interest and penalties. The Company files income tax returns as prescribed by the tax laws of the jurisdictions in which it operates. In the normal course of business, the Company is subject to examination by federal and state jurisdictions, where applicable. As of December 31, 2020, there were no pending tax examinations. The Company is open to future tax examination under statute from 2016 to the present.

License Agreements

License Agreements	12 Months Ended
	Dec. 31, 2020
License Agreements
License Agreements	11. License Agreements License Agreement with Massachusetts Eye and Ear In October 2017, the Company entered into a license agreement with Massachusetts Eye and Ear Infirmary and the Schepens Eye Research Institute, Inc. (collectively referred to as “MEE”) (the “MEE License”). Under the MEE License, the Company received an exclusive, non‑transferable, sublicensable, worldwide, royalty‑bearing license to certain patent rights and know‑how, including rights related to adeno-associated virus, or AAV, ancestral technology, including AAVAnc80, to use, research, develop, manufacture, and commercialize licensed products in the treatment, diagnosis, and prevention of any and all balance disorders, including hearing disorders of the inner ear, in each case, with a total prevalence in the United States of less than 3,000 patients, and an exclusive, non‑transferable, sublicensable right and license under MEE’s rights, title, and interest in certain patents co‑owned by MEE and Children’s Medical Center Corporation to use, research, develop, manufacture, and commercialize licensed products. The Company is obligated to use commercially reasonable efforts to develop and commercialize the MEE licensed products, including filing an investigational new drug application (“IND”) in the United States or investigational medicinal product dossier (“IMPD”) in any country in the European Union or an equivalent application in any country within 18 months of completion of specified toxicology studies for a licensed product. Upon entering into the MEE License in 2017, the Company issued to MEE shares of the Company’s common stock then having a fair value of $0.1 million. The Company is obligated to make aggregate milestone payments to MEE of up to $17.7 million upon the achievement of specified development, regulatory, and sales milestones. The Company is also obligated to pay tiered royalties of a mid to high single‑digit percentage based on annual net sales of licensed products by the Company and any of its affiliates and sublicensees. Royalties will be paid by the Company on a licensed product‑by‑licensed product and country‑by‑country basis beginning after the first commercial sale of an MEE licensed product and lasting until the later of (i) the expiration of the last valid claim in the licensed patents or (ii) ten years after the first commercial sale of such MEE licensed product (the “MEE Royalty Term”). The MEE License remains in effect until the last expiration date of the last to expire MEE Royalty Term, unless terminated earlier. The Company has the right to terminate the MEE License at will, with or without cause, by 90 days’ advance written notice to MEE or upon MEE’s material breach of the MEE License, provided that MEE does not cure such material breach within a specified period. MEE has the right to terminate the MEE License in its entirety if (i) the Company fails to make any payment due within a specified period after MEE notifies the Company of such failure, (ii) the Company or its affiliates challenge the validity of the licensed patent rights, (iii) the Company fails to maintain required insurance, or (iv) the Company becomes insolvent or bankrupt. MEE also has the right to terminate the Company’s rights to specific intellectual property rights it has licensed to the Company under the MEE License if the Company materially breaches certain diligence obligations and does not cure within a specified period after written notice from MEE. During the years ended December 31, 2020 and 2019, the Company did not make any payments to MEE or recognize any research and development expenses under the MEE License. Sublicense Agreement with Lonza Houston, Inc. In October 2017, the Company entered into a sublicense agreement with Lonza Houston, Inc. (“Lonza”), as amended in December 2018 (the “Lonza Sublicense”). Under the agreement, the Company received an exclusive, non‑transferable, sublicensable, worldwide, royalty‑bearing sublicense to certain patent rights and know‑how related to AAV ancestral technology, including AAVAnc80, to use, research, develop, manufacture, and commercialize licensed products for the treatment, diagnosis, and prevention of any and all balance disorders or diseases pertaining to the inner ear and/or any and all hearing diseases or disorders, including hearing disorders of the inner ear, but excluding all such disorders or diseases with a total prevalence in the United States of less than 3,000 patients. The Company is obligated to use commercially reasonable efforts to develop and commercialize the Lonza sublicensed products, including filing an IND in the United States or IMPD in any country in the European Union or an equivalent application in any country within 18 months of completion of specified toxicology studies for a licensed product. Upon entering into the Lonza Sublicense in 2017, the Company issued to Lonza shares of the Company’s common stock then having a fair value of $0.1 million. The Company is obligated to make aggregate milestone payments to Lonza of up to $18.5 million upon the achievement of specified development, regulatory, and sales milestones. The Company is also obligated to pay tiered royalties of a mid to high single‑digit percentage based on annual net sales of licensed products by the Company and any of its affiliates and sublicensees as denoted in the Lonza Sublicense. Royalties will be paid by the Company on a licensed product‑by‑licensed product and country‑by‑country basis beginning after the first commercial sale of a Lonza sublicensed product and lasting until the later of (i) the expiration of the last valid claim in the patent or (ii) ten years after the first commercial sale of such Lonza sublicensed product (the “Lonza Royalty Term”). The Lonza Sublicense remains in effect until the last expiration date of the last to expire Lonza Royalty Term, unless terminated earlier. The Company has the right to terminate the Lonza Sublicense at will, with or without cause, by 90 days’ advance written notice to Lonza or upon Lonza’s material breach of the Lonza Sublicense, provided that Lonza does not cure such material breach within a specified period. Lonza has the right to terminate the Lonza Sublicense in its entirety if (i) the Company fails to make any payment due within a specified period after Lonza notifies the Company of such failure, (ii) the Company or its affiliates challenge the validity of the sublicensed patent rights, (iii) the Company fails to maintain required insurance, or (iv) the Company becomes insolvent or bankrupt. Lonza also has the right to terminate the Company’s rights to specific intellectual property rights it has sublicensed to the Company under the Lonza Sublicense if the Company materially breaches certain diligence obligations and does not cure within a specified period after written notice from Lonza. During the years ended December 31, 2020 and 2019 the Company did not make any payments to Lonza or recognize any research and development expenses under the Lonza Sublicense.

Leases

Leases	12 Months Ended
	Dec. 31, 2020
Leases
Leases	12. Leases The Company leases its office and laboratory facility in Boston, Massachusetts pursuant to a lease agreement entered into in December 2018 (the “2018 Lease”), which includes a lease incentive, fixed payment escalations, and a rent holiday. The 2018 Lease term commenced in May 2019, which was the point at which the Company obtained control of the leased premises, and expires in February 2028. Under the 2018 Lease, the Company is entitled to one option to extend the lease term for an additional five years. The option to extend the lease term was not included in the right‑of‑use asset and lease liability as it was not reasonably certain of being exercised. The Company classified the 2018 Lease as an operating lease under ASC 842; Leases . The initial annual base rent under the 2018 Lease is $2.3 million, with such base rent increasing annually during the initial term by 3% and with lease payments beginning in February 2020. Additionally, the 2018 Lease requires the Company to pay its portion of real estate taxes and costs related to the premises, including costs of operations, maintenance, repair, replacement, and management of the new leased premises. In connection with the 2018 Lease, the Company is required to maintain a letter of credit for the benefit of the landlord in an amount of approximately $1.3 million, based on a specified formula. The 2018 Lease included a landlord‑provided tenant improvement allowance of up to $6.6 million to be applied to the costs of the construction of leasehold improvements. The Company determined that it owns the leasehold improvements related to the 2018 Lease and, as such, reflected the $6.6 million lease incentive as a reduction of the rental payments used to measure the operating lease liability, and, in turn, the operating lease right‑of‑use asset as of the lease commencement date in May 2019. Between the lease commencement date and December 31, 2019, the Company recorded increases of $6.6 million to the operating lease liability and to leasehold improvements as and when such leasehold improvements were paid for by the lessor. The Company also leases laboratory equipment under an agreement which is classified as a finance lease. The components of the Company’s lease expense are as follows (in thousands): December 31, 2020 Operating lease cost $ 1,613 Variable lease cost $ 2,557 Finance lease cost: Amortization of lease assets $ 109 Interest on lease liabilities 6 Total finance lease cost $ 115 Supplemental disclosure of cash flow information related to leases was as follows (in thousands): December 31, 2020 Cash paid for amounts included in the measurement of operating lease liabilities (operating cash flows) $ 2,004 Cash paid for amounts included in the measurement of finance lease liabilities (operating cash flows) $ 6 Cash paid for amounts included in the measurement of finance lease liabilities (financing cash flows) $ 253 The weighted‑average remaining lease term and discount rate were as follows: December 31, December 31, 2020 2019 Weighted‑average remaining lease term (in years) used for: Operating leases 7.17 8.14 Finance leases 0.75 1.75 Weighted‑average discount rate used for: Operating leases 10.00 % 10.00 % Finance leases 1.99 % 1.99 % Because the interest rate implicit in the lease was not readily determinable, the Company’s incremental borrowing rate was used to calculate the present value of the 2018 Lease. In determining its incremental borrowing rate, the Company considered its credit quality and assessed interest rates available in the market for similar borrowings, adjusted for the impact of collateral over the term of the lease. The present value of the Company’s laboratory equipment lease was calculated using the rate implicit in the lease. Future annual lease payments under the 2018 Lease and the laboratory equipment finance lease as of December 31, 2020 were as follows (in thousands): Years Ending December 31, Operating Leases Finance Leases 2021 $ 2,380 $ 189 2022 2,457 — 2023 2,535 — 2024 2,601 — 2025 2,684 — Thereafter 6,012 — Total future lease payments 18,669 189 Less: Imputed interest (5,527) (1) Total lease liabilities $ 13,142 $ 188 The following table presents lease assets and liabilities and their classification on the consolidated balance sheet (in thousands): December 31, December 31, Leases Consolidated Balance Sheet Classification (unaudited) 2020 2019 Assets: Operating lease assets Operating lease right‑of‑use assets $ 5,964 $ 6,214 Finance lease assets Property and equipment, net 300 409 Total lease assets $ 6,264 $ 6,623 Liabilities: Current: Operating lease liabilities Operating lease liabilities $ 1,115 $ 641 Finance lease liabilities Accrued expenses and other current liabilities 188 247 Non‑current: Operating lease liabilities Operating lease liabilities, net of current portion 12,027 13,143 Finance lease liabilities Other long‑term liabilities — 188 Total lease liabilities $ 13,330 $ 14,219

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2020
Commitments and Contingencies
Commitments and Contingencies	13. Commitments and Contingencies Leases The Company’s commitments under its leases are described in Note 12. 401(k) Plan The Company has a defined‑contribution plan under Section 401(k) of the Code (the “401(k) Plan”). The 401(k) Plan covers all employees who meet defined minimum age and service requirements and allows participants to defer a portion of their annual compensation on a pre‑tax basis. As currently established, the Company is not required to make, and to date has not made, any contributions to the 401(k) Plan. Indemnification Agreements In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, contract research organizations, business partners, and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with members of its board of directors and certain of its executive officers that will 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. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. The Company has not incurred any material costs as a result of such indemnifications and is not currently aware of any indemnification claims. Legal Proceedings The Company is not currently party to any material legal proceedings. At each reporting date, the Company evaluates whether or not a potential loss amount or a potential range of loss is probable and reasonably estimable under the provisions of the authoritative guidance that addresses accounting for contingencies. The Company expenses as incurred the costs related to such legal proceedings.

Net Loss per Share and Unaudite

Net Loss per Share and Unaudited Pro Forma Net Loss per Share	12 Months Ended
	Dec. 31, 2020
Net Loss per Share
Net Loss per Share and Unaudited Pro Forma Net Loss per Share	14. Net Loss per Share Net Loss per Share Basic and diluted net loss per share attributable to common stockholders was calculated as follows (in thousands, except share and per share amounts): Year Ended December 31, 2020 2019 Numerator: Net loss attributable to common stockholders $ (48,600) $ (25,741) Denominator: Weighted‑average common shares outstanding, basic and diluted 17,550,847 605,824 Net loss per share attributable to common stockholders, basic and diluted $ (2.77) $ (42.49) The Company’s potential dilutive securities have been excluded from the computation of diluted net loss per share as the effect would be to reduce the net loss per share. Therefore, the weighted‑average number of common shares outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders is the same. The Company excluded the following potential common shares, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti‑dilutive effect: December 31, 2020 2019 Convertible preferred stock (as converted to common stock) — 8,463,388 Unvested restricted common stock 141,639 312,638 Stock options to purchase common stock 3,674,687 961,751 3,816,326 9,737,777

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2020
Subsequent Events
Subsequent Events	15. Subsequent Events Boston Facility Expansion On January 29, 2021, the Company entered into an amendment to a property lease agreement for its office and laboratory space located at 645 Summer Street in Boston, Massachusetts. The amendment expands the 2018 leased premises to include an additional 37,500 square feet for a total of approximately 75,000 square feet and extends the term of the 2018 lease for a new ten-year term. The monthly base rent for the expansion premises will start at $61,667 and will increase over the first 27 months of the term to $245,333 per month according to a prescribed schedule, following which it will increase every 12 months by approximately 3% per annum, up to a maximum monthly base rent of $310,781 during the term. The Company’s expected contractual obligation for the base rent over the term is approximately $30.1 million with the annual fixed payments escalating each period. The Company has posted a customary letter of credit in the amount of approximately $1.2 million as a security deposit pursuant to the Lease Amendment. In addition to the fixed rent during the lease term, the Company will be responsible for certain customary operating expenses and real estate taxes specified in the agreement.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2020
Summary of Significant Accounting Policies
Use of Estimates	Use of Estimates The preparation of the Company’s consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements, and the reported amounts of revenue and expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual of research and development expenses. The Company bases its estimates on historical experience, known trends, and other market‑specific or other relevant factors that it believes to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates, as there are changes in circumstances, facts, and experience. Changes in estimates are recorded in the period in which they become known. Actual results may differ from those estimates or assumptions.
Concentrations of Credit Risk and of Significant Suppliers	Concentrations of Credit Risk and of Significant Suppliers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash, cash equivalents and marketable securities. The Company maintains its cash, cash equivalents and marketable securities at one accredited financial institution. The Company’s cash equivalents as of December 31, 2020 consisted of U.S. government money market funds. Marketable securities as of December 31, 2020 consisted of U.S. Treasury notes with maturities of less than 12 months. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. The Company is dependent on third‑party vendors for its product candidates. In particular, the Company relies, and expects to continue to rely, on a small number of vendors to manufacture materials and components required for the production of its product candidates. These programs could be adversely affected by a significant interruption in the manufacturing process.
Cash Equivalents	Cash Equivalents The Company considers all highly liquid investments with a remaining maturity when purchased of three months or less to be cash equivalents.
Restricted Cash	Restricted Cash In connection with the Company’s lease agreement entered into in December 2018 (see Note 12), the Company is required to maintain a letter of credit of $1.3 million for the benefit of the landlord. As of the years ended December 31, 2020 and 2019, this amount was classified as restricted cash (non‑current) in the consolidated balance sheets. The Company is required to maintain a separate cash balance of less than $0.1 million pledged as collateral for a credit card account. As of December 31, 2020 and 2019, this amount was classified as restricted cash (non‑current) on the consolidated balance sheets.
Marketable Securities	Marketable Securities Marketable securities represent holdings of available-for-sale marketable debt securities in accordance with the Company’s investment policy. The Company has classified its investments with maturities beyond one year as current, based on their highly liquid nature and because such marketable securities represent the investment of cash that is available for current operations. Investments in marketable securities are recorded at fair value, with any unrealized gains and losses reported within accumulated other comprehensive income (loss) as a separate component of stockholders’ equity (deficit) until realized or until a determination is made that an other-than-temporary decline in market value has occurred. The amortized cost of debt securities is adjusted for amortization of premiums and accretion of discounts to maturity. Such amortization and accretion, together with interest on securities sold, is determined based on the specific identification method and any realized gains or losses on the sale of investments are reflected as a component of other income (expense).
Fair Value Measurements	Fair Value Measurements Certain assets and liabilities of the Company are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and financial liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: · Level 1—Quoted prices in active markets for identical assets or liabilities. · Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. · Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies, and similar techniques. The Company’s cash equivalents and marketable securities are carried at fair value, determined according to the fair value hierarchy described above (see Note 3). The carrying values of the Company’s accounts payable and accrued expenses approximate their fair values due to the short‑term nature of these liabilities.
Property and Equipment	Property and Equipment Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization expense is recognized using the straight‑line method over the estimated useful life of each asset as follows: Estimated Useful Life Laboratory equipment 5 years Furniture and fixtures 4 years Leasehold improvements Shorter of term of lease or 15 years Costs for capital assets not yet placed into service are capitalized as construction‑in‑progress and depreciated once placed into service. Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation and amortization are removed from the accounts and any resulting gain or loss is included in loss from operations. Expenditures for repairs and maintenance that do not improve or extend the life of the respective assets are charged to expense as incurred.
Classification and Accretion of Convertible Preferred Stock	Classification and Accretion of Convertible Preferred Stock The Company’s convertible preferred stock was classified outside of stockholders’ equity (deficit) on the consolidated balance sheets because the holders of such shares had liquidation rights in the event of a deemed liquidation that, in certain situations, was not solely within the control of the Company and would require the redemption of the then‑outstanding convertible preferred stock. The Series Seed convertible preferred stock (the “Series Seed preferred stock”), Series Seed 1 convertible preferred stock (the “Series Seed 1 preferred stock”), Series A convertible preferred stock (the “Series A preferred stock”), and Series B convertible preferred stock (the “Series B preferred stock,” and collectively with the Series Seed preferred stock, the Series Seed 1 preferred stock, and the Series A preferred stock, the “Preferred Stock”) of the Company were not redeemable, except in the event of a deemed liquidation. Because the occurrence of a deemed liquidation event was not currently probable, the carrying values of the convertible preferred stock were not being accreted to their redemption values. Subsequent adjustments to the carrying values to the convertible preferred stock would be made only when a deemed liquidation event becomes probable. Upon the closing of the Company’s initial public offering on June 30, 2020, all outstanding shares of convertible preferred stock automatically converted into 18,969,672 shares of common stock.
Impairment of Long-Lived Assets	Impairment of Long‑Lived Assets Long‑lived assets consist of property and equipment. Long‑lived assets to be held and used are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends, and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long‑lived asset group for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long‑lived asset group to its carrying value. An impairment loss would be recognized in loss from operations when estimated undiscounted future cash flows expected to result from the use of an asset group are less than its carrying amount. Impairment is measured based on the excess of the carrying value of the related assets over the fair value of such assets. The Company did not record any impairment losses on long‑lived assets during the years ended December 31, 2020 and 2019.
Segment Information	Segment Information The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. The Company’s focus is on developing gene therapies with the potential to restore, improve, and preserve high-acuity physiologic hearing for individuals with hearing loss. All of the Company’s long‑lived assets are located in the United States.
Research and Development Costs	Research and Development Costs Costs for research and development activities are expensed in the period in which they are incurred. Research and development expenses consist of costs incurred in performing research and development activities, including salaries and bonuses, stock‑based compensation, employee benefits, facilities costs, laboratory supplies, depreciation and amortization, manufacturing expenses, and external costs of vendors engaged to conduct research and preclinical development activities as well as the cost of licensing technology. Upfront payments under license agreements are expensed upon receipt of the license, and annual maintenance fees under license agreements are expensed in the period in which they are incurred. Milestone payments under license agreements are accrued, with a corresponding expense being recognized, in the period in which the milestone is determined to be probable of achievement and the related amount is reasonably estimable. Nonrefundable advance payments for goods or services to be received in the future for use in research and development activities are recorded as prepaid expenses. The prepaid amounts are expensed as the related goods are delivered or the services are performed.
Research, Development, and Manufacturing Contract Costs and Accruals	Research, Development, and Manufacturing Contract Costs and Accruals The Company has entered into various research, development, and manufacturing contracts with research institutions and other companies. These agreements are generally cancelable, and related costs are recorded as research and development expenses as incurred. The Company records accruals for estimated ongoing research and development costs. When billing terms under these contracts do not coincide with the timing of when the work is performed, the Company is required to make estimates of outstanding obligations to those third parties as of period end. Any accrual estimates are based on a number of factors, including the Company’s knowledge of the progress towards completion of the research, development, and manufacturing activities, invoicing to date under the contracts, communication from the research institutions and other companies of any actual costs incurred during the period that have not yet been invoiced, and the costs included in the contracts. Significant judgments and estimates may be made in determining the accrued balances at the end of any reporting period. Actual results could differ from the estimates made by the Company. The historical accrual estimates made by the Company have not been materially different from the actual costs.
Patent Costs	Patent Costs All patent‑related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses.
Stock-Based Compensation	Stock‑Based Compensation The Company measures stock options with service‑based vesting granted to employees, non‑employees, and directors based on the fair value on the date of grant using the Black‑Scholes option‑pricing model. The Company measures restricted common stock awards using the difference, if any, between the purchase price per share of the award and the fair value of the Company’s common stock at the date of grant. Compensation expense for employee awards is recognized over the requisite service period, which is generally the vesting period of the award. Compensation expense for non‑employee awards is recognized in the same manner as if the Company had paid cash in exchange for the goods or services, which is generally the vesting period of the award. The Company uses the straight‑line method to record the expense of awards with only service‑based vesting conditions. The Company has not granted awards with performance‑ or market‑based vesting conditions. The Company accounts for forfeitures of share‑based awards as they occur. The Company classifies stock‑based compensation expense in its consolidated statements of operations and comprehensive loss in the same manner in which the award recipient’s payroll costs are classified or in which the award recipient’s service payments are classified.
Comprehensive Loss	Comprehensive Income (Loss) Comprehensive income (loss) includes net loss as well as other changes in stockholders’ equity (deficit) that result from transactions and economic events other than those with stockholders. For the year ended December 31, 2020, comprehensive income (loss) included less than $0.1 million of unrealized gains on marketable securities. For the year ended December 31, 2019, there was no difference between net loss and comprehensive loss.
Income Taxes	Income Taxes The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the consolidated financial statements or in the Company’s tax returns. Deferred tax assets and liabilities are determined on the basis of the differences between the financial statements and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of the deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies. The Company accounts for uncertainty in income taxes recognized in the consolidated financial statements by applying a two‑step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more‑likely‑than‑not to be sustained, the tax position is then assessed to determine the amount of benefit to recognize in the consolidated financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties. The Company’s policy is to recognize interest and/or penalties related to income tax matters in income tax expense. The Company had no amounts accrued for interest and penalties on its consolidated balance sheets at December 31, 2020 and 2019.
Net Loss per Share	Net Loss per Share The Company follows the two‑class method when computing net income (loss) per share as the Company has issued shares that meet the definition of participating securities. The two‑class method determines net income (loss) per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two‑class method requires income (loss) available to common stockholders for the period to be allocated between common and participating securities based upon their respective rights to share in undistributed earnings as if all income (loss) for the period had been distributed. Basic net income (loss) per share attributable to common stockholders is computed by dividing net income (loss) attributable to common stockholders by the weighted‑average number of common shares outstanding for the period. Diluted net income (loss) per share attributable to common stockholders is computed by adjusting net income (loss) attributable to common stockholders to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net income (loss) per share attributable to common stockholders is computed by dividing the diluted net income (loss) attributable to common stockholders by the weighted‑average number of common shares outstanding for the period, including potential dilutive common shares. For purposes of this calculation, the Company’s outstanding stock options, unvested restricted common stock, and convertible preferred stock are considered potential dilutive common shares. The Company’s participating securities contractually entitle the holders of such securities to participate in dividends but do not contractually require the holders of such securities to participate in losses of the Company. Accordingly, in periods in which the Company reports a net loss, such losses are not allocated to such participating securities. In periods in which the Company reports a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive common shares are not assumed to have been issued if their effect is anti‑dilutive. The Company reported a net loss attributable to common stockholders for the years ended December 31, 2020 and 2019.
Leases	Leases The Company determines if an arrangement is a lease at inception. A contract contains a lease if the contract conveys the right to control the use of an identified asset for a period of time in exchange for consideration. The Company classifies leases at the lease commencement date as operating or finance leases and records a right‑of‑use asset and a lease liability on the consolidated balance sheet for all leases with an initial lease term of greater than 12 months. Leases with an initial term of 12 months or less are not recorded on the balance sheet, but payments are recognized as expense on a straight‑line basis over the lease term. The Company often enters into contracts that contain both lease and non‑lease components. Non‑lease components may include maintenance, utilities, and other operating costs. The Company combines the lease and non‑lease components of fixed costs in its lease arrangements as a single lease component. Variable costs, such as utilities or maintenance costs, are not included in the measurement of right‑of‑use assets and lease liabilities, but rather are expensed when the event determining the amount of variable consideration to be paid occurs. Lease liabilities and their corresponding right‑of‑use assets are recorded based on the present value of future lease payments over the expected lease term. The present value of future lease payments is determined by using the interest rate implicit in the lease if that rate is readily determinable; otherwise, the Company uses its estimated secured incremental borrowing rate for that lease term. The Company estimates its secured incremental borrowing rate for each lease based on the rate of interest that the Company would have to pay to borrow an amount equal to the lease payments on a collateralized basis over a similar term. Certain of the Company’s leases include options to extend or terminate the lease. The amounts determined for the Company’s right‑of‑use assets and lease liabilities generally do not assume that renewal options or early‑termination provisions, if any, are exercised, unless it is reasonably certain that the Company will exercise such options.
Recently Adopted and Issued Accounting Pronouncements	Recently Adopted Accounting Pronouncements In August 2018, the FASB issued ASU No. 2018‑13, Fair Value Measurement (Topic 820): Disclosure Requirements for Fair Value Measurement (“ASU 2018‑13”), which modifies the existing disclosure requirements for fair value measurements in Topic 820. The new disclosure requirements include disclosure related to changes in unrealized gains or losses included in other comprehensive income (loss) for recurring Level 3 fair value measurements held at the end of each reporting period and the explicit requirement to disclose the range and weighted‑average of significant unobservable inputs used for Level 3 fair value measurements. The other provisions of ASU 2018‑13 include eliminated and modified disclosure requirements. An entity is permitted to early adopt any removed or modified disclosures upon issuance of ASU 2018‑13 and delay adoption of the additional disclosures until their effective date. For all entities, this guidance is required to be adopted for annual periods beginning after December 15, 2019, including interim periods within those fiscal years. Early adoption is permitted. The Company adopted ASU 2018‑13 as of the required effective date of January 1, 2020, and the adoption did not have a material impact on the Company’s consolidated financial statements. Recently Issued Accounting Pronouncements From time to time, new accounting pronouncements are issued by the FASB or other standard‑setting bodies that the Company adopts as of the specified effective date. The Company qualifies as an “emerging growth company” as defined in the Jumpstart Our Business Startups Act of 2012 and has elected not to “opt out” of the extended transition related to complying with new or revised accounting standards, which means that when a standard is issued or revised and it has different application dates for public and non‑public companies, the Company can adopt the new or revised standard at the time non‑public companies adopt the new or revised standard and can do so until such time that the Company either (i) irrevocably elects to “opt out” of such extended transition period or (ii) no longer qualifies as an emerging growth company. The Company may choose to early adopt any new or revised accounting standards whenever such early adoption is permitted for non‑public companies. In June 2016, the FASB issued ASU No. 2016‑13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments (“ASU 2016‑13”), which requires the measurement and recognition of expected credit losses for financial assets held at amortized cost. ASU 2016‑13 replaces the existing incurred loss impairment model with an expected loss model. It also eliminates the concept of other‑than‑temporary impairment and requires credit losses related to available‑for‑sale debt securities to be recorded through an allowance for credit losses rather than as a reduction in the amortized cost basis of the securities. These changes may result in earlier recognition of credit losses. In November 2018, the FASB issued ASU No. 2018‑19, Codification Improvements to Topic 326, Financial Instruments—Credit Losses , which narrowed the scope and changed the effective date for non‑public entities for ASU 2016‑13. The FASB subsequently issued supplemental guidance within ASU No. 2019‑05, Financial Instruments—Credit Losses (Topic 326): Targeted Transition Relief , which provides an option to irrevocably elect the fair value option for certain financial assets previously measured at amortized cost basis. For public entities that are Securities and Exchange Commission filers, excluding entities eligible to be smaller reporting companies, ASU 2016‑13 is effective for annual periods beginning after December 15, 2019, including interim periods within those fiscal years. For all other entities, ASU 2016‑13 is effective for annual periods beginning after December 15, 2022, including interim periods within those fiscal years. Early adoption is permitted. The Company is currently evaluating the impact that the adoption of ASU 2016‑13 will have on its consolidated financial statements

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2020
Summary of Significant Accounting Policies
Schedule of estimated useful lives	Estimated Useful Life Laboratory equipment 5 years Furniture and fixtures 4 years Leasehold improvements Shorter of term of lease or 15 years

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2020
Fair Value Measurements
Schedule of fair value hierarchy for its assets and liabilities that are measured at fair value on a recurring basis	The following tables present the Company’s fair value hierarchy for its assets and liabilities that are measured at fair value on a recurring basis and indicate the level within the fair value hierarchy of the valuation techniques the Company utilized to determine such fair value (in thousands): Fair Value Measurements at December 31, 2020 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 68,016 $ — $ — $ 68,016 Restricted cash: Money market funds 1,292 — — 1,292 Marketable securities: U.S. Treasury notes — 239,078 — 239,078 $ 69,308 $ 239,078 $ — $ 308,386 Fair Value Measurements at December 31, 2019 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 24,281 $ — $ — $ 24,281 Restricted cash: Money market funds 1,292 — — 1,292 $ 25,573 $ — $ — $ 25,573

Marketable Securities (Tables)

Marketable Securities (Tables)	12 Months Ended
	Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Schedule of the fair value of available-for-sale marketable debt securities by type of security	December 31, 2020 Gross Gross Amortized Unrealized Unrealized Fair Cost Gain Loss Value Assets Marketable securities: U.S. Treasury notes $ 239,065 $ 16 $ (3) $ 239,078 $ 239,065 $ 16 $ (3) $ 239,078

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2020
Property and Equipment, Net
Schedule of property and equipment, net	Property and equipment, net consisted of the following (in thousands): 2020 2019 Laboratory equipment $ 4,576 $ 2,536 Furniture and fixtures 461 358 Leasehold improvements 8,915 8,915 Construction in progress 3,755 77 17,707 11,886 Less: Accumulated depreciation and amortization (2,319) (394) $ 15,388 $ 11,492

Accrued Expenses and Other Cu_2

Accrued Expenses and Other Current Liabilities (Tables)	12 Months Ended
	Dec. 31, 2020
Accrued Expenses and Other Current Liabilities
Schedule of accrued expenses and other current liabilities	Accrued expenses and other current liabilities consisted of the following (in thousands): 2020 2019 Accrued external research, development, and manufacturing expenses $ 2,442 $ 1,157 Accrued employee compensation and benefits 2,438 2,605 Accrued professional fees 1,175 67 Payments due for leasehold improvements 2,071 756 Other 573 377 $ 8,699 $ 4,962

Convertible Preferred Stock (Ta

Convertible Preferred Stock (Tables)	12 Months Ended
	Dec. 31, 2020
Convertible Preferred Stock
Schedule of Preferred stock	December 31, 2019 Preferred Stock Common Stock Preferred Stock Issued and Carrying Liquidation Issuable Upon Authorized Outstanding Value Preference Conversion Series Seed preferred stock 25,622,520 25,622,520 $ 6,989 $ 7,100 1,215,893 Series Seed 1 preferred stock 2,058,855 2,058,855 570 485 97,701 Series A preferred stock 150,667,630 150,667,630 51,131 50,100 7,149,794 178,349,005 178,349,005 $ 58,690 $ 57,685 8,463,388

Stock_Based Compensation (Table

Stock‑Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2020
Stock‑Based Compensation
Schedule of assumptions used in the Black Scholes option pricing model to determine the fair value of stock options granted	Year Ended December 31, 2020 2019 Risk‑free interest rate 0.51 % 1.61 % Expected volatility 89.4 % 86.1 % Expected dividend yield — — Expected term (in years) 6.0 6.0
Schedule of stock option activity	Weighted‑ Weighted‑ Average Average Aggregate Number of Exercise Contractual Intrinsic Shares Price Term Value (in years) (in thousands) Outstanding as of December 31, 2019 961,751 $ 1.69 9.5 $ 524 Granted 2,812,584 12.44 Exercised (41,882) 1.58 Forfeited (57,766) 3.32 Outstanding as of December 31, 2020 3,674,687 $ 9.92 $ 38,170 Vested and expected to vest as of December 31, 2020 3,674,687 $ 9.92 $ 38,170 Options exercisable as of December 31, 2020 588,000 $ 4.94 $ 8,797
Schedule of unvested common stock from option early exercises	Shares Unvested restricted common stock as of December 31, 2019 102,896 Vested (55,165) Unvested restricted common stock as of December 31, 2020 47,731
Schedule of restricted common stock award activity	Weighted‑ Average Grant‑Date Shares Fair Value Unvested restricted common stock as of December 31, 2019 209,742 $ 0.0443 Vested (115,834) 0.0427 Unvested restricted common stock as of December 31, 2020 93,908 $ 0.0529
Schedule of stock based compensation expense	The Company recorded stock‑based compensation expense in the following expense categories of its consolidated statements of operations and comprehensive loss (in thousands): December 31, 2020 2019 Research and development expenses $ 1,486 $ 76 General and administrative expenses 3,009 62 $ 4,495 $ 138

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2020
Income Taxes
Schedule of reconciliation of the U.S. federal statutory income tax rate	Year Ended December 31, 2020 2019 Federal statutory income tax rate (21.0) % (21.0) % State income taxes, net of federal benefit (6.0) (5.7) Federal and state research and development tax credits (2.5) (3.8) Non‑deductible items 1.0 2.0 Other — 0.9 Change in deferred tax asset valuation allowance 28.5 27.6 Effective income tax rate 0.0 % 0.0 %
Schedule Of net deferred tax assets	December 31, 2020 2019 Deferred tax assets: Net operating loss carryforwards $ 19,558 $ 7,676 Research and development tax credit carryforwards 2,843 1,615 Stock-based compensation 626 4 Operating lease liabilities 3,642 3,884 Other 792 516 Total deferred tax assets 27,461 13,695 Deferred tax liabilities: Property and equipment (1,934) (1,923) Operating lease right‑of‑use assets (1,711) (1,809) Total deferred tax liabilities (3,645) (3,732) Valuation allowance (23,816) (9,963) Net deferred tax assets $ — $ —
Schedule of changes in the valuation allowance	Year Ended December 31, 2020 2019 Valuation allowance at beginning of year $ 9,963 $ 2,869 Increases recorded to income tax provision 13,853 7,094 Valuation allowance at end of year $ 23,816 $ 9,963

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2020
Leases
Schedule of lease costs, supplemental disclosure, weighted average lease term and discount rate and lease classification	The components of the Company’s lease expense are as follows (in thousands): December 31, 2020 Operating lease cost $ 1,613 Variable lease cost $ 2,557 Finance lease cost: Amortization of lease assets $ 109 Interest on lease liabilities 6 Total finance lease cost $ 115 Supplemental disclosure of cash flow information related to leases was as follows (in thousands): December 31, 2020 Cash paid for amounts included in the measurement of operating lease liabilities (operating cash flows) $ 2,004 Cash paid for amounts included in the measurement of finance lease liabilities (operating cash flows) $ 6 Cash paid for amounts included in the measurement of finance lease liabilities (financing cash flows) $ 253 The weighted‑average remaining lease term and discount rate were as follows: December 31, December 31, 2020 2019 Weighted‑average remaining lease term (in years) used for: Operating leases 7.17 8.14 Finance leases 0.75 1.75 Weighted‑average discount rate used for: Operating leases 10.00 % 10.00 % Finance leases 1.99 % 1.99 %
Schedule of lease assets and liabilities and their classification on the consolidated balance sheet	The following table presents lease assets and liabilities and their classification on the consolidated balance sheet (in thousands): December 31, December 31, Leases Consolidated Balance Sheet Classification (unaudited) 2020 2019 Assets: Operating lease assets Operating lease right‑of‑use assets $ 5,964 $ 6,214 Finance lease assets Property and equipment, net 300 409 Total lease assets $ 6,264 $ 6,623 Liabilities: Current: Operating lease liabilities Operating lease liabilities $ 1,115 $ 641 Finance lease liabilities Accrued expenses and other current liabilities 188 247 Non‑current: Operating lease liabilities Operating lease liabilities, net of current portion 12,027 13,143 Finance lease liabilities Other long‑term liabilities — 188 Total lease liabilities $ 13,330 $ 14,219
Schedule of future annual payments under operating lease	Future annual lease payments under the 2018 Lease and the laboratory equipment finance lease as of December 31, 2020 were as follows (in thousands): Years Ending December 31, Operating Leases Finance Leases 2021 $ 2,380 $ 189 2022 2,457 — 2023 2,535 — 2024 2,601 — 2025 2,684 — Thereafter 6,012 — Total future lease payments 18,669 189 Less: Imputed interest (5,527) (1) Total lease liabilities $ 13,142 $ 188
Schedule of future annual payments under finance lease	Years Ending December 31, Operating Leases Finance Leases 2021 $ 2,380 $ 189 2022 2,457 — 2023 2,535 — 2024 2,601 — 2025 2,684 — Thereafter 6,012 — Total future lease payments 18,669 189 Less: Imputed interest (5,527) (1) Total lease liabilities $ 13,142 $ 188

Net Loss per Share (Tables)

Net Loss per Share (Tables)	12 Months Ended
	Dec. 31, 2020
Net Loss per Share
Schedule of net loss per share	Year Ended December 31, 2020 2019 Numerator: Net loss attributable to common stockholders $ (48,600) $ (25,741) Denominator: Weighted‑average common shares outstanding, basic and diluted 17,550,847 605,824 Net loss per share attributable to common stockholders, basic and diluted $ (2.77) $ (42.49)
Schedule of securities excluded from computation of net loss per share	December 31, 2020 2019 Convertible preferred stock (as converted to common stock) — 8,463,388 Unvested restricted common stock 141,639 312,638 Stock options to purchase common stock 3,674,687 961,751 3,816,326 9,737,777

Nature of the Business and Ba_2

Nature of the Business and Basis of Presentation (Details) $ / shares in Units, $ in Thousands	Jun. 30, 2020USD ($)$ / sharesshares	Jun. 18, 2020	Dec. 31, 2020USD ($)shares	Dec. 31, 2019USD ($)
Temporary Equity [Line Items]
Value of shares issued			$ 223,850
Accumulated deficit			$ (81,724)	$ (33,124)
Common Stock
Temporary Equity [Line Items]
Reverse stock split ratio		21.073
Shares issued | shares	14,375,000		14,375,000
Share price | $ / shares	$ 17
Value of shares issued	$ 223,800		$ 1
Shares issued, gross proceeds	$ 244,400
Conversion of Stock, Shares Issued | shares	18,969,672		18,969,672

Summary of Significant Accoun_4

Summary of Significant Accounting Policies (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)Institution
Summary of Significant Accounting Policies
Number of financial institutions in which the company maintains cash and cash equivalents | Institution	1
Letter of credit related to lease agreement	$ 1.3
Cash collateral for credit card account	$ 0.1

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2020
Laboratory equipment
Property and Equipment
Property, Plant and Equipment, Useful Life	5 years
Furniture and fixtures
Property and Equipment
Property, Plant and Equipment, Useful Life	4 years
Leasehold improvements
Property and Equipment
Property, Plant and Equipment, Useful Life	15 years

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Preferred stock, etc. (Details) - USD ($)	Jun. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Summary of Significant Accounting Policies [Line Items]
Impairment losses on long-lived assets		$ 0	$ 0
Unrealized gain on marketable securities		13,000
Amounts accrued for interest and penalties		$ 0	$ 0
Lessee, Operating Lease, Option to Extend		True
Lessee, Operating Lease, Option to Terminate		True
Lessee, Finance Lease, Option to Extend		True
Lessee, Finance Lease, Option to Terminate		True
Maximum
Summary of Significant Accounting Policies [Line Items]
Unrealized gain on marketable securities		$ 100,000
Common Stock
Summary of Significant Accounting Policies [Line Items]
Common stock shares converted	18,969,672	18,969,672

Fair Value Measurements (Detail

Fair Value Measurements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Transfers Between Level 1 and Level 2, Description and Policy [Abstract]
Transfer of fair value asset, Level 1 into Level 2	$ 0	$ 0
Transfer of fair value asset, Level 2 into Level 1	0	0
Transfer of fair value asset, into Level 3	0	0
Transfer of fair value asset, out of Level 3	0	0
Recurring
Assets:
Restricted cash	1,292
Assets	308,386	25,573
Recurring | Money market funds
Assets:
Cash equivalents	68,016	24,281
Restricted cash		1,292
Recurring | US Treasury Securities [Member]
Assets:
Marketable securities	239,078
Recurring | Level 1
Assets:
Restricted cash	1,292
Assets	69,308	25,573
Recurring | Level 1 | Money market funds
Assets:
Cash equivalents	68,016	24,281
Restricted cash		$ 1,292
Recurring | Level 2
Assets:
Assets	239,078
Recurring | Level 2 | US Treasury Securities [Member]
Assets:
Marketable securities	$ 239,078

Marketable Securities (Details)

Marketable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Marketable securities, Amortized Cost	$ 239,065
Unrealized gain	16
Unrealized loss	(3)
Fair market value	239,078	$ 0
US Treasury Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Marketable securities, Amortized Cost	239,065
Unrealized gain	16
Unrealized loss	(3)
Fair market value	$ 239,078

Property and Equipment, Net (De

Property and Equipment, Net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Property and Equipment
Property and equipment, Gross	$ 17,707	$ 11,886
Less: Accumulated depreciation and amortization	(2,319)	(394)
Property and Equipment, Net	15,388	11,492
Depreciation and amortization expense	1,925	361
Assets under finance leases, gross	500	500
Assets under finance leases, accumulated amortization	200	100
Laboratory equipment
Property and Equipment
Property and equipment, Gross	4,576	2,536
Furniture and fixtures
Property and Equipment
Property and equipment, Gross	461	358
Leasehold improvements
Property and Equipment
Property and equipment, Gross	8,915	8,915
Construction in progress
Property and Equipment
Property and equipment, Gross	$ 3,755	$ 77

Accrued Expenses and Other Cu_3

Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Accrued Expenses and Other Current Liabilities
Accrued external research, development, and manufacturing expenses	$ 2,442	$ 1,157
Accrued employee compensation and benefits	2,438	2,605
Accrued professional fees	1,175	67
Payments due for leasehold improvements	2,071	756
Other	573	377
Total	$ 8,699	$ 4,962

Convertible Preferred Stock (De

Convertible Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	Jun. 30, 2020	Feb. 29, 2020	Sep. 30, 2019	Dec. 31, 2020	Dec. 31, 2019	Aug. 31, 2019	Dec. 31, 2018
Temporary Equity [Line Items]
Conversion right liability						$ 4,800
Convertible preferred stock, Shares authorized				0	178,349,005
Convertible preferred stock, Par value				$ 0.0001	$ 0.0001
Convertible preferred stock, Shares issued				0	178,349,005
Convertible preferred stock, Shares outstanding				0	178,349,005		103,015,190
Gross proceeds received from sale of convertible preferred stock				$ 104,837	$ 25,016
Issuance costs				$ 3,419
Preferred stock, Shares issued				0	0
Preferred stock, Shares outstanding				0	0
Series A Preferred Stock [Member]
Temporary Equity [Line Items]
Requisite holders percentage				55.00%
Convertible preferred stock, Shares authorized					150,667,630
Shares issued			75,333,815
Share price			$ 0.33252
Gross proceeds received from sale of convertible preferred stock			$ 25,100
Issuance costs			100		$ 34
Amount reclassified			$ 6,000
Series B Preferred Stock [Member]
Temporary Equity [Line Items]
Shares issued		221,399,223
Share price		$ 0.47455
Gross proceeds received from sale of convertible preferred stock		$ 105,100
Issuance costs		$ 200		$ 228
Common Stock
Temporary Equity [Line Items]
Shares issued	14,375,000			14,375,000
Share price	$ 17
Common stock shares converted	18,969,672			18,969,672

Convertible Preferred Stock - B

Convertible Preferred Stock - Balances (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Temporary Equity [Line Items]
Preferred Stock Authorized	0	178,349,005
Preferred Stock Issued and Outstanding		178,349,005
Carrying Value		$ 58,690
Liquidation Preference		$ 57,685
Common Stock Issuable Upon Conversion		8,463,388
Series Seed preferred stock
Temporary Equity [Line Items]
Preferred Stock Authorized		25,622,520
Preferred Stock Issued and Outstanding		25,622,520
Carrying Value		$ 6,989
Liquidation Preference		$ 7,100
Common Stock Issuable Upon Conversion		1,215,893
Series Seed 1 preferred stock
Temporary Equity [Line Items]
Preferred Stock Authorized		2,058,855
Preferred Stock Issued and Outstanding		2,058,855
Carrying Value		$ 570
Liquidation Preference		$ 485
Common Stock Issuable Upon Conversion		97,701
Series A Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Stock Authorized		150,667,630
Preferred Stock Issued and Outstanding		150,667,630
Carrying Value		$ 51,131
Liquidation Preference		$ 50,100
Common Stock Issuable Upon Conversion		7,149,794

Convertible Preferred Stock - V

Convertible Preferred Stock - Voting (Details)	12 Months Ended
	Dec. 31, 2020item
Series A Preferred Stock [Member]
Temporary Equity [Line Items]
Requisite holders percentage	55.00%
Number of directors eligible to be elected	2
Series B Preferred Stock [Member]
Temporary Equity [Line Items]
Number of directors eligible to be elected	1

Convertible Preferred Stock - C

Convertible Preferred Stock - Conversion, Dividends, Liquidation (Details)	Jun. 30, 2020$ / shares	Dec. 31, 2020USD ($)shares	Feb. 29, 2020$ / shares	Dec. 31, 2019USD ($)shares	Sep. 30, 2019$ / shares	Dec. 31, 2018shares
Temporary Equity [Line Items]
Convertible preferred stock, Shares outstanding | shares		0		178,349,005		103,015,190
Conversion ratio	21.073
Dividends | $		$ 0		$ 0
Original issue price multiplier		3
Minimum
Temporary Equity [Line Items]
Requisite holders percentage	71.00%
Series A Preferred Stock [Member]
Temporary Equity [Line Items]
Requisite holders percentage		55.00%
Original Issue price					$ 0.33252
Series B Preferred Stock [Member]
Temporary Equity [Line Items]
Original Issue price			$ 0.47455
Common Stock
Temporary Equity [Line Items]
Original Issue price	$ 17

Stockholders' Equity (Details)

Stockholders' Equity (Details) $ / shares in Units, $ in Thousands	Jun. 30, 2020USD ($)$ / sharesshares	Jun. 18, 2020	Dec. 31, 2020USD ($)$ / sharesshares	Dec. 31, 2019$ / sharesshares
Class of Stock [Line Items]
Common Stock, Shares Authorized			200,000,000	235,000,000
Common Stock, Par or Stated Value Per Share | $ / shares			$ 0.0001	$ 0.0001
Issuance of common stock upon completion of initial public offering, net of commissions, underwriting discounts and offering costs | $			$ 223,850
Preferred Stock, Shares Authorized			5,000,000	0
Preferred Stock, Par or Stated Value Per Share | $ / shares			$ 0.0001	$ 0.0001
Preferred Stock, Shares Issued			0	0
Preferred Stock, Shares Outstanding			0	0
Common Stock
Class of Stock [Line Items]
Common Stock, Shares Authorized			200,000,000	235,000,000
Common Stock, Par or Stated Value Per Share | $ / shares			$ 0.0001	$ 0.0001
Reverse stock split ratio		21.073
Issuance of common stock upon completion of initial public offering, net of commissions, underwriting discounts and offering costs, shares	14,375,000		14,375,000
Share Price | $ / shares	$ 17
Shares issued, gross proceeds | $	$ 244,400
Issuance of common stock upon completion of initial public offering, net of commissions, underwriting discounts and offering costs | $	$ 223,800		$ 1

Stock-Based Compensation (Detai

Stock-Based Compensation (Details) - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
2016 Plan
Stock-Based Compensation
Number of shares authorized for issuance	3,722,685	1,508,669
Remaining shares available for issuance	0	363,350
2016 Plan | Stock option
Stock-Based Compensation
Vesting period	4 years
Expiration period	10 years
2020 Stock Plan
Stock-Based Compensation
Remaining shares available for issuance	4,212,479
Number of shares authorized, fixed portion	4,294,594
Number of shares authorized, variable portion	3,622,691
Annual increase in shares reserved for issuance, as a percent of stock outstanding	4.00%
2020 Stock Plan | Maximum
Stock-Based Compensation
Annual increase in shares reserved for issuance	2,728,610
2020 Employee Stock Purchase Plan
Stock-Based Compensation
Number of shares authorized for issuance	360,651
Remaining shares available for issuance	360,651
Annual increase in shares reserved for issuance	640,630
Annual increase in shares reserved for issuance, as a percent of stock outstanding	1.00%

Stock_Based Compensation - Opti

Stock‑Based Compensation - Options Valuation (Details) - Stock option	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Stock-Based Compensation
Risk-free interest rate	0.51%	1.61%
Expected volatility	89.40%	86.10%
Expected dividend yield
Expected term (in years)	6 years	6 years

Stock_Based Compensation - Stoc

Stock‑Based Compensation - Stock option activity (Details) - Stock option - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Number of Shares
Outstanding as at beginning of period (in shares)	961,751
Granted (in shares)	2,812,584
Exercised (in shares)	(41,882)
Forfeited (in shares)	(57,766)
Outstanding as at end of period (in shares)	3,674,687	961,751
Vested and expected to vest as at end of period (in shares)	3,674,687
Options exercisable as at end of period (in shares)	588,000
Weighted Average Exercise Price
Weighted average exercise price outstanding as at beginning of period (in dollars per share)	$ 1.69
Weighted average exercise price, Granted (in dollars per share)	12.44
Weighted average exercise price, Exercised (in dollars per share)	1.58
Weighted average exercise price, Forfeited (in dollars per share)	3.32
Weighted average exercise price, Outstanding as at end of period (in dollars per share)	9.92	$ 1.69
Vested and expected to vest as at end of period (in dollars per share)	9.92
Options exercisable as at end of period (in dollars per share)	$ 4.94
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Contractual term, Outstanding (in years)	9 years 1 month 6 days	9 years 6 months
Contractual term, Vested and expected to vest (in years)	9 years 1 month 6 days
Contractual term, Options exercisable (in years)	8 years 8 months 12 days
Average intrinsic value, Outstanding	$ 38,170	$ 524
Average intrinsic value, Vested and expected to vest	38,170
Average intrinsic value, Exercisable	8,797
Aggregate intrinsic value of stock options exercised	$ 800
Weighted-average grant-date fair value of stock options granted	$ 9.93	$ 1.57
Maximum
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Aggregate intrinsic value of stock options exercised		$ 100

Stock_Based Compensation - Earl

Stock‑Based Compensation - Early exercise of stock options into restricted stock (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Restricted stock from early exercise of options
Stock-Based Compensation
Unvested restricted common stock at beginning of period	102,896
Vested	(55,165)
Unvested restricted common stock at end of period	47,731	102,896
Stock option
Stock-Based Compensation
Liability for unvested shares		$ 0.1
Stock option | Maximum
Stock-Based Compensation
Liability for unvested shares	$ 0.1

Stock_Based Compensation - Rest

Stock‑Based Compensation - Restricted common stock award activity (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Restricted stock awards
Shares
Unvested restricted common stock at beginning of period	209,742
Vested	(115,834)
Unvested restricted common stock at end of period	93,908	209,742
Weighted Average Grant Date Fair Value
Unvested restricted common stock at beginning of period	$ 0.0443
Vested	0.0427
Unvested restricted common stock at end of period	$ 0.0529	$ 0.0443
Total fair value of restricted common stock vested		$ 0.2
Restricted stock awards | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Liability for unvested awards	$ 0.1	$ 0.1
Weighted Average Grant Date Fair Value
Total fair value of restricted common stock vested	$ 0.1
Service-based restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	4 years
Issued in period	0	0

Stock_Based Compensation - St_2

Stock‑Based Compensation - Stock based compensation expense (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Jul. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Stock-Based Compensation
Stock-based compensation expense		$ 4,495	$ 138
Accelerated vesting, expense recognized		1,100
Total unrecognized stock-based compensation expense related to the unvested stock‑based awards		$ 25,500
Unrecognized stock-based compensation expense, weighted average period		2 years 3 months 18 days
Stock option
Stock-Based Compensation
Accelerated vesting, number of shares	60,948
Stock option | Terminated Employee [Member]
Stock-Based Compensation
Accelerated vesting, number of shares	22,684
Restricted stock awards
Stock-Based Compensation
Accelerated vesting, number of shares	1,541
Research and development expenses
Stock-Based Compensation
Stock-based compensation expense		$ 1,486	76
General and administrative expenses
Stock-Based Compensation
Stock-based compensation expense		$ 3,009	$ 62

Income Taxes - Rate reconciliat

Income Taxes - Rate reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Income Taxes
Income tax benefits	$ 0	$ 0
Federal statutory income tax rate	(21.00%)	(21.00%)
State income taxes, net of federal benefit	(6.00%)	(5.70%)
Federal and state research and development tax credits	(2.50%)	(3.80%)
Non‑deductible items	1.00%	2.00%
Other		0.90%
Change in deferred tax asset valuation allowance	28.50%	27.60%
Effective income tax rate	0.00%	0.00%

Income Taxes - Net deferred tax

Income Taxes - Net deferred tax assets (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Net operating loss carryforwards	$ 19,558	$ 7,676
Research and development tax credit carryforwards	2,843	1,615
Stock based compensation	626	4
Operating lease liabilities	3,642	3,884
Other	792	516
Total deferred tax assets	27,461	13,695
Deferred tax liabilities:
Property and equipment	(1,934)	(1,923)
Operating lease right-of-use assets	(1,711)	(1,809)
Total deferred tax liabilities	(3,645)	(3,732)
Valuation allowance	$ (23,816)	$ (9,963)	$ (2,869)

Income Taxes - Operating loss c

Income Taxes - Operating loss carryforwards, etc. (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)
Income Tax [Line Items]
Federal net operating loss carryforwards	$ 71.7
Net operating loss carryforwards subject to expiration	0.4
Net operating loss carryforwards not subject to expiration	71.3
State operating loss carryforwards	71.3
Uncertain tax position	0
Federal
Income Tax [Line Items]
Research and development tax credit carryforwards	2.2
State
Income Tax [Line Items]
Research and development tax credit carryforwards	$ 0.8

Income Taxes - Valuation allowa

Income Taxes - Valuation allowance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Income Taxes
Valuation allowance at beginning of year	$ 9,963	$ 2,869
Valuation allowance, increases recorded to income tax provision	13,853	7,094
Valuation allowance at end of year	$ 23,816	$ 9,963

License Agreements (Details)

License Agreements (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Oct. 31, 2017USD ($)item	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)
License Agreements
Research and development		$ 34,297	$ 20,473
MEE License
License Agreements
Number of domestic patients | item	3,000
IND filing period	18 months
Fair value of stock	$ 100
Period for start commercial sale of product	10 years
Number of days for advance written notice	90 days
Research and development		0	0
MEE License | Maximum
License Agreements
Milestone payment amount	$ 17,700
Lonza Sublicense
License Agreements
Number of domestic patients | item	3,000
IND filing period	18 months
Fair value of stock	$ 100
Period for start commercial sale of product	10 years
Number of days for advance written notice	90 days
Research and development		$ 0	$ 0
Lonza Sublicense | Maximum
License Agreements
Milestone payment amount	$ 18,500

Leases (Details)

Leases (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Leases
Option to extend	true
Lease extension period	5 years
Annual base rent	$ 2,300
Percentage of annual increase	3.00%
Letter of credit related to lease agreement	$ 1,300
Amount of tenant lease improvement allowance	6,600
Operating lease liabilities arising from obtaining right-of-use assets	$ 6,600	$ 6,550

Leases - Components of lease co

Leases - Components of lease cost (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)
Lease cost
Operating lease cost	$ 1,613
Variable lease cost	2,557
Finance lease cost:
Amortization of lease assets	109
Interest on lease liabilities	6
Total finance lease cost	$ 115

Leases - Supplemental disclosur

Leases - Supplemental disclosure of cash flow information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Cash flow information related to leases
Cash paid for amounts included in the measurement of operating lease liabilities (operating cash flows)	$ 2,004
Cash paid for amounts included in the measurement of finance lease liabilities (operating cash flows)	6
Cash paid for amounts included in the measurement of finance lease liabilities (financing cash flows)	253	$ 99
Operating lease liabilities arising from obtaining right-of-use assets	$ 6,600	$ 6,550

Leases - Weighted average infor

Leases - Weighted average information (Details)	Dec. 31, 2020	Dec. 31, 2019
Leases
Weighted-average remaining lease term (in years) used for: Operating Lease	7 years 2 months 1 day	8 years 1 month 21 days
Weighted-average remaining lease term (in years) used for: Finance Leases	9 months	1 year 9 months
Weighted-average discount rate used for: Operating leases	10.00%	10.00%
Weighted-average discount rate used for: Finance leases	1.99%	1.99%

Leases - Future annual payments

Leases - Future annual payments (Details) $ in Thousands	Dec. 31, 2020USD ($)
Operating lease
2021	$ 2,380
2022	2,457
2023	2,535
2024	2,601
2025	2,684
Thereafter	6,012
Total future lease payments	18,669
Less: Imputed interest	(5,527)
Total lease liabilities	13,142
Finance Leases
2021	189
Total future lease payments	189
Less: Imputed interest	(1)
Total lease liabilities	$ 188

Leases - Assets and Liabilities

Leases - Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Leases
Operating lease assets	$ 5,964	$ 6,214
Finance lease assets	$ 300	$ 409
Finance lease assets, Balance sheet location	Property and equipment, net	Property and equipment, net
Total lease assets	$ 6,264	$ 6,623
Current:
Operating lease liabilities	1,115	641
Finance lease liabilities	$ 188	$ 247
Finance lease liabilities, Balance sheet location	Accrued expenses and other current liabilities	Accrued expenses and other current liabilities
Non‑current:
Operating lease liabilities, net of current portion	$ 12,027	$ 13,143
Finance lease liabilities		$ 188
Finance lease liabilities, Balance sheet location	Other long‑term liabilities	Other long‑term liabilities
Total lease liabilities	$ 13,330	$ 14,219

Net Loss per Share (Details)

Net Loss per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Net Loss per Share
Net loss attributable to common stockholders	$ (48,600)	$ (25,741)
Weighted-average common shares outstanding, basic and diluted	17,550,847	605,824
Net loss per share attributable to common stockholders, basic and diluted	$ (2.77)	$ (42.49)

Net Loss per Share - Antidiluti

Net Loss per Share - Antidilutive securities (Details) - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Net Loss per Share and Unaudited Pro Forma Net Loss per Share
Securities excluded from computation of earnings per share amount	3,816,326	9,737,777
Convertible preferred stock
Net Loss per Share and Unaudited Pro Forma Net Loss per Share
Securities excluded from computation of earnings per share amount		8,463,388
Restricted stock awards
Net Loss per Share and Unaudited Pro Forma Net Loss per Share
Securities excluded from computation of earnings per share amount	141,639	312,638
Stock option
Net Loss per Share and Unaudited Pro Forma Net Loss per Share
Securities excluded from computation of earnings per share amount	3,674,687	961,751

Subsequent Events (Details)

Subsequent Events (Details)	Jan. 29, 2021USD ($)ft²	Dec. 31, 2020USD ($)
Subsequent Event [Line Items]
Lease extension period		5 years
Aggregate total fixed rent, undiscounted		$ 18,669,000
Aggregate total fixed rent, present value		13,142,000
Letter of credit related to lease agreement		$ 1,300,000
Subsequent event
Subsequent Event [Line Items]
Increase in area under lease | ft²	37,500
Area under lease | ft²	75,000
Lease extension period	10 years
Monthly base rent, initial period	$ 61,667
Scheduled increase period	27 months
Monthly base rent, at end of scheduled increase period	$ 245,333
Monthly rent, annual percentage increase after scheduled period	3.00%
Monthly base rent, at end of percentage increase period	$ 310,781
Aggregate total fixed rent, undiscounted	30,100,000
Letter of credit related to lease agreement	$ 1,200,000