aTyr Pharma (ATYR) 10-K 24 Mar 21 2020 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2020	Mar. 19, 2021	Jun. 30, 2020
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Trading Symbol	LIFE
Entity Registrant Name	aTYR PHARMA INC
Entity Central Index Key	0001339970
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false
ICFR Auditor Attestation Flag	false
Entity Common Stock, Shares Outstanding		16,011,385
Entity Public Float			$ 25,388,988
Document Annual Report	true
Document Transition Report	false
Entity File Number	001-37378
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	20-3435077
Entity Address, Postal Zip Code	92121
City Area Code	858
Local Phone Number	731-8389
Entity Address, Address Line One	3545 John Hopkins Court
Entity Address, Address Line Two	Suite #250
Entity Address, City or Town	San Diego
Entity Address, State or Province	CA
Security Exchange Name	NASDAQ
Title of 12(b) Security	Common Stock, par value $0.001 per share
Entity Interactive Data Current	Yes
Documents Incorporated by Reference	Portions of the registrant’s proxy statement to be filed with the Securities and Exchange Commission (SEC), pursuant to Regulation 14A in connection with the registrant’s 2021 Annual Meeting of Stockholders, which will be filed subsequent to the date hereof, are incorporated by reference into Part III of this annual report on Form 10-K. Such proxy statement will be filed with the SEC not later than 120 days following the end of the registrant’s fiscal year ended December 31, 2020

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 16,952	$ 9,210
Available-for-sale investments	14,737	21,934
Other receivables	2,039	100
Prepaid expenses	1,803	681
Total current assets	35,531	31,925
Property and equipment, net	899	1,270
Right-of-use assets	2,083	2,821
Other assets	213	172
Total assets	38,726	36,188
Current liabilities:
Accounts payable	1,431	847
Accrued expenses	3,572	2,376
Contract liability		208
Current portion of operating lease liability	861	755
Term loans, net of issuance costs and discount (Note 6)		8,737
Total current liabilities	5,864	12,923
Long-term operating lease liability, net of current portion	1,378	2,239
Commitments and contingencies (Note 6)
Stockholders’ equity:
Preferred stock, $0.001 par value per share; 5,000,000 undesignated authorized shares; Class X Convertible Preferred Stock issued and outstanding shares – 0 and 1,643,961 as of December 31, 2020 and December 31, 2019, respectively		2
Common stock, $0.001 par value per share; 21,425,000 and 10,714,286 authorized shares as of December 31, 2020 and December 31, 2019, respectively; issued and outstanding shares – 11,018,954 and 3,891,787 as of December 31, 2020 and December 31, 2019, respectively	11	4
Additional paid-in capital	370,210	343,524
Accumulated other comprehensive loss	(43)	(40)
Accumulated deficit	(338,528)	(322,304)
Total aTyr Pharma stockholders’ equity	31,650	21,186
Noncontrolling interest in Pangu BioPharma Limited	(166)	(160)
Total stockholders' equity	31,484	21,026
Total liabilities and stockholders’ equity	$ 38,726	$ 36,188

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2020	Dec. 31, 2019
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	5,000,000	5,000,000
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	21,425,000	10,714,286
Common stock, shares issued	11,018,954	3,891,787
Common stock, shares outstanding	11,018,954	3,891,787
Class X Convertible Preferred Stock [Member]
Preferred stock, shares issued	0	1,643,961
Preferred stock, shares outstanding	0	1,643,961

Consolidated Statements of Oper

Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues:
Total revenues	$ 10,455	$ 422
Operating expenses:
Research and development	17,291	14,048	$ 20,385
General and administrative	9,075	9,352	12,435
Total operating expenses	26,366	23,400	32,820
Loss from operations	(15,911)	(22,978)	(32,820)
Total other expense, net	(319)	(785)	(1,695)
Consolidated net loss	(16,230)	(23,763)	(34,515)
Net loss attributable to noncontrolling interest in Pangu BioPharma Limited	6	160
Net loss attributable to aTyr Pharma, Inc.	$ (16,224)	$ (23,603)	$ (34,515)
Net loss per share, basic and diluted	$ (1.77)	$ (7.03)	$ (16.11)
Shares used in computing net loss per share, basic and diluted	9,160,269	3,355,600	2,141,961
License Revenues [Member]
Revenues:
Total revenues	$ 10,455	$ 422

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement Of Income And Comprehensive Income [Abstract]
Consolidated net loss	$ (16,230)	$ (23,763)	$ (34,515)
Other comprehensive gain (loss):
Change in unrealized gain (loss) on available-for-sale investments, net of tax	(3)	20	60
Comprehensive loss	(16,233)	(23,743)	(34,455)
Comprehensive loss attributable to noncontrolling interest Pangu BioPharma Limited	6	160
Comprehensive loss attributable to aTyr Pharma, Inc. common stockholders	$ (16,227)	$ (23,583)	$ (34,455)

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Convertible Preferred Stock	Common Stock [Member]	Additional Paid-In Capital [Member]	Other Comprehensive Gain/(Loss) [Member]	Accumulated Deficit [Member]	Noncontrolling Interest [Member]
Beginning balance at Dec. 31, 2017	$ 64,245	$ 2	$ 2	$ 328,547	$ (120)	$ (264,186)
Beginning balance, Shares at Dec. 31, 2017		2,285,952	2,129,968
Exercise of common stock options and release of restricted stock units	14			14
Issuance of common stock upon release of restricted stock units, Shares			3,670
Issuance of common stock pursuant to employee stock purchase plan	36			36
Issuance of common stock pursuant to employee stock purchase plan, Shares			3,028
Issuance of common stock from at the market offerings, net of offering costs	379			379
Issuance of common stock from at the market offerings, net of offering costs, Shares			49,723
Stock-based compensation	3,431			3,431
Change in unrealized gain (loss) on available-for-sale investments, net of tax	60				60
Net loss	(34,515)
Net loss	(34,515)					(34,515)
Ending balance at Dec. 31, 2018	33,650	$ 2	$ 2	332,407	(60)	(298,701)
Ending balance, Shares at Dec. 31, 2018		2,285,952	2,186,389
Conversion of preferred stock to common stock, shares		(641,991)	229,283
Issuance of common stock upon release of restricted stock units, Shares			7,487
Issuance of common stock pursuant to employee stock purchase plan	13			13
Issuance of common stock pursuant to employee stock purchase plan, Shares			3,117
Issuance of common stock from at the market offerings, net of offering costs	4,405		$ 1	4,404
Issuance of common stock from at the market offerings, net of offering costs, Shares			805,357
Issuance of common stock from registered direct offering, net of offering costs	4,918		$ 1	4,917
Issuance of common stock from registered direct offering, net of offering costs, Shares			660,154
Stock-based compensation	1,783			1,783
Change in unrealized gain (loss) on available-for-sale investments, net of tax	20				20
Net loss	(23,763)					(23,603)	$ (160)
Net loss	(23,603)
Ending balance at Dec. 31, 2019	21,186
Ending balance, Shares at Dec. 31, 2019		1,643,961	3,891,787
Ending balance at Dec. 31, 2019	21,026	$ 2	$ 4	343,524	(40)	(322,304)	(160)
Conversion of preferred stock to common stock		$ (2)	$ 1	1
Conversion of preferred stock to common stock, shares		(1,643,961)	587,444
Issuance of common stock from underwritten follow-on offering, net of offering costs	18,779		$ 4	18,775
Issuance of common stock from underwritten follow-on offering, net of offering costs, Shares			4,870,588
Issuance of common stock upon release of restricted stock units, Shares			8,678
Issuance of common stock pursuant to employee stock purchase plan	10			10
Issuance of common stock pursuant to employee stock purchase plan, Shares			3,382
Issuance of common stock from at the market offerings, net of offering costs	6,437		$ 2	6,435
Issuance of common stock from at the market offerings, net of offering costs, Shares			1,657,075
Stock-based compensation	1,465			1,465
Change in unrealized gain (loss) on available-for-sale investments, net of tax	(3)				(3)
Net loss	(16,230)					(16,224)	(6)
Net loss	(16,224)
Ending balance at Dec. 31, 2020	31,650
Ending balance, Shares at Dec. 31, 2020			11,018,954
Ending balance at Dec. 31, 2020	$ 31,484		$ 11	$ 370,210	$ (43)	$ (338,528)	$ (166)

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities:
Consolidated net loss	$ (16,230)	$ (23,763)	$ (34,515)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	569	635	746
Stock-based compensation	1,465	1,783	3,431
Debt discount accretion and non-cash interest expense	346	707	966
Amortization (accretion) of premium (discount) of available-for-sale investment securities	110	(284)	(261)
Amortization of right-of-use assets	697	731
Loss (gain) on disposal of property and equipment	3	(28)	18
Changes in operating assets and liabilities:
Other receivables	(1,939)	58	(53)
Prepaid expenses and other assets	(1,122)	276	663
Accounts payable and accrued expenses	1,763	162	(2,058)
Contract liability	(208)	208
Operating lease liability	(755)	(498)
Net cash used in operating activities	(15,301)	(20,013)	(31,063)
Cash flows from investing activities:
Purchases of property and equipment	(202)	(79)	(594)
Purchases of available-for-sale investment securities	(21,066)	(40,647)	(40,299)
Maturities of available-for-sale investment securities	28,150	45,600	78,065
Proceeds from sale of property and equipment	18	51
Net cash provided by investing activities	6,900	4,925	37,172
Cash flows from financing activities:
Proceeds from issuance of common stock through employee stock purchase plan	10	13	36
Proceeds from issuance of common stock through option exercises			14
Proceeds from issuance of common stock through at the market offerings, net of offering costs	6,437	4,405	379
Proceeds from issuance of common stock through registered direct offering, net of offering costs		4,918
Proceeds from issuance of common stock through underwritten follow-on offering, net of offering costs	18,779
Repayments on borrowings	(9,083)	(8,000)	(4,667)
Net cash provided by (used in) financing activities	16,143	1,336	(4,238)
Net change in cash and cash equivalents	7,742	(13,752)	1,871
Cash and cash equivalents at beginning of period	9,210	22,962	21,091
Cash and cash equivalents at the end of period	16,952	9,210	22,962
Supplemental disclosure of cash flow information:
Interest paid	308	$ 1,122	1,700
Purchase of fixed assets included in accounts payable	$ 17		$ 4

Organization, Business and Basi

Organization, Business and Basis of Presentation	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Organization, Business and Basis of Presentation	1. Organization, Business and Basis of Presentation Organization and Business We were incorporated in the state of Delaware on September 8, 2005. We are focused on the discovery and development of innovative medicines based on novel immunological pathways. Principles of Consolidation Our consolidated financial statements include our accounts and our 98% majority-owned subsidiary in Hong Kong, Pangu BioPharma Limited (Pangu BioPharma). All intercompany transactions and balances are eliminated in consolidation. Liquidity and Financial Condition Other than the net income generated in the three months ended March 31, 2020, we have incurred losses and negative cash flows from operations since our inception. As of December 31, 2020, we had an accumulated deficit of $338.5 million and we expect to continue to incur net losses for the foreseeable future. As of December 31, 2020, our cash, cash equivalents and available-for-sale investments were $31.7 million. Subsequent to December 31, 2020, we received $2.0 million related to the Kyorin Agreement, approximately $9.9 million in gross proceeds from our at-the-market offering (ATM Offering Program) before deducting commissions and offering expenses and approximately $15.3 million in gross proceeds from our purchase agreement (Purchase Agreement) before deducting offering expenses payable by us. We believe that our current cash, cash equivalents and available-for-sale investments, will be sufficient to meet our anticipated cash requirements for a period of at least one year from the date of this Annual Report. We do not expect to generate any revenues from product sales unless and until we successfully complete development and obtain regulatory approval for one or more of our product candidates, which we expect will take a number of years at a minimum. If we obtain regulatory approval for any of our product candidates, we expect to incur significant commercialization expenses related to product sales, marketing, manufacturing and distribution. Accordingly, we will need to raise substantial additional capital to fund our operations. The amount and timing of our future funding requirements will depend on many factors, including the pace and results of our preclinical and clinical development efforts and the timing and nature of the regulatory approval process for our product candidates. We anticipate that we will seek to fund our operations through equity offerings, grant funding, collaborations, strategic partnerships and/or licensing arrangements, and when we are closer to commercialization of our product candidates potentially through debt financings. However, we may be unable to raise additional capital or enter into such arrangements when needed on favorable terms or at all. Our failure to raise capital or enter into such arrangements when needed would have a negative impact on our financial condition and ability to develop our product candidates. Use of Estimates Our consolidated financial statements are prepared in accordance with generally accepted accounting principles (GAAP). The preparation of our consolidated financial statements requires us to make estimates and assumptions that impact the reported amounts of assets, liabilities and expenses and the disclosure for these items in our consolidated financial statements and accompanying notes. The most significant estimates in our consolidated financial statements relate to the fair value of equity issuances and awards, and clinical trial and research and development expenses. Although these estimates are based on our knowledge of current events and actions we may undertake in the future, actual results may ultimately differ materially from these estimates and assumptions. Reclassifications Certain reclassifications have been made to prior year amounts to conform to the current year presentation. The reclassifications were not material to the consolidated financial statements. Segment Reporting Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision-maker in making decisions regarding resource allocation and assessing performance. We view our operations and manage our business in one operating segment.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Cash and Cash Equivalents Cash and cash equivalents consist primarily of readily available checking, money market accounts and money market funds. We consider all highly liquid investments that mature in three months or less when purchased to be cash equivalents. Investment Securities Investment securities primarily consist of investment grade corporate debt securities, asset-backed securities and commercial paper. We classify all investment securities as available-for-sale. Investment securities are carried at fair value, with the unrealized gains and losses, if any, reported as a component of other comprehensive income (loss) in stockholders’ equity until realized. Realized gains and losses from the sale of investment securities, if any, are determined on a specific identification basis. A decline in the market value of any investment security below cost that is determined to be other than temporary will result in an impairment charge to earnings and a new cost basis for the security is established. No such impairment charges were recorded for any period presented. Premiums and discounts are amortized or accreted over the life of the related security as an adjustment to yield using the straight-line method and are included in interest income. Interest income is recognized when earned. As of December 31, 2020, we held an aggregate total of $14.7 million of investment securities which consisted of corporate debt securities, asset-backed securities, and commercial paper all of which will mature in less than one year, and there was an unrealized gain of approximately $7,000 between the amortized cost and fair value of these investment securities. As of December 31, 2019, we held $21.9 million of corporate debt securities, asset-backed securities and commercial paper, all of which mature in less than one year, and there was an unrealized gain of approximately $10,000 between the amortized cost and fair value of these investment securities. Concentration of Credit Risk Financial instruments that potentially subject us to significant concentration of credit risk consist primarily of cash, cash equivalents and investment securities. We have established guidelines regarding diversification of investments and their maturities, which are designed to maintain principal and maximize liquidity. We maintain deposits in federally insured financial institutions in excess of federally insured limits. We have not experienced any losses in such accounts and we believe that we are not exposed to significant credit risk due to the financial position of the depository institutions in which those deposits are held. Property and Equipment Property and equipment are stated at cost and depreciated on a straight-line basis over the estimated useful life of the related assets (generally three to seven years). Leasehold improvements are stated at cost and amortized on a straight-line basis over the lesser of the remaining term of the related lease or the estimated useful life of the leasehold improvements. Repairs and maintenance costs are charged to expense as incurred . Impairment of Long-Lived Assets Long-lived assets consist primarily of property and equipment. An impairment loss is recorded if and when events and circumstances indicate that assets might be impaired and the undiscounted cash flows estimated to be generated by those assets are less than the carrying amount of those assets. While our current and historical operating losses are indicators of impairment, we believe that future cash flows to be received support the carrying value of our long-lived assets and, accordingly, have not recognized any impairment losses since inception. Accrued Expenses Expenses related to clinical studies, preclinical development activities and product manufacturing are based on estimates of the services received and efforts expended pursuant to our contractual arrangements. There may be instances in which payments made to our service providers will temporarily exceed the level of services provided and result in a prepayment of the expense. Payments under some of these contracts depend on factors such as, for clinical studies, the successful enrollment of patients, site initiation and the completion of clinical milestones. We make estimates of our accrued expenses as of each balance sheet date based on facts and circumstances known at the time. In accruing service fees, we estimate the time period over which services will be performed and the level of effort to be expended in each period. If the actual timing of the performance of services or the level of effort varies from our estimate, we adjust the accrual or prepaid expense balance accordingly. Historically, our estimated accrued liabilities have materially approximated actual expenses incurred. Leases We follow Accounting Standards Codification (ASC) Topic 842, Leases We do not separate lease and non-lease components for our long-term leases. Rent expense for the operating lease is recognized on a straight-line basis over the lease term and is included in operating expenses in our consolidated statements of operations. Revenue Recognition We evaluate our agreements under ASC Topic 606, Revenue from Contracts with Customers and ASC Topic 808, Collaborative Arrangements . . In determining the appropriate amount of revenue to be recognized as we fulfill our obligations under our agreement, we perform the following steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when (or as) we satisfy each performance obligation. As part of the accounting for these arrangements, we must develop assumptions that require judgment to determine the stand-alone selling price for each performance obligation identified in the contract. We use key assumptions to determine the stand-alone selling price, which may include forecasted revenues, development timelines, reimbursement rates for personnel costs, discount rates and probabilities of technical and regulatory success. We recognize revenue in one of two ways, over time or at a point in time. We recognize revenue over time when we are executing on our performance obligation over time and our partner receives benefit over time. For example, we recognize revenue over time when we provide research and development services. We recognize revenue at a point in time when we transfer control of a distinct performance obligation to our partner. For example, if Research and Development Costs Research and development costs are expensed as incurred. Research and development costs include: salaries and employee-related expenses, including stock-based compensation and benefits for personnel in research and product development functions; costs associated with conducting our preclinical, development and regulatory activities, including fees paid to third-party professional consultants, service providers and our scientific, therapeutic and clinical advisors; costs to acquire, develop and manufacture preclinical study and clinical trial materials; costs incurred under clinical trial agreements with clinical research organizations Patent Costs Costs related to filing and pursuing patent applications are recorded as general and administrative expense and expensed as incurred since recoverability of such expenditures is uncertain. Stock-Based Compensation Stock-based compensation expense represents the grant date fair value of employee stock option grants recognized as expense over the requisite service period of the awards (usually the vesting period) on a straight-line basis. We estimate fair value of stock option grants using the Black-Scholes option pricing model. We estimate the fair value using assumptions, including the risk-free interest rate, the expected volatility of a peer group of similar companies, the expected term of the awards and the expected dividend yield. These estimates involve inherent uncertainties and the application of management’s judgment. If factors change and different assumptions are used, our stock-based compensation expense could be materially different in the future. We follow ASC Topic 718, Compensation – Stock Compensation Income Taxes We account for income taxes under 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 included in the financial statements. Under this method, 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. The effect of a change in tax rates on deferred tax assets and liabilities is recognized as income in the period that includes the enactment date. We recognize net deferred tax assets to the extent that we believe these assets are more likely than not to be realized. In making such a determination, we consider all available positive and negative evidence, including future reversals of existing taxable temporary differences, projected future taxable income, tax-planning strategies and results of recent operations. If we determine that we would be able to realize the deferred tax assets in the future in excess of their net recorded amount, we would make an adjustment to the deferred tax asset valuation allowance, which would reduce the provision for income taxes. We record uncertain tax positions on the basis of a two-step process whereby (1) we determine whether it is more likely than not that the tax positions will be sustained on the basis of the technical merits of the position and (2) for those tax positions that meet the more-likely-than-not recognition threshold, we recognize the largest amount of tax benefit that is more than 50% likely to be realized upon ultimate settlement with the related tax authority. We recognize interest and penalties related to unrecognized tax benefits within income tax expense. Any accrued interest and penalties are included within the related tax liability. Net Loss Per Share Basic net loss per share is calculated by dividing the net loss attributable to common stockholders by the weighted average number of common shares outstanding for the period, without consideration for common stock equivalents. Diluted net loss per share attributable to common stockholders is calculated by dividing the net loss attributable to common stockholders by the weighted average number of common stock and common stock equivalents outstanding for the period determined using the treasury-stock method. Dilutive common stock equivalents are comprised of convertible preferred stock, warrants for common stock, options and restricted stock units outstanding under our stock option plan and estimated shares to be purchased under our employee stock purchase plan. For all periods presented, there is no difference in the number of shares used to calculate basic and diluted shares outstanding due to our net loss position. Potentially dilutive securities not included in the calculation of diluted net loss per share because to do so would be anti-dilutive are as follows (in common share equivalents): Years Ended December 31, 2020 2019 2018 Class X Preferred Stock (if-converted to common stock) — 587,445 816,851 Common stock warrants 13,904 13,904 477,639 Common stock options and restricted stock units 584,211 363,553 371,823 Employee stock purchase plan 1,602 1,958 1,610 Total 599,717 966,860 1,667,923 The following table summarizes our net loss per share (in thousands, except per share data): Years Ended December 31, 2020 2019 2018 Numerator: Net loss attributable to aTyr Pharma, Inc. $ (16,224 ) $ (23,603 ) $ (34,515 ) Denominator: Shares used in computing net loss per share, basic and diluted 9,160,269 3,355,600 2,141,961 Net loss per share - basic and diluted $ (1.77 ) $ (7.03 ) $ (16.11 ) Derivative Financial Instruments We do not use derivative instruments to hedge exposures to cash flow, market, or foreign currency risks. We evaluate all of our financial instruments, including warrants, to determine if such instruments are derivatives or contain features that qualify as embedded derivatives. We generally use the Black-Scholes option-pricing model to value the derivative instruments at inception and subsequent valuation dates. The classification of derivative instruments, including whether such instruments should be recorded as liabilities or as equity, is re-assessed at the end of each reporting period. Recent Accounting Pronouncements In June 2016, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) No. 2016-13, Financial Instruments – Credit Losses to provide financial statement users with more decision-useful information about the expected credit losses on financial instruments and other commitments to extend credit held by a reporting entity at each reporting date. To achieve this objective, the amendments in Topic 326 replace the incurred loss impairment methodology in current GAAP with a methodology that reflects expected credit losses and requires consideration of a broader range of reasonable and supportable information to inform credit loss estimates. Topic 326 is effective for fiscal years beginning after December 15, 2022, including interim periods within those fiscal years for smaller reporting companies. We expect the adoption of the amendments in ASU 2016-13 to not have an effect on its financial position and the results of its operations when such amendment is adopted. In December 2019, the FASB issued ASU No. 2019-12, Income Taxes Simplifying the Accounting for Income Taxes

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. Fair Value Measurements The carrying amounts of cash equivalents, prepaid and other assets, accounts payable and accrued liabilities are considered to be representative of their respective fair values because of the short-term nature of those instruments. Investment securities are recorded at fair value. The accounting guidance defines fair value, establishes a consistent framework for measuring fair value and expands disclosure for each major asset and liability category measured at fair value on either a recurring or nonrecurring basis. Fair value is defined as an exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or liability. As a basis for considering such assumptions, the accounting guidance establishes a three-tier fair value hierarchy, which prioritizes the inputs used in measuring fair value as follows: Level 1: Observable inputs such as quoted prices in active markets. Level 2: Inputs, other than the quoted prices in active markets that are observable either directly or indirectly. Level 3: Unobservable inputs in which there is little or no market data, which require the reporting entity to develop its own assumptions. Financial assets measured at fair value on a recurring basis consist of investment securities. Investment securities are recorded at fair value, defined as the exit price in the principal market in which we would transact, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. Level 2 securities are valued using quoted market prices for similar instruments, non-binding market prices that are corroborated by observable market data, or discounted cash flow techniques and include our investments in asset-backed securities, commercial paper, and corporate debt securities. We have no financial liabilities measured at fair value on a recurring basis. None of our non-financial assets and liabilities is recorded at fair value on a non-recurring basis. No transfers between levels have occurred during the periods presented. Assets measured at fair value on a recurring basis are as follows (in thousands): Fair Value Measurements Using Total Quoted Active for Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) As of December 31, 2020 Assets: Current: Cash equivalents $ 13,708 $ 13,708 $ — $ — Available-for-sale investments: Asset-backed securities 2,219 — 2,219 — Commercial paper 5,494 — 5,494 — Corporate debt securities 7,024 — 7,024 — Total available-for-sale investments 14,737 — 14,737 — Total assets measured at fair value $ 28,445 $ 13,708 $ 14,737 $ — Fair Value Measurements Using Total Quoted Active for Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) As of December 31, 2019 Assets: Current: Cash equivalents $ 8,248 $ 8,248 $ — $ — Available-for-sale investments: Asset-backed securities 6,304 — 6,304 — Commercial paper 7,568 — 7,568 — Corporate debt securities 8,062 — 8,062 — Total available-for-sale investments 21,934 — 21,934 — Total assets measured at fair value $ 30,182 $ 8,248 $ 21,934 $ — As of December 31, 2020 and 2019, available-for-sale investments are detailed as follows (in thousands): December 31, 2020 Gross Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Market Value Available-for-sale investments: Asset-backed securities $ 2,218 $ 1 $ — $ 2,219 Commercial paper 5,491 3 — 5,494 Corporate debt securities 7,021 3 — 7,024 $ 14,730 $ 7 $ — $ 14,737 December 31, 2019 Gross Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Market Value Available-for-sale investments: Asset-backed securities $ 6,299 $ 5 $ — $ 6,304 Commercial paper 7,568 — — 7,568 Corporate debt securities 8,057 5 — 8,062 $ 21,924 $ 10 $ — $ 21,934 At each reporting date, we perform an evaluation of impairment to determine if the unrealized losses are other-than-temporary. Factors considered in determining whether a loss is other-than-temporary include the length of time and extent to which fair value has been less than the cost basis, the financial condition of the issuer, and our intent and ability to hold the investment until recovery of its amortized cost basis. As of December 31, 2020, except of one investment security, all available-for-sale investments were in a gross unrealized gain position. The unrealized loss for one investment security was immaterial.

License, Collaboration and Othe

License, Collaboration and Other Agreements	12 Months Ended
	Dec. 31, 2020
License Agreement [Abstract]
License, Collaboration and Other Agreements	4. License, Collaboration and Other Agreements Kyorin Pharmaceutical Co., Ltd. In January 2020, we entered into a collaboration and license agreement with Kyorin Pharmaceutical Co., Ltd. (Kyorin) for the development and commercialization of ATYR1923 for ILD in Japan. Under the agreement (the Kyorin Agreement), Kyorin received an exclusive right to develop and commercialize ATYR1923 in Japan for all forms of interstitial lung diseases (ILD). Under the terms of the Kyorin Agreement, Kyorin is obligated to fund all research, development, regulatory, marketing and commercialization activities in Japan. In September 2020, Kyorin began dosing patients in a Phase 1 clinical trial of ATYR1923 (known as KRP-R120 in Japan) and completed the last subject visit in December 2020. The Phase 1 clinical trial, which was conducted and funded by Kyorin, is a placebo-controlled study to evaluate the safety, pharmacokinetics and immunogenicity of ATYR1923. Results from this study are intended to enable Kyorin to initiate clinical trials in ILD in Japan. We received an $8.0 million upfront payment in January 2020 and a $2.0 milestone payment in January 2021 upon completion of enrollment in the Phase 1 clinical trial, and we are eligible to receive up to an additional $165.0 million in the aggregate upon achievement of certain development, regulatory and sales milestones, as well as tiered royalties ranging from the mid-single digits to mid-teens on net sales in Japan. As of December 31, 2021, the $2.0 million milestone payment is recorded as Other receivables in the Consolidated Balance Sheets. Following the first anniversary of the effective date of the Kyorin Agreement, Kyorin has the right to terminate the agreement for any reason upon 90 days advance written notice. Either party may terminate the Kyorin Agreement in the event that the other party breaches the agreement and fails to cure the breach, becomes insolvent or challenges certain of the intellectual property rights licensed under the agreement . We assessed our license and collaboration with Kyorin in accordance with Topic 606 and concluded that Kyorin is a customer. We identified the following performance obligations under the Kyorin Agreement: 1) the license of ATYR1923 for ILD in Japan; and 2) free clinical trial material for Kyorin’s Phase 1 clinical trial. The $8.0 million upfront payment received from Kyorin is non-refundable and non-creditable and is considered fixed consideration. We determined that the relative stand-alone selling price was $7.9 million when the license was delivered to Kyorin in January 2020. We determined that the relative standalone selling price was $0.1 million for the free clinical trial material delivered to Kyorin in June 2020, using the “expected cost plus a margin” approach. In December 2020, Kyorin completed the last subject visit in its Phase 1 trial of ATYR1923. This achievement triggered a $2.0 million milestone payment, which we received in January 2021. For the year ended December 31, 2020, we recognized $10.0 million as license and collaboration agreement revenue for the upfront payment and milestone achievement. Both the milestones and royalty payments under the Kyorin Agreement are variable consideration. Since milestone payments are binary in nature, we will use the “most-likely” method to evaluate whether the milestones should be included as revenue. We will apply constraint to these amounts until the milestone is probable of being achieved. The royalties are dependent on future sales by Kyorin which are at the full discretion of Kyorin. Accordingly, we will apply a constraint to these amounts until the future sale sales have occurred. CSL Behring In March 2019, we entered into a research collaboration and option agreement with CSL Behring (CSL) for the development of product candidates derived from up to four tRNA synthetases from our preclinical pipeline (CSL Agreement). Under the terms of the CSL Agreement, CSL will fund all research and development activities related to the development of the applicable product candidates for the duration of the collaboration. CSL reimburses us for all research and development activities. The research and development activities will be performed in six phases by both parties. The first phase totaling $0.6 million was funded in May 2019 and future phases will be funded on a quarterly basis. In June 2020, the CSL Agreement was amended to extend the work on the first phase of the research program through September 30, 2020 and provided $0.2 million of additional funding for research and development activities. In February 2021, the CSL Agreement was terminated. We assessed our research collaboration with CSL in accordance with Topic 606 and concluded that CSL is a customer. We identified the following performance obligations under the CSL Agreement: 1) research services; and 2) participation in the Joint Steering Committee. We concluded that the performance obligations are interrelated and do not have a standalone basis. CSL has the right to terminate the research collaboration upon 45 days notice, which is considered to be the legally enforceable contract term. Therefore, during the first phase of research services, we have a 45 day performance obligation and all research services beyond the initial 45 days performance obligation are considered a material right. In addition, each phase of research services represents a separate customer option since CSL must provide written notice of its intent to advance to the next phase. Under the CSL Agreement, CSL is obligated to pay us for the costs incurred by us under the research programs. The payment of $0.6 million for the first phase of the research program received in May 2019 as well as the $0.2 million related to the amendment in June 2020 were considered fixed consideration and we will recognize revenue on the payment for the research service performance obligation as the services are performed. We are utilizing a cost-based input method to measure proportional performance and to calculate the corresponding amount of revenue to recognize. We believe this is the best measure of progress because other measures do not reflect how we transfer the performance obligation to our counterparty. In applying the cost-based input methods of revenue recognition, we use actual costs incurred relative to budgeted costs to fulfill the combined performance obligation. These costs consist primarily of third-party contract costs and internal full-time equivalent effort. A cost-based input method of revenue recognition requires us to make estimates of costs to complete the performance obligations. The cumulative effect of revisions to estimated costs to complete the performance obligations will be recorded in the period in which changes are identified and amounts can be reasonably estimated. A significant change in these assumptions and estimates could have a material impact on the timing and amount of revenue recognized in future periods. As of December 31, 2020, all activities under the CSL Agreement were complete and the contract liability had been recognized. For the years ended December 31, 2020 and 2019, we recognized $0.5 million and $0.4 million, respectively, as license and collaboration agreement revenues under the CSL Agreement. Hong Kong University of Science and Technology In March 2020, our subsidiary, Pangu BioPharma, together with the Hong Kong University of Science and Technology (HKUST) was awarded a grant of approximately $750,000 to build a high-throughput platform for the development of bi-specific antibodies. The two-year project is being funded by the Hong Kong Government’s Innovation and Technology Commission (ITC) under the Partnership Research Program (PRP). The PRP aims to support research and development projects undertaken by companies in collaboration with local universities and public research institutions. The grant will fund approximately 50% of the total estimated project cost, with aTyr contributing the remaining 50%. The research grant agreement between Pangu BioPharma, HKUST and the Government of the Hong Kong Special Administration Region was effective April 1, 2020. All the contributions provided by the ITC are paid to HKUST and we record expenses under this grant award when incurred. Expenses for the year ended December 31, 2020 were $0.2 million.

Balance Sheet Details

Balance Sheet Details	12 Months Ended
	Dec. 31, 2020
Balance Sheet Details [Abstract]
Balance Sheet Details	5. Balance Sheet Details Prepaid expenses consist of the following (in thousands): December 31, 2020 2019 Prepaid clinical and research expense $ 583 $ 22 Prepaid manufacturing expenses $ 513 $ 172 Other prepaid expenses 707 487 $ 1,803 $ 681 Property and equipment consist of the following (in thousands): December 31, 2020 2019 Computer and office equipment $ 552 $ 543 Scientific and laboratory equipment 5,270 5,241 Tenant improvements 1,701 1,700 7,523 7,484 Less accumulated depreciation and amortization (6,624 ) (6,214 ) $ 899 $ 1,270 As of December 31, 2020, 2019 and 2018, depreciation expense was $0.6 million, $0.6 million and $0.7 million, respectively. Accrued expenses consist of the following (in thousands): December 31, 2020 2019 Accrued salaries, wages and benefits $ 1,809 $ 1,275 Other accrued expenses (1) 1,763 1,101 $ 3,572 $ 2,376 (1) Other accrued expenses include expenses for clinical research organizations and contract manufacturing organizations.

Debt, Commitments and Contingen

Debt, Commitments and Contingencies	12 Months Ended
	Dec. 31, 2020
Debt Instruments [Abstract]
Debt, Commitments and Contingencies	6. Debt, Commitments and Contingencies Term Loans In November 2016, we entered into a loan and security agreement and subsequently entered amendments (collectively, the Loan Agreement), for term loans with Silicon Valley Bank (SVB) and Solar Capital Ltd. (Solar), to borrow up to $20.0 million issuable in three separate tranches (the Term Loans), $10.0 million of which was funded in November 2016, $5.0 million of which was funded in June 2017 and $5.0 million of which was funded in December 2017. In November 2020, the Term Loans were fully repaid, including the final payment equal to 8.75% of the funded amounts. In connection with the first tranche, we issued warrants to each of SVB and Solar to purchase an aggregate of 3,415 shares of our common stock with an exercise price of $43.93 per share. In connection with the second tranche, we issued warrants to each of SVB and Solar to purchase an aggregate of 1,489 shares of our common stock with an exercise price of $50.37 per share. In connection with the third tranche, we issued warrants to each of SVB and Solar to purchase an aggregate of 1,443 shares of our common stock with an exercise price of $51.98 per share. The warrants are immediately exercisable and have a maximum contractual term of seven years. The aggregate fair value of the warrants was determined to be $0.5 million using the Black-Scholes option pricing model and was recorded as debt discount which were accreted to interest expense over the life of Term Loans. Facility Lease We have a non-cancelable facility lease that is subject to base lease payments, which escalate over the term of the lease, additional charges for common area maintenance and other costs. In July 2018, we entered into a lease amendment that reduced the space we lease from 24,494 square feet to 20,508 square feet and extended the lease term to May 2023. With the lease amendment, we do not have an option to extend the lease. Operating lease expense for each of the years ended December 31, 2020, 2019 and 2018 was $1.0 million. A s of December 31, 2020 and 2019, the weighted average remaining lease term was 2.4 years and 3.4 years, respectively, and the weighted average discount rate for each years was 9.6%. Future minimum payments under the non-cancelable facility lease and reconciliation to the operating lease liability as of December 31, 2020 were as follows (in thousands): Operating Lease 2021 $ 1,031 2022 1,062 2023 404 Less: Amount representing interest (258 ) Present value of lease payments 2,239 Less: Current portion of operating lease liability (861 ) Long-term operating lease liability, net of current portion $ 1,378

Stockholders' Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Stockholders' Equity	7. Stockholders’ Equity At the Market Offering Program In June 2016, we entered into a sales agreement with Cowen and Company, LLC (Cowen) for at the market offerings (ATM Offering Program), under which we were able to offer and sell shares of our common stock having an aggregate offering price of up to $35.0 million from time to time. In May 2019, we terminated the ATM Offering Program with Cowen. During the year ended December 31, 2019 and prior to termination in May 2019, we sold an aggregate of 193,670 shares of common stock at an average price of $7.35 per common share for net proceeds of $1.4 million under the ATM Offering Program with Cowen In May 2019, we entered into a sales agreement with H.C. Wainwright & Co., LLC (Wainwright) to create an ATM Offering Program under which we may offer and sell shares of our common stock having an aggregate offering price of up to $10.0 million. In November 2020, we amended our sales agreement with Wainwright to increase the amount of the ATM Offering Program up to $20.0 million. Wainwright is entitled to a commission at a fixed commission rate equal to 3% of the gross proceeds. During the year ended December 31, 2020, we sold an aggregate of 1,657,075 shares of common stock at an average price of $4.07 per share for gross proceeds of $6.8 million under the ATM Offering Program. During the year ended December 31, 2019, we sold an aggregate of 611,687 shares of common stock at an average price of $5.43 per common share for Underwritten Follow-On Public Offering In February 2020, we completed an underwritten follow-on public offering of 4,235,294 shares of our common stock at a price to the public of $4.25 per share. In March 2020, the underwriters fully exercised their option to purchase additional shares resulting in the issuance of an additional 635,294 shares of common stock. The total gross proceeds from the underwritten follow-on public offering, including the underwriters’ option to purchase additional shares, was approximately $20.7 million, before deducting underwriting discounts, commissions and offering expenses payable by us. Purchase Agreement In September 2020 , 2014 Stock Plan We adopted a stock option plan in 2007 (the 2007 Plan), which was subsequently amended, restated and renamed in July 2014 (the 2014 Plan) to provide for the incentive stock options, nonstatutory stock options, stock and rights to purchase restricted stock to eligible recipients. Recipients of incentive stock options are eligible to purchase shares of our common stock at an exercise price equal to no less than the estimated fair market value of such stock on the date of grant. The maximum term of options under the 2014 Plan is ten years. Options granted generally vest over four years. Shares underlying any awards under the 2014 Plan that are forfeited, canceled, reacquired by us prior to vesting, satisfied without the issuance of stock or otherwise terminated (other than by exercise) will be added to shares available for issuance under the 2015 Plan. 2015 Stock Plan In April 2015, our board of directors adopted, and our stockholders approved, the 2015 Stock Plan (the 2015 Plan). The 2015 Plan became effective on May 6, 2015 and we ceased granting any new awards under our 2014 Plan. Awards granted under the 2014 Plan prior to our IPO that are forfeited, canceled, reacquired by us prior to vesting satisfied without the issuance of stock or otherwise terminated (other than by exercise) will be added to shares available for issuance under the 2015 Plan. A total of 112,469 shares of our common stock were initially reserved for issuance under the 2015 Plan. In addition, the number of shares reserved and available for issuance under the 2015 Plan automatically increased each January 1, beginning on January 1, 2016 and thereafter until January 1, 2019, by the lesser of (i) 131,428 shares, (ii) 4% of the outstanding number of shares of our common stock on the immediately preceding December 31 or (iii) an amount determined by our board of directors. Pursuant to this provision, through January 1, 2019, an aggregate of 307,949 additional shares were reserved for issuance under the 2015 Plan. At our 2020 Annual Meeting of Stockholders, our stockholders approved an amendment to increase the number of shares of common stock reserved under the 2015 Plan by 350,000 shares. Total shares available for issuance under the 2015 Plan as of December 31, 2020 were 381,663. Shares underlying any awards under the 2015 Plan that are forfeited, canceled, reacquired by us prior to vesting, satisfied without the issuance of stock or otherwise terminated (other than by exercise) will be added to shares available for issuance under the 2015 Plan. The maximum term of options granted under 2015 Plan is ten years. For an initial grant to an employee, 25% of the options generally vest on the first anniversary of the original vesting date, with the balance vesting monthly over the remaining three years. For subsequent grants to an employee, the options generally vest monthly over a four-year term. Inducement Grants Options under inducement grants vest over a period of four years, with 25% vesting on the one year anniversary of the grant date and the remaining 75% vesting on a monthly basis over three years thereafter, subject to continuous employment. These options were inducement grants issued outside of the 2015 Plan in accordance with Nasdaq Listing Rule 5635(c)(4). In addition, from time to time, we may make inducement grants of stock options to new employees. During the year ended December 31, 2020, we did not make inducement grants of stock options. Employee Stock Purchase Plan In April 2015, our board of directors adopted, and our stockholders approved, our 2015 Employee Stock Purchase Plan (the 2015 ESPP). The 2015 ESPP became effective on May 6, 2015. A total of 16,258 shares of our common stock were initially reserved for issuance under the 2015 ESPP. In addition, the number of shares reserved and available for purchase under the 2015 ESPP automatically increased each January 1, beginning on January 1, 2016 and thereafter until January 1, 2019, by 1% of the outstanding number of shares of our common stock on the immediately preceding December 31 or such lesser number of shares as determined by the administrator of the 2015 ESPP. Pursuant to this provision, through January 1, 2019, an aggregate of 76,988 additional shares were reserved for issuance under the 2015 ESPP. As of December 31, 2020, total shares reserved for issuance under the 2015 ESPP were 75,315. Stock-based Compensation Stock Options Stock option activity is summarized as follows: Number of Outstanding Stock Options Weighted Average Exercise Price Weighted Remaining Contractual Term Aggregate Intrinsic Value Outstanding as of December 31, 2019 351,078 $ 51.34 Granted 412,432 $ 4.18 Canceled/forfeited/expired (186,976 ) $ 33.73 Outstanding as of December 31, 2020 576,534 $ 23.33 7.89 $ 15,087 Options vested and expected to vest as of December 31, 2020 576,534 $ 23.33 7.89 $ 15,087 Options exercisable as of December 31, 2020 268,039 $ 41.06 6.70 $ 1,233 The assumptions used in the Black-Scholes option pricing model to determine the fair value of the employee stock option grants were as follows: Years Ended December 31, 2020 2019 2018 Expected term (in years) 5.50– 6.08 5.51– 6.07 5.50 – 6.08 Risk-free interest rate 0.3% – 1.5 % 1.4% – 2.6 % 2.3% – 3.0 % Expected volatility 102.2% – 109.7 % 97.2% – 105.4 % 87.9% – 98.4 % Expected dividend yield 0.0 % 0.0 % 0.0 % The assumptions used in the Black-Scholes option pricing model to determine the fair value of the ESPP offering were as follows: Years Ended December 31, 2020 2019 2018 Expected term (in years) 0.50 0.50 0.50 Risk-free interest rate 0.1% – 1.6 % 1.6% – 2.5 % 1.4% – 2.1 % Expected volatility 89.7% – 143.2 % 99.7% – 141.7 % 71.5% – 99.7 % Expected dividend yield 0.0 % 0.0 % 0.0 % Expected term . The expected term represents the period of time that options are expected to be outstanding. Because we do not have sufficient history of exercise behavior, we determine the expected life assumption using the simplified method, which is an average of the contractual term of the option and its vesting period. Risk-free interest rate. We base the risk-free interest rate assumption on the U.S. Treasury’s rates for U.S. Treasury zero-coupon bonds with maturities similar to those of the expected term of the award being valued. Expected volatility. The expected volatility assumption is based on volatilities of a peer group of similar companies whose share prices are publicly available. The peer group was developed based on companies in the biotechnology industry. Expected dividend yield. We base the expected dividend yield assumption on the fact that we have never paid cash dividends and have no present intention to pay cash dividends. Restricted Stock Units Occasionally, we grant restricted stock units to employees. The fair value of restricted stock is determined by the closing price of the Company's common stock reported on the Nasdaq Global Select Market on the date of grant. Number of Outstanding Restricted Stock Units Weighted Average Grant Date Fair Value Balance as of December 31, 2019 12,475 $ 9.90 Granted 5,000 $ 4.29 Released (8,678 ) $ 10.47 Forfeited (1,120 ) $ 11.90 Balance as of December 31, 2020 7,677 $ 5.32 The allocation of stock-based compensation for all options, including performance options with market condition and restricted stock units is as follows (in thousands): Years Ended December 31, 2020 2019 2018 Research and development $ 254 $ 354 $ 1,216 General and administrative 1,211 1,429 2,215 Total stock-based compensation expense $ 1,465 $ 1,783 $ 3,431 The weighted–average grant date fair value per share of stock options granted by us, during the years ended December 31, 2020, 2019 and 2018 was $3.39, $5.67 and $25.76, respectively. The total grant date fair value of restricted stock units granted by us during the years ended December 31, 2020, 2019 and 2018 was $21,000, $39,000 and $0.2 million, respectively. The aggregate intrinsic value of stock options exercised during the years ended December 31, 2018 was $6,000. We did not have any options exercised during the years ended December 31, 2020 and 2019. The aggregate intrinsic value of restricted stock units released during the years ended December 31, 2020, 2019 and 2018 was $34,000, $31,000 and $19,000, respectively. As of December 31, 2020, total unrecognized share-based compensation expense related to unvested stock options and restricted stock units was approximately $1.8 million and $17,000, respectively. These unrecognized costs for options and restricted stock units are expected to be recognized ratably over a weighted-average period of approximately 2.8 years and 1.7 years, respectively. Warrants Warrants outstanding for the purchase of common stock as of December 31, 2020 were as follows: Number Exercise Price Expiration Outstanding Per Share Date 144 $ 104.65 March 2021 1,066 $ 281.50 July 2023 6,830 $ 43.93 November 2023 2,978 $ 50.37 June 2024 2,886 $ 51.98 December 2024 13,904 Common Stock Reserved for Future Issuance Common stock reserved for future issuance was as follows: December 31, 2020 Common stock warrants 13,904 Common stock options and restricted stock units 584,211 Shares available under the 2015 equity incentive plan 381,663 Shares available under the employee stock purchase plan 75,315 1,055,093

Income Tax

Income Tax	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Income Tax	8. Income Tax Pretax losses were generated by both domestic and foreign operations as follows (in thousands): Years Ended December 31, 2020 2019 2018 United States $ (15,950 ) $ (23,315 ) $ (34,021 ) Foreign (280 ) (448 ) (494 ) $ (16,230 ) $ (23,763 ) $ (34,515 ) For the years ended December 31, 2020, 2019, and 2018, we did not record a provision for income taxes due to a full valuation allowance against our deferred taxes. A reconciliation of the expected statutory federal income tax provision to the actual income tax provision is summarized as follows (in thousands): Years Ended December 31, 2020 2019 2018 Expected income taxes benefit at federal statutory rate $ (3,408 ) $ (4,990 ) $ (7,248 ) State income taxes, net of federal benefit (12 ) (19 ) (14 ) Permanent items and other 169 49 (80 ) Stock compensation 804 701 850 Research credits (835 ) (817 ) (1,222 ) Unrecognized tax benefits 334 327 489 Foreign rate differential 13 20 22 Change in tax rate (7 ) (49 ) (11 ) Change in valuation allowance 2,942 4,778 7,214 Income tax (benefit) expense $ — $ — $ — Deferred income taxes are provided for temporary differences in recognizing certain income and expense items for financial and tax reporting purposes. The deferred tax assets consisted primarily of the income tax benefits from net operating loss (NOL) carryforwards, research and development credits and capitalized research and development expenses, along with other accruals and reserves. Valuation allowances of $74.6 million and $71.7 million as of December 31, 2020 and 2019, respectively, have been recorded to offset deferred tax assets as realization of such assets does not meet the more-likely-than-not threshold under ASC 740, Accounting for Income Taxes Significant components of our deferred tax assets are summarized as follows (in thousands): December 31, 2020 2019 Deferred tax assets: Net operating loss carryforwards $ 41,203 $ 38,180 Capitalized research and development expenses 17,007 16,900 Research credits and other state credits 12,954 12,452 Intangible assets 1,655 1,841 Reserve and accruals 544 481 Share-based compensation expense 1,253 1,829 Lease liability 472 631 Valuation allowance (74,646 ) (71,702 ) Total deferred tax assets $ 442 $ 612 Deferred tax liabilities: Right of use lease assets (442 ) (612 ) Total deferred tax liabilities (442 ) (612 ) Net deferred tax assets $ — $ — As of December 31, 2020, we had federal NOL carryforwards of approximately $177.8 million, with $65.3 million of NOLs generated after December 31, 2017 carrying forward indefinitely and $112.5 million of NOLs that will begin to expire in 2025. NOLs generated after January 1, 2018 are subject to an 80% of taxable income limitation when utilized after December 31, 2020 in accordance with the Tax Cuts and Jobs Act of 2017 as modified by the Coronavirus Aid, Relief and Economic Security Act (CARES Act). We had state net operating loss carryforwards of approximately $164.1 million, and foreign net operating loss carryforwards of $8.3 million. The state net operating losses will begin to expire in 2021. The foreign net operating losses carry over indefinitely. As of December 31, 2020, we had federal and state research and development credit carryforwards of approximately $5.5 million and $4.5 million, respectively, which begin to expire in 2026 for federal purposes and carry over indefinitely for state purposes. We had $12.5 million of federal Orphan Drug Credits as of December 31, 2020, which will begin to expire in 2035. Utilization of the domestic NOL and research and development credit carryforwards may be subject to a substantial annual limitation due to ownership change limitations that may have occurred or that could occur in the future, as required by Sections 382 and 383 of the Internal Revenue Code of 1986, as amended (the Code), as well as similar state and foreign provisions. These ownership changes may limit the amount of NOL and research and development credit carryforwards that can be utilized annually to offset future taxable income and tax, respectively. In general, an “ownership change” as defined by Section 382 of the Code results from a transaction or series of transactions over a three-year period resulting in an ownership change of more than 50 percentage points of the outstanding stock of a company by certain stockholders. Since the Company’s formation, we raised capital through the issuance of capital stock on several occasions which on its own or combined with the purchasing stockholders’ subsequent disposition of those shares, has resulted in such an ownership change, and could result in an ownership change in the future. Upon the occurrence of an ownership change under Section 382 as outlined above, utilization of the NOL and research and development credit carryforwards become subject to an annual limitation under Section 382 of the Code, which is determined by first multiplying the value of our stock at the time of the ownership change by the applicable long-term, tax-exempt rate, which could be subject to additional adjustments. Any limitation may result in expiration of a portion of our NOL or research and development credit carryforwards before utilization. Due to the existence of the valuation allowance, any impact to the NOL and research and development tax credit carryforwards from Section 382 analysis will be offset by a corresponding adjustment to valuation allowance, resulting in no tax provision impact. We recognize a tax benefit from an uncertain tax position when it is more likely than not that the position will be sustained upon examination, including resolutions of any related appeals or litigation processes, based on the technical merits. Income tax positions must meet a more-likely-than-not recognition threshold to be recognized. Our practice is to recognize interest and penalties related to income tax matters in income tax expense. We had no accrual for interest and penalties on our balance sheet and had not recognized interest or penalties in the consolidated statements of operations for the years ended December 31, 2020, 2019 and 2018. Due to the existence of the valuation allowance, future changes in unrecognized tax benefits will not impact our effective tax rate. Uncertain tax positions are evaluated based upon the facts and circumstances that exist at each reporting period. Subsequent changes in judgment based upon new information may lead to changes in recognition, derecognition, and measurement. Adjustments may result, for example, upon resolution of an issue with the taxing authorities, or expiration of a statute of limitations barring an assessment for an issue. The activity related to our unrecognized tax benefits is summarized as follows (in thousands): December 31, 2020 2019 2018 Balance as of beginning of year $ 21,302 $ 19,643 $ 16,558 Increase related to prior year tax positions 3 — 2 Increase related to current year tax positions 402 1,659 3,083 Balance as of end of year $ 21,707 $ 21,302 $ 19,643 We do not anticipate that the amount of unrecognized tax benefits as of December 31, 2020 will change within the next twelve months. We are subject to taxation in the United States, Hong Kong and state jurisdictions. Our tax years from inception are subject to examination by the United States, Hong Kong and California authorities due to carry forward of unutilized NOLs and research and development credits. On March 27, 2020, the CARES Act was enacted and signed into law in response to COVID-19. The CARES Act, among other things, included several significant provisions that impacted corporate taxpayers’ accounting for income taxes. Prior to the enactment of the CARES Act, the 2017 Tax Cuts and Jobs Act generally eliminated the ability to carryback net operating losses (NOLs), and permitted the NOLs arising in tax years beginning after December 31, 2017 to be carried forward indefinitely, limited to 80% of the taxpayer’s income. The CARES Act amended the NOL rules, suspending the 80% limitation on the utilization of NOLs generated after December 31, 2017 and before January 1, 2021. Additionally, the CARES Act allows corporate NOLs arising in taxable years beginning after December 31, 2017 and before January 1, 2021, to be carried back to each of the five taxable years preceding the taxable year of the loss. Also, the CARES Act allows companies to defer making certain payroll tax payments until future years. With the enactment of the CARES Act, we do not expect a material impact on our consolidated financial positions or results of operations. On December 27, 2020, the United States enacted the Consolidated Appropriations Act of 2021 (CAA). The CAA includes provisions extending certain CARES Act provisions and adds coronavirus relief, tax and health extenders. We will continue to evaluate the impact of the CAA and its impact on our consolidated financial positions or results of operations in 2021 and future years. In June 29, 2020, the state of California passed Assembly Bill 85 (AB 85) which suspends the California net operating loss deduction during tax years 2020 through 2022. AB 85 also applies limitation to the amount of tax that can be offset by business credits to $5.0 million for tax years 2020 through 2022. These suspensions were considered in preparation of our 2020 consolidated financial positions or results of operations.

Employee Benefits

Employee Benefits	12 Months Ended
	Dec. 31, 2020
Compensation And Retirement Disclosure [Abstract]
Employee Benefits	9. Employee Benefits 401(k) Plan We maintain a defined contribution 401(k) plan available to eligible employees. Employee contributions are voluntary and are determined on an individual basis, limited to the maximum amount allowable under federal tax regulations. In April 2015, our Board of Directors approved a policy, beginning on June 1, 2015, to match employee contributions equal to 50% of the participant’s contribution of up to a maximum of 6% of the participant’s annual salary. We made discretionary contributions totaling $0.2 million during each of the years ended December 31, 2020, 2019 and 2018.

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2020
Subsequent Events [Abstract]
Subsequent Events	10. Subsequent Events From January 1, 2021 through March 22, 2021 we sold an aggregate of 1,988,254 shares of common stock at a weighted average price of $4.99 per share through our ATM Offering Program for gross proceeds of $9.9 million. From January 1, 2021 through March 22, 2021 we sold an aggregate of 3,000,000 shares of common stock at a weighted average price of $5.09 per share through our Purchase Agreement with Aspire Capital for gross proceeds of $15.3 million. On March 23, 2021, we entered into Capital on Demand TM Sales Agreement

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Organization and Business	Organization and Business We were incorporated in the state of Delaware on September 8, 2005. We are focused on the discovery and development of innovative medicines based on novel immunological pathways.
Principles of Consolidation	Principles of Consolidation Our consolidated financial statements include our accounts and our 98% majority-owned subsidiary in Hong Kong, Pangu BioPharma Limited (Pangu BioPharma). All intercompany transactions and balances are eliminated in consolidation.
Liquidity and Financial Condition	Liquidity and Financial Condition Other than the net income generated in the three months ended March 31, 2020, we have incurred losses and negative cash flows from operations since our inception. As of December 31, 2020, we had an accumulated deficit of $338.5 million and we expect to continue to incur net losses for the foreseeable future. As of December 31, 2020, our cash, cash equivalents and available-for-sale investments were $31.7 million. Subsequent to December 31, 2020, we received $2.0 million related to the Kyorin Agreement, approximately $9.9 million in gross proceeds from our at-the-market offering (ATM Offering Program) before deducting commissions and offering expenses and approximately $15.3 million in gross proceeds from our purchase agreement (Purchase Agreement) before deducting offering expenses payable by us. We believe that our current cash, cash equivalents and available-for-sale investments, will be sufficient to meet our anticipated cash requirements for a period of at least one year from the date of this Annual Report. We do not expect to generate any revenues from product sales unless and until we successfully complete development and obtain regulatory approval for one or more of our product candidates, which we expect will take a number of years at a minimum. If we obtain regulatory approval for any of our product candidates, we expect to incur significant commercialization expenses related to product sales, marketing, manufacturing and distribution. Accordingly, we will need to raise substantial additional capital to fund our operations. The amount and timing of our future funding requirements will depend on many factors, including the pace and results of our preclinical and clinical development efforts and the timing and nature of the regulatory approval process for our product candidates. We anticipate that we will seek to fund our operations through equity offerings, grant funding, collaborations, strategic partnerships and/or licensing arrangements, and when we are closer to commercialization of our product candidates potentially through debt financings. However, we may be unable to raise additional capital or enter into such arrangements when needed on favorable terms or at all. Our failure to raise capital or enter into such arrangements when needed would have a negative impact on our financial condition and ability to develop our product candidates.
Use of Estimates	Use of Estimates Our consolidated financial statements are prepared in accordance with generally accepted accounting principles (GAAP). The preparation of our consolidated financial statements requires us to make estimates and assumptions that impact the reported amounts of assets, liabilities and expenses and the disclosure for these items in our consolidated financial statements and accompanying notes. The most significant estimates in our consolidated financial statements relate to the fair value of equity issuances and awards, and clinical trial and research and development expenses. Although these estimates are based on our knowledge of current events and actions we may undertake in the future, actual results may ultimately differ materially from these estimates and assumptions.
Reclassifications	Reclassifications Certain reclassifications have been made to prior year amounts to conform to the current year presentation. The reclassifications were not material to the consolidated financial statements.
Segment Reporting	Segment Reporting Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision-maker in making decisions regarding resource allocation and assessing performance. We view our operations and manage our business in one operating segment.
Cash and Cash Equivalents	Cash and Cash Equivalents Cash and cash equivalents consist primarily of readily available checking, money market accounts and money market funds. We consider all highly liquid investments that mature in three months or less when purchased to be cash equivalents.
Investment Securities	Investment Securities Investment securities primarily consist of investment grade corporate debt securities, asset-backed securities and commercial paper. We classify all investment securities as available-for-sale. Investment securities are carried at fair value, with the unrealized gains and losses, if any, reported as a component of other comprehensive income (loss) in stockholders’ equity until realized. Realized gains and losses from the sale of investment securities, if any, are determined on a specific identification basis. A decline in the market value of any investment security below cost that is determined to be other than temporary will result in an impairment charge to earnings and a new cost basis for the security is established. No such impairment charges were recorded for any period presented. Premiums and discounts are amortized or accreted over the life of the related security as an adjustment to yield using the straight-line method and are included in interest income. Interest income is recognized when earned. As of December 31, 2020, we held an aggregate total of $14.7 million of investment securities which consisted of corporate debt securities, asset-backed securities, and commercial paper all of which will mature in less than one year, and there was an unrealized gain of approximately $7,000 between the amortized cost and fair value of these investment securities. As of December 31, 2019, we held $21.9 million of corporate debt securities, asset-backed securities and commercial paper, all of which mature in less than one year, and there was an unrealized gain of approximately $10,000 between the amortized cost and fair value of these investment securities.
Concentration of Credit Risk	Concentration of Credit Risk Financial instruments that potentially subject us to significant concentration of credit risk consist primarily of cash, cash equivalents and investment securities. We have established guidelines regarding diversification of investments and their maturities, which are designed to maintain principal and maximize liquidity. We maintain deposits in federally insured financial institutions in excess of federally insured limits. We have not experienced any losses in such accounts and we believe that we are not exposed to significant credit risk due to the financial position of the depository institutions in which those deposits are held.
Property and Equipment	Property and Equipment Property and equipment are stated at cost and depreciated on a straight-line basis over the estimated useful life of the related assets (generally three to seven years). Leasehold improvements are stated at cost and amortized on a straight-line basis over the lesser of the remaining term of the related lease or the estimated useful life of the leasehold improvements. Repairs and maintenance costs are charged to expense as incurred .
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets Long-lived assets consist primarily of property and equipment. An impairment loss is recorded if and when events and circumstances indicate that assets might be impaired and the undiscounted cash flows estimated to be generated by those assets are less than the carrying amount of those assets. While our current and historical operating losses are indicators of impairment, we believe that future cash flows to be received support the carrying value of our long-lived assets and, accordingly, have not recognized any impairment losses since inception.
Accrued Expenses	Accrued Expenses Expenses related to clinical studies, preclinical development activities and product manufacturing are based on estimates of the services received and efforts expended pursuant to our contractual arrangements. There may be instances in which payments made to our service providers will temporarily exceed the level of services provided and result in a prepayment of the expense. Payments under some of these contracts depend on factors such as, for clinical studies, the successful enrollment of patients, site initiation and the completion of clinical milestones. We make estimates of our accrued expenses as of each balance sheet date based on facts and circumstances known at the time. In accruing service fees, we estimate the time period over which services will be performed and the level of effort to be expended in each period. If the actual timing of the performance of services or the level of effort varies from our estimate, we adjust the accrual or prepaid expense balance accordingly. Historically, our estimated accrued liabilities have materially approximated actual expenses incurred.
Leases	Leases We follow Accounting Standards Codification (ASC) Topic 842, Leases We do not separate lease and non-lease components for our long-term leases. Rent expense for the operating lease is recognized on a straight-line basis over the lease term and is included in operating expenses in our consolidated statements of operations.
Revenue Recognition	Revenue Recognition We evaluate our agreements under ASC Topic 606, Revenue from Contracts with Customers and ASC Topic 808, Collaborative Arrangements . . In determining the appropriate amount of revenue to be recognized as we fulfill our obligations under our agreement, we perform the following steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when (or as) we satisfy each performance obligation. As part of the accounting for these arrangements, we must develop assumptions that require judgment to determine the stand-alone selling price for each performance obligation identified in the contract. We use key assumptions to determine the stand-alone selling price, which may include forecasted revenues, development timelines, reimbursement rates for personnel costs, discount rates and probabilities of technical and regulatory success. We recognize revenue in one of two ways, over time or at a point in time. We recognize revenue over time when we are executing on our performance obligation over time and our partner receives benefit over time. For example, we recognize revenue over time when we provide research and development services. We recognize revenue at a point in time when we transfer control of a distinct performance obligation to our partner. For example, if
Research and Development Costs	Research and Development Costs Research and development costs are expensed as incurred. Research and development costs include: salaries and employee-related expenses, including stock-based compensation and benefits for personnel in research and product development functions; costs associated with conducting our preclinical, development and regulatory activities, including fees paid to third-party professional consultants, service providers and our scientific, therapeutic and clinical advisors; costs to acquire, develop and manufacture preclinical study and clinical trial materials; costs incurred under clinical trial agreements with clinical research organizations
Patent Costs	Patent Costs Costs related to filing and pursuing patent applications are recorded as general and administrative expense and expensed as incurred since recoverability of such expenditures is uncertain.
Stock-Based Compensation	Stock-Based Compensation Stock-based compensation expense represents the grant date fair value of employee stock option grants recognized as expense over the requisite service period of the awards (usually the vesting period) on a straight-line basis. We estimate fair value of stock option grants using the Black-Scholes option pricing model. We estimate the fair value using assumptions, including the risk-free interest rate, the expected volatility of a peer group of similar companies, the expected term of the awards and the expected dividend yield. These estimates involve inherent uncertainties and the application of management’s judgment. If factors change and different assumptions are used, our stock-based compensation expense could be materially different in the future. We follow ASC Topic 718, Compensation – Stock Compensation
Income Taxes	Income Taxes We account for income taxes under 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 included in the financial statements. Under this method, 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. The effect of a change in tax rates on deferred tax assets and liabilities is recognized as income in the period that includes the enactment date. We recognize net deferred tax assets to the extent that we believe these assets are more likely than not to be realized. In making such a determination, we consider all available positive and negative evidence, including future reversals of existing taxable temporary differences, projected future taxable income, tax-planning strategies and results of recent operations. If we determine that we would be able to realize the deferred tax assets in the future in excess of their net recorded amount, we would make an adjustment to the deferred tax asset valuation allowance, which would reduce the provision for income taxes. We record uncertain tax positions on the basis of a two-step process whereby (1) we determine whether it is more likely than not that the tax positions will be sustained on the basis of the technical merits of the position and (2) for those tax positions that meet the more-likely-than-not recognition threshold, we recognize the largest amount of tax benefit that is more than 50% likely to be realized upon ultimate settlement with the related tax authority. We recognize interest and penalties related to unrecognized tax benefits within income tax expense. Any accrued interest and penalties are included within the related tax liability.
Net Loss Per Share	Net Loss Per Share Basic net loss per share is calculated by dividing the net loss attributable to common stockholders by the weighted average number of common shares outstanding for the period, without consideration for common stock equivalents. Diluted net loss per share attributable to common stockholders is calculated by dividing the net loss attributable to common stockholders by the weighted average number of common stock and common stock equivalents outstanding for the period determined using the treasury-stock method. Dilutive common stock equivalents are comprised of convertible preferred stock, warrants for common stock, options and restricted stock units outstanding under our stock option plan and estimated shares to be purchased under our employee stock purchase plan. For all periods presented, there is no difference in the number of shares used to calculate basic and diluted shares outstanding due to our net loss position. Potentially dilutive securities not included in the calculation of diluted net loss per share because to do so would be anti-dilutive are as follows (in common share equivalents): Years Ended December 31, 2020 2019 2018 Class X Preferred Stock (if-converted to common stock) — 587,445 816,851 Common stock warrants 13,904 13,904 477,639 Common stock options and restricted stock units 584,211 363,553 371,823 Employee stock purchase plan 1,602 1,958 1,610 Total 599,717 966,860 1,667,923 The following table summarizes our net loss per share (in thousands, except per share data): Years Ended December 31, 2020 2019 2018 Numerator: Net loss attributable to aTyr Pharma, Inc. $ (16,224 ) $ (23,603 ) $ (34,515 ) Denominator: Shares used in computing net loss per share, basic and diluted 9,160,269 3,355,600 2,141,961 Net loss per share - basic and diluted $ (1.77 ) $ (7.03 ) $ (16.11 )
Derivative Financial Instruments	Derivative Financial Instruments We do not use derivative instruments to hedge exposures to cash flow, market, or foreign currency risks. We evaluate all of our financial instruments, including warrants, to determine if such instruments are derivatives or contain features that qualify as embedded derivatives. We generally use the Black-Scholes option-pricing model to value the derivative instruments at inception and subsequent valuation dates. The classification of derivative instruments, including whether such instruments should be recorded as liabilities or as equity, is re-assessed at the end of each reporting period.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In June 2016, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) No. 2016-13, Financial Instruments – Credit Losses to provide financial statement users with more decision-useful information about the expected credit losses on financial instruments and other commitments to extend credit held by a reporting entity at each reporting date. To achieve this objective, the amendments in Topic 326 replace the incurred loss impairment methodology in current GAAP with a methodology that reflects expected credit losses and requires consideration of a broader range of reasonable and supportable information to inform credit loss estimates. Topic 326 is effective for fiscal years beginning after December 15, 2022, including interim periods within those fiscal years for smaller reporting companies. We expect the adoption of the amendments in ASU 2016-13 to not have an effect on its financial position and the results of its operations when such amendment is adopted. In December 2019, the FASB issued ASU No. 2019-12, Income Taxes Simplifying the Accounting for Income Taxes

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Schedule of Potentially Dilutive Securities Not Included in Calculation of Diluted Net Loss Per Share	Potentially dilutive securities not included in the calculation of diluted net loss per share because to do so would be anti-dilutive are as follows (in common share equivalents): Years Ended December 31, 2020 2019 2018 Class X Preferred Stock (if-converted to common stock) — 587,445 816,851 Common stock warrants 13,904 13,904 477,639 Common stock options and restricted stock units 584,211 363,553 371,823 Employee stock purchase plan 1,602 1,958 1,610 Total 599,717 966,860 1,667,923
Summary of Net Loss Per Share	The following table summarizes our net loss per share (in thousands, except per share data): Years Ended December 31, 2020 2019 2018 Numerator: Net loss attributable to aTyr Pharma, Inc. $ (16,224 ) $ (23,603 ) $ (34,515 ) Denominator: Shares used in computing net loss per share, basic and diluted 9,160,269 3,355,600 2,141,961 Net loss per share - basic and diluted $ (1.77 ) $ (7.03 ) $ (16.11 )

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Schedule of Assets Measured at Fair Value on Recurring Basis	Assets measured at fair value on a recurring basis are as follows (in thousands): Fair Value Measurements Using Total Quoted Active for Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) As of December 31, 2020 Assets: Current: Cash equivalents $ 13,708 $ 13,708 $ — $ — Available-for-sale investments: Asset-backed securities 2,219 — 2,219 — Commercial paper 5,494 — 5,494 — Corporate debt securities 7,024 — 7,024 — Total available-for-sale investments 14,737 — 14,737 — Total assets measured at fair value $ 28,445 $ 13,708 $ 14,737 $ — Fair Value Measurements Using Total Quoted Active for Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) As of December 31, 2019 Assets: Current: Cash equivalents $ 8,248 $ 8,248 $ — $ — Available-for-sale investments: Asset-backed securities 6,304 — 6,304 — Commercial paper 7,568 — 7,568 — Corporate debt securities 8,062 — 8,062 — Total available-for-sale investments 21,934 — 21,934 — Total assets measured at fair value $ 30,182 $ 8,248 $ 21,934 $ —
Schedule of Available-for-sale Investments	As of December 31, 2020 and 2019, available-for-sale investments are detailed as follows (in thousands): December 31, 2020 Gross Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Market Value Available-for-sale investments: Asset-backed securities $ 2,218 $ 1 $ — $ 2,219 Commercial paper 5,491 3 — 5,494 Corporate debt securities 7,021 3 — 7,024 $ 14,730 $ 7 $ — $ 14,737 December 31, 2019 Gross Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Market Value Available-for-sale investments: Asset-backed securities $ 6,299 $ 5 $ — $ 6,304 Commercial paper 7,568 — — 7,568 Corporate debt securities 8,057 5 — 8,062 $ 21,924 $ 10 $ — $ 21,934

Balance Sheet Details (Tables)

Balance Sheet Details (Tables)	12 Months Ended
	Dec. 31, 2020
Balance Sheet Details [Abstract]
Schedule of Prepaid Expenses	Prepaid expenses consist of the following (in thousands): December 31, 2020 2019 Prepaid clinical and research expense $ 583 $ 22 Prepaid manufacturing expenses $ 513 $ 172 Other prepaid expenses 707 487 $ 1,803 $ 681
Summary of Property and Equipment	Property and equipment consist of the following (in thousands): December 31, 2020 2019 Computer and office equipment $ 552 $ 543 Scientific and laboratory equipment 5,270 5,241 Tenant improvements 1,701 1,700 7,523 7,484 Less accumulated depreciation and amortization (6,624 ) (6,214 ) $ 899 $ 1,270
Summary of Accrued Expenses	Accrued expenses consist of the following (in thousands): December 31, 2020 2019 Accrued salaries, wages and benefits $ 1,809 $ 1,275 Other accrued expenses (1) 1,763 1,101 $ 3,572 $ 2,376 (1) Other accrued expenses include expenses for clinical research organizations and contract manufacturing organizations.

Debt, Commitments and Conting_2

Debt, Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2020
Debt Instruments [Abstract]
Schedule of Future Minimum Payments under Non-cancelable Operating Lease and Reconciliation to Operating Lease Liability	Future minimum payments under the non-cancelable facility lease and reconciliation to the operating lease liability as of December 31, 2020 were as follows (in thousands): Operating Lease 2021 $ 1,031 2022 1,062 2023 404 Less: Amount representing interest (258 ) Present value of lease payments 2,239 Less: Current portion of operating lease liability (861 ) Long-term operating lease liability, net of current portion $ 1,378

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2020
Summary of Stock Option Activity	Stock option activity is summarized as follows: Number of Outstanding Stock Options Weighted Average Exercise Price Weighted Remaining Contractual Term Aggregate Intrinsic Value Outstanding as of December 31, 2019 351,078 $ 51.34 Granted 412,432 $ 4.18 Canceled/forfeited/expired (186,976 ) $ 33.73 Outstanding as of December 31, 2020 576,534 $ 23.33 7.89 $ 15,087 Options vested and expected to vest as of December 31, 2020 576,534 $ 23.33 7.89 $ 15,087 Options exercisable as of December 31, 2020 268,039 $ 41.06 6.70 $ 1,233
Summary of Assumptions Used to Determine Fair Value of Employee Stock Purchase Plan	The assumptions used in the Black-Scholes option pricing model to determine the fair value of the ESPP offering were as follows: Years Ended December 31, 2020 2019 2018 Expected term (in years) 0.50 0.50 0.50 Risk-free interest rate 0.1% – 1.6 % 1.6% – 2.5 % 1.4% – 2.1 % Expected volatility 89.7% – 143.2 % 99.7% – 141.7 % 71.5% – 99.7 % Expected dividend yield 0.0 % 0.0 % 0.0 %
Schedule of Restricted Stock Units Activity	Occasionally, we grant restricted stock units to employees. The fair value of restricted stock is determined by the closing price of the Company's common stock reported on the Nasdaq Global Select Market on the date of grant. Number of Outstanding Restricted Stock Units Weighted Average Grant Date Fair Value Balance as of December 31, 2019 12,475 $ 9.90 Granted 5,000 $ 4.29 Released (8,678 ) $ 10.47 Forfeited (1,120 ) $ 11.90 Balance as of December 31, 2020 7,677 $ 5.32
Schedule of Allocation of Stock-Based Compensation for All Options Including Performance Options with Market Condition and Restricted Stock Units	The allocation of stock-based compensation for all options, including performance options with market condition and restricted stock units is as follows (in thousands): Years Ended December 31, 2020 2019 2018 Research and development $ 254 $ 354 $ 1,216 General and administrative 1,211 1,429 2,215 Total stock-based compensation expense $ 1,465 $ 1,783 $ 3,431
Summary of Warrants Outstanding	Warrants outstanding for the purchase of common stock as of December 31, 2020 were as follows: Number Exercise Price Expiration Outstanding Per Share Date 144 $ 104.65 March 2021 1,066 $ 281.50 July 2023 6,830 $ 43.93 November 2023 2,978 $ 50.37 June 2024 2,886 $ 51.98 December 2024 13,904
Summary of Common Stock Reserved for Future Issuance	Common stock reserved for future issuance was as follows: December 31, 2020 Common stock warrants 13,904 Common stock options and restricted stock units 584,211 Shares available under the 2015 equity incentive plan 381,663 Shares available under the employee stock purchase plan 75,315 1,055,093
Employee Stock Option [Member]
Summary of Assumptions Used to Determine Fair Value of Employee Stock Option Grants and Performance Options with Market Condition	The assumptions used in the Black-Scholes option pricing model to determine the fair value of the employee stock option grants were as follows: Years Ended December 31, 2020 2019 2018 Expected term (in years) 5.50– 6.08 5.51– 6.07 5.50 – 6.08 Risk-free interest rate 0.3% – 1.5 % 1.4% – 2.6 % 2.3% – 3.0 % Expected volatility 102.2% – 109.7 % 97.2% – 105.4 % 87.9% – 98.4 % Expected dividend yield 0.0 % 0.0 % 0.0 %

Income Tax (Tables)

Income Tax (Tables)	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Schedule of Pretax Losses for Domestic and Foreign Operations	Pretax losses were generated by both domestic and foreign operations as follows (in thousands): Years Ended December 31, 2020 2019 2018 United States $ (15,950 ) $ (23,315 ) $ (34,021 ) Foreign (280 ) (448 ) (494 ) $ (16,230 ) $ (23,763 ) $ (34,515 )
Schedule of Reconciliations of the Expected Statutory Federal Income Tax	A reconciliation of the expected statutory federal income tax provision to the actual income tax provision is summarized as follows (in thousands): Years Ended December 31, 2020 2019 2018 Expected income taxes benefit at federal statutory rate $ (3,408 ) $ (4,990 ) $ (7,248 ) State income taxes, net of federal benefit (12 ) (19 ) (14 ) Permanent items and other 169 49 (80 ) Stock compensation 804 701 850 Research credits (835 ) (817 ) (1,222 ) Unrecognized tax benefits 334 327 489 Foreign rate differential 13 20 22 Change in tax rate (7 ) (49 ) (11 ) Change in valuation allowance 2,942 4,778 7,214 Income tax (benefit) expense $ — $ — $ —
Schedule of Significant Components of Deferred Tax Assets	Significant components of our deferred tax assets are summarized as follows (in thousands): December 31, 2020 2019 Deferred tax assets: Net operating loss carryforwards $ 41,203 $ 38,180 Capitalized research and development expenses 17,007 16,900 Research credits and other state credits 12,954 12,452 Intangible assets 1,655 1,841 Reserve and accruals 544 481 Share-based compensation expense 1,253 1,829 Lease liability 472 631 Valuation allowance (74,646 ) (71,702 ) Total deferred tax assets $ 442 $ 612 Deferred tax liabilities: Right of use lease assets (442 ) (612 ) Total deferred tax liabilities (442 ) (612 ) Net deferred tax assets $ — $ —
Schedule of Activity Related to Unrecognized Tax Benefits	The activity related to our unrecognized tax benefits is summarized as follows (in thousands): December 31, 2020 2019 2018 Balance as of beginning of year $ 21,302 $ 19,643 $ 16,558 Increase related to prior year tax positions 3 — 2 Increase related to current year tax positions 402 1,659 3,083 Balance as of end of year $ 21,707 $ 21,302 $ 19,643

Organization, Business and Ba_2

Organization, Business and Basis of Presentation - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)Segment	Dec. 31, 2019USD ($)
Description Of Business [Line Items]
Accumulated deficit	$ 338,528	$ 322,304
Cash, cash equivalents and available-for-sale investments	31,700
Milestone payment to be received	2,039	$ 100
Gross proceeds from market offering	$ 9,900
Number of operating segment | Segment	1
Purchase Agreement [Member]
Description Of Business [Line Items]
Gross proceeds from agreement	$ 15,300
Pangu BioPharma [Member] | Hong Kong [Member]
Description Of Business [Line Items]
Majority-owned subsidiary percentage	98.00%

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Significant Accounting Policies [Line Items]
Impairment charges on investment securities	$ 0
Aggregate value of investment securities	14,737,000	$ 21,934,000
Unrealized gain between amortized cost and fair value of investment securities	7,000	$ 10,000
Impairment of long lived assets	$ 0
Maximum [Member]
Significant Accounting Policies [Line Items]
Maturity period of investment securities	1 year	1 year
Estimated useful life of property and equipment	7 years
Minimum [Member]
Significant Accounting Policies [Line Items]
Estimated useful life of property and equipment	3 years
Percentage of tax benefit to be realized upon settlement	50.00%

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Schedule of Potentially Dilutive Securities Not Included in Calculation of Diluted Net Loss Per Share (Detail) - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities not included in calculation of diluted net loss per share	599,717	966,860	1,667,923
Class X Preferred Stock (if-converted to common stock) [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities not included in calculation of diluted net loss per share		587,445	816,851
Common Stock Warrants [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities not included in calculation of diluted net loss per share	13,904	13,904	477,639
Common Stock Options and Restricted Stock Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities not included in calculation of diluted net loss per share	584,211	363,553	371,823
Employee Stock Purchase Plan [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities not included in calculation of diluted net loss per share	1,602	1,958	1,610

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Summary of Net Loss Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Numerator:
Net loss	$ (16,224)	$ (23,603)	$ (34,515)
Denominator:
Shares used in computing net loss per share, basic and diluted	9,160,269	3,355,600	2,141,961
Net loss per share - basic and diluted	$ (1.77)	$ (7.03)	$ (16.11)

Fair Value Measurements - Sched

Fair Value Measurements - Schedule of Assets Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 13,708	$ 8,248
Total assets measured at fair value	28,445	30,182
Available-for-sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	14,737	21,934
Available-for-sale [Member] | Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	2,219	6,304
Available-for-sale [Member] | Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	5,494	7,568
Available-for-sale [Member] | Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	7,024	8,062
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	13,708	8,248
Total assets measured at fair value	13,708	8,248
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	14,737	21,934
Significant Other Observable Inputs (Level 2) [Member] | Available-for-sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	14,737	21,934
Significant Other Observable Inputs (Level 2) [Member] | Available-for-sale [Member] | Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	2,219	6,304
Significant Other Observable Inputs (Level 2) [Member] | Available-for-sale [Member] | Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	5,494	7,568
Significant Other Observable Inputs (Level 2) [Member] | Available-for-sale [Member] | Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	$ 7,024	$ 8,062

Fair Value Measurements - Sch_2

Fair Value Measurements - Schedule of Available-for-sale Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	$ 14,730	$ 21,924
Gross Unrealized Gains	7	10
Market Value	14,737	21,934
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	5,491	7,568
Gross Unrealized Gains	3
Market Value	5,494	7,568
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	2,218	6,299
Gross Unrealized Gains	1	5
Market Value	2,219	6,304
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	7,021	8,057
Gross Unrealized Gains	3	5
Market Value	$ 7,024	$ 8,062

License, Collaboration and Ot_2

License, Collaboration and Other Agreements - Additional Information (Detail)	1 Months Ended						12 Months Ended
	Jan. 31, 2021USD ($)	Jun. 30, 2020USD ($)	Mar. 31, 2020USD ($)	Jan. 31, 2020USD ($)	May 31, 2019USD ($)	Mar. 31, 2019Program	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2021USD ($)
Research And Development Arrangement Contract To Perform For Others [Line Items]
Upfront payment received				$ 8,000,000
Milestone payment to be received							$ 2,039,000	$ 100,000
Total revenues							$ 10,455,000	422,000
Other Receivables [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Milestone payment to be received									$ 2,000,000
Subsequent Event [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Milestone payments received	$ 2,000,000
CSL Behring [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Agreement termination							2021-02
Hong Kong University of Science and Technology [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Collaborative agreement, funding received			$ 750,000
Funding of project cost, description			The grant will fund approximately 50% of the total estimated project cost, with aTyr contributing the remaining 50%. The research grant agreement between Pangu BioPharma, HKUST and the Government of the Hong Kong Special Administration Region was effective April 1, 2020.
Expenses							$ 200,000
Development Regulatory And Sales Milestones [Member] | Kyorin Pharmaceutical Co Ltd [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Remaining receivable based on achievement of research milestones				165,000,000
Development Regulatory And Sales Milestones [Member] | Kyorin Pharmaceutical Co Ltd [Member] | Collaboration Revenue [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Stand-alone selling price of license		$ 100,000		$ 7,900,000
License and collaboration agreement revenue							10,000,000
Research Collaboration and Option Agreement [Member] | CSL Behring [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Upfront payment received					$ 600,000
Number of programs | Program						4
Additional proceeds from research and development reimbursement from amendment		200,000
Additional upfront payment received		$ 200,000
Research Collaboration and Option Agreement [Member] | CSL Behring [Member] | Collaboration Revenue [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Total revenues							$ 500,000	$ 400,000

License, Collaboration and Ot_3

License, Collaboration and Other Agreements - Additional Information (Detail 1)	Dec. 31, 2020
Research Collaboration and Option Agreement [Member] | CSL Behring [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2020-01-01
Research And Development Arrangement Contract To Perform For Others [Line Items]
Performance obligations period	45 days

Balance Sheet Details - Summary

Balance Sheet Details - Summary of Prepaid Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Prepaid Expenses [Line Items]
Prepaid expenses	$ 1,803	$ 681
Prepaid Clinical and Research Expense [Member]
Prepaid Expenses [Line Items]
Prepaid expenses	583	22
Prepaid Manufacturing Expenses [Member]
Prepaid Expenses [Line Items]
Prepaid expenses	513	172
Other Prepaid Expenses [Member]
Prepaid Expenses [Line Items]
Prepaid expenses	$ 707	$ 487

Balance Sheet Details - Summa_2

Balance Sheet Details - Summary of Property and Equipment (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Property Plant And Equipment [Line Items]
Property and equipment, Gross	$ 7,523	$ 7,484
Less accumulated depreciation and amortization	(6,624)	(6,214)
Property and equipment, Net	899	1,270
Computer and Office Equipment [Member]
Property Plant And Equipment [Line Items]
Property and equipment, Gross	552	543
Scientific and Laboratory Equipment [Member]
Property Plant And Equipment [Line Items]
Property and equipment, Gross	5,270	5,241
Tenant Improvements [Member]
Property Plant And Equipment [Line Items]
Property and equipment, Gross	$ 1,701	$ 1,700

Balance Sheet Details - Additio

Balance Sheet Details - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property Plant And Equipment [Abstract]
Depreciation expense	$ 569	$ 635	$ 746

Balance Sheet Details - Summa_3

Balance Sheet Details - Summary of Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Accrued Liabilities Current [Abstract]
Accrued salaries, wages and benefits	$ 1,809	$ 1,275
Other accrued expenses	1,763	1,101
Accrued expenses	$ 3,572	$ 2,376

Debt, Commitments and Conting_3

Debt, Commitments and Contingencies - Additional Information (Detail)	1 Months Ended					12 Months Ended
	Nov. 30, 2020	Jul. 31, 2018ft²	Dec. 31, 2017USD ($)$ / sharesshares	Jun. 30, 2017USD ($)$ / sharesshares	Nov. 30, 2016USD ($)Tranche$ / sharesshares	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Debt Instrument [Line Items]
Loan and security agreement, payment term						Issuable in three separate tranches (the Term Loans), $10.0 million of which was funded in November 2016, $5.0 million of which was funded in June 2017 and $5.0 million of which was funded in December 2017.
Lease extended maturity date		May 31, 2023
Lease, option to extend		false
Operating lease expense						$ 1,000,000	$ 1,000,000	$ 1,000,000
Operating lease, weighted average remaining lease term						2 years 4 months 24 days	3 years 4 months 24 days
Operating lease, weighted average discount rate						9.60%	9.60%
Maximum [Member]
Debt Instrument [Line Items]
Lease space | ft²		24,494
Minimum [Member]
Debt Instrument [Line Items]
Lease space | ft²		20,508
Silicon Valley Bank and Solar Capital, Ltd. [Member]
Debt Instrument [Line Items]
Available credit under loan and security agreement					$ 20,000,000
Number of tranches | Tranche					3
Warrants expiration year						7 years
Aggregate fair value of warrants using black scholes option pricing model			$ 500,000
Silicon Valley Bank and Solar Capital, Ltd. [Member] | Term Loans Tranche One [Member]
Debt Instrument [Line Items]
Available credit under loan and security agreement					$ 10,000,000
Loan and security agreement funded date					Nov. 30, 2016
Exercise price of warrant per share | $ / shares					$ 43.93
Silicon Valley Bank and Solar Capital, Ltd. [Member] | Term Loans Tranche One [Member] | Common Stock [Member]
Debt Instrument [Line Items]
Warrants to purchase number of common stock, shares | shares					3,415
Silicon Valley Bank and Solar Capital, Ltd. [Member] | Term Loans Tranche Two [Member]
Debt Instrument [Line Items]
Available credit under loan and security agreement				$ 5,000,000
Loan and security agreement funded date				Jun. 30, 2017
Exercise price of warrant per share | $ / shares				$ 50.37
Silicon Valley Bank and Solar Capital, Ltd. [Member] | Term Loans Tranche Two [Member] | Common Stock [Member]
Debt Instrument [Line Items]
Warrants to purchase number of common stock, shares | shares				1,489
Silicon Valley Bank and Solar Capital, Ltd. [Member] | Term Loans Tranche Three [Member]
Debt Instrument [Line Items]
Available credit under loan and security agreement			$ 5,000,000
Loan and security agreement funded date			Dec. 31, 2017
Exercise price of warrant per share | $ / shares			$ 51.98
Silicon Valley Bank and Solar Capital, Ltd. [Member] | Term Loans Tranche Three [Member] | Common Stock [Member]
Debt Instrument [Line Items]
Warrants to purchase number of common stock, shares | shares			1,443
Loan Amendment Agreement with SVB and Solar [Member]
Debt Instrument [Line Items]
Percentage of funded amounts for final payment	8.75%

Debt, Commitments and Conting_4

Debt, Commitments and Contingencies - Schedule of Future Minimum Payments under Non-cancelable Operating Lease and Reconciliation to Operating Lease Liability (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Leases Operating [Abstract]
2021	$ 1,031
2022	1,062
2023	404
Less: Amount representing interest	(258)
Present value of lease payments	2,239
Less: Current portion of operating lease liability	(861)	$ (755)
Long-term operating lease liability, net of current portion	$ 1,378	$ 2,239

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Detail) - USD ($)	1 Months Ended			12 Months Ended
	Sep. 30, 2020	Feb. 29, 2020	Apr. 30, 2015	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Nov. 30, 2020	Mar. 31, 2020	May 31, 2019	Jan. 01, 2019	Jun. 30, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of common stock shares reserved for issuance				1,055,093
Weighted average grant date fair value of options granted				$ 3.39	$ 5.67	$ 25.76
Total grant date fair value of restricted stock units vested				$ 21,000	$ 39,000	$ 200,000
Aggregate intrinsic value of stock options exercised				0	0	6,000
Aggregate intrinsic value of restricted stock units				34,000	$ 31,000	$ 19,000
Unrecognized share based compensation expense related to unvested stock options				1,800,000
Unrecognized share based compensation expense related to restricted stock units				$ 17,000
Employee Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized cost expected to be recognized over a weighted average period				2 years 9 months 18 days
Restricted Stock Unit [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized cost expected to be recognized over a weighted average period				1 year 8 months 12 days
2014 Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Maximum term for stock option plan grant				10 years
Options vesting period				4 years
2015 Stock Option and Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Maximum term for stock option plan grant			10 years
Options vesting period			4 years
Options vesting period, description				For an initial grant to an employee, 25% of the options generally vest on the first anniversary of the original vesting date, with the balance vesting monthly over the remaining three years. For subsequent grants to an employee, the options generally vest monthly over a four-year term.
2015 Stock Option and Incentive Plan [Member] | One Year Anniversary [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Option vesting percentage			25.00%
2015 Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of common stock shares reserved for issuance				75,315
Aspire Capital [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares issued during the period				0
Number of shares issued and sold				0
Commitment to purchase shares	$ 20,000,000
Long-term purchase commitment, period	30 months
Long-term purchase commitment, description	upon the terms and subject to the conditions and limitations set forth therein, Aspire Capital is committed to purchase up to an aggregate of $20.0 million of shares of our common stock at our request from time to time during the 30 month term of the Purchase Agreement.
ATM Offering Program [Member] | Cowen Company, LLC (Cowen) [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Net proceeds from issuance of common stock				$ 1,400,000
ATM Offering Program [Member] | Cowen Company, LLC (Cowen) [Member] | Sale Agreement [Member] | Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Agreed upon value of sale of common stock per transaction											$ 35,000,000
ATM Offering Program [Member] | Common Stock [Member] | Cowen Company, LLC (Cowen) [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares issued during the period				193,670
Shares issued, price per share					$ 7.35
Number of shares issued and sold				193,670
ATM Offering Program [Member] | H.C. Wainwright & Co., LLC [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Net proceeds from issuance of common stock				$ 6,800,000	$ 3,300,000
Commission rate equal to gross proceeds									3.00%
ATM Offering Program [Member] | H.C. Wainwright & Co., LLC [Member] | Sale Agreement [Member] | Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Agreed upon value of sale of common stock per transaction							$ 20,000,000		$ 10,000,000
ATM Offering Program [Member] | Common Stock [Member] | H.C. Wainwright & Co., LLC [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares issued during the period				1,657,075	611,687
Shares issued, price per share				$ 4.07	$ 5.43
Number of shares issued and sold				1,657,075	611,687
Public Offerings [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Net proceeds from issuance of common stock		$ 20,700,000
Public Offerings [Member] | Common Stock [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares issued during the period		4,235,294
Shares issued, price per share		$ 4.25
Number of shares issued and sold		4,235,294
Issuance of additional share of common stock								635,294
IPO [Member] | 2015 Stock Option and Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Effective date of plan				May 6, 2015
Number of common stock shares reserved for issuance			112,469	381,663
Stock option grants description				The number of shares reserved and available for issuance under the 2015 Plan will automatically increase each January 1, beginning on January 1, 2016 and thereafter until January 1, 2019, by the lesser of (i) 1,840,000 shares, (ii) 4% of the outstanding number of shares of our common stock on the immediately preceding December 31 or (iii) an amount determined by our board of directors.
Percentage threshold of outstanding shares as of December 31 of each year for calculation of annual increase in authorized shares under the plan			4.00%
Additional number of common stock shares reserved for issuance				350,000						307,949
IPO [Member] | 2015 Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Effective date of plan				May 6, 2015
Number of common stock shares reserved for issuance			16,258	75,315
Percentage threshold of outstanding shares as of December 31 of each year for calculation of annual increase in authorized shares under the plan			1.00%
Additional number of common stock shares reserved for issuance										76,988
IPO [Member] | Maximum [Member] | 2015 Stock Option and Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Annual increase in shares authorized under plan, shares threshold			131,428

Stockholders' Equity - Summary

Stockholders' Equity - Summary of Stock Option Activity (Detail)	12 Months Ended
	Dec. 31, 2020USD ($)$ / sharesshares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Outstanding Options, Beginning Balance | shares	351,078
Number of Outstanding Options, Granted | shares	412,432
Number of Outstanding Options, Canceled/forfeited/expired | shares	(186,976)
Number of Outstanding Options, Ending Balance | shares	576,534
Number of Outstanding Options, Vested and Expected to Vest, Ending Balance | shares	576,534
Number of Outstanding Options, Exercisable, Ending Balance | shares	268,039
Weighted Average Exercise Price, Beginning Balance | $ / shares	$ 51.34
Weighted Average Exercise Price, Granted | $ / shares	4.18
Weighted Average Exercise Price, Canceled/forfeited/expired | $ / shares	33.73
Weighted Average Exercise Price, Ending Balance | $ / shares	23.33
Weighted Average Exercise Price, Options Vested and Expected to Vest, Ending Balance | $ / shares	23.33
Weighted Average Exercise Price, Options Exercisable, Ending Balance | $ / shares	$ 41.06
Weighted Average Contractual Term, Outstanding	7 years 10 months 21 days
Weighted Average Contractual Term, Options Vested and Expected to Vest	7 years 10 months 21 days
Weighted Average Contractual Term, Options Exercisable	6 years 8 months 12 days
Aggregate Intrinsic Value, Outstanding | $	$ 15,087
Aggregate Intrinsic Value, Options Vested and Expected to Vest | $	15,087
Aggregate Intrinsic Value, Options Exercisable | $	$ 1,233

Stockholders' Equity - Summar_2

Stockholders' Equity - Summary of Assumptions Used to Determine Fair Value of Employee Stock Option Grants, Employee Stock Purchase Plan and Performance Options with Market Condition (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
ESPP [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	6 months	6 months	6 months
Risk-free interest rate, minimum	0.10%	1.60%	1.40%
Risk-free interest rate, maximum	1.60%	2.50%	2.10%
Expected volatility, minimum	89.70%	99.70%	71.50%
Expected volatility, maximum	143.20%	141.70%	99.70%
Expected dividend yield	0.00%	0.00%	0.00%
Employee Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate, minimum	0.30%	1.40%	2.30%
Risk-free interest rate, maximum	1.50%	2.60%	3.00%
Expected volatility, minimum	102.20%	97.20%	87.90%
Expected volatility, maximum	109.70%	105.40%	98.40%
Expected dividend yield	0.00%	0.00%	0.00%
Employee Stock Option [Member] | Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	5 years 6 months	5 years 6 months 3 days	5 years 6 months
Employee Stock Option [Member] | Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	6 years 29 days	6 years 26 days	6 years 29 days

Stockholders' Equity - Schedule

Stockholders' Equity - Schedule of Restricted Stock Unit Activity (Detail) - Restricted Stock Unit [Member]	12 Months Ended
	Dec. 31, 2020$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Number of Outstanding Restricted Stock Units, Beginning Balance | shares	12,475
Number of Outstanding Restricted Stock Units, Granted | shares	5,000
Number of Outstanding Restricted Stock Units, Released | shares	(8,678)
Number of Outstanding Restricted Stock Units, Forfeited | shares	(1,120)
Number of Outstanding Restricted Stock Units, Ending Balance | shares	7,677
Weighted Average Grant Date Fair Value, Beginning Balance | $ / shares	$ 9.90
Weighted Average Grant Date Fair Value, Granted | $ / shares	4.29
Weighted Average Grant Date Fair Value, Released | $ / shares	10.47
Weighted Average Grant Date Fair Value, Forfeited | $ / shares	11.90
Weighted Average Grant Date Fair Value, Ending Balance | $ / shares	$ 5.32

Stockholders' Equity - Schedu_2

Stockholders' Equity - Schedule of Allocation of Stock-Based Compensation for All Options Including Performance Options with Market Condition and Restricted Stock Units (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 1,465	$ 1,783	$ 3,431
Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	254	354	1,216
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 1,211	$ 1,429	$ 2,215

Stockholders' Equity - Summar_3

Stockholders' Equity - Summary of Warrants Outstanding (Detail)	12 Months Ended
	Dec. 31, 2020$ / sharesshares
Class Of Warrant Or Right [Line Items]
Number Outstanding	13,904
Warrant One [Member]
Class Of Warrant Or Right [Line Items]
Number Outstanding	144
Exercise Price Per Share | $ / shares	$ 104.65
Expiration Date	2021-03
Warrant Two [Member]
Class Of Warrant Or Right [Line Items]
Number Outstanding	1,066
Exercise Price Per Share | $ / shares	$ 281.50
Expiration Date	2023-07
Warrant Three [Member]
Class Of Warrant Or Right [Line Items]
Number Outstanding	6,830
Exercise Price Per Share | $ / shares	$ 43.93
Expiration Date	2023-11
Warrant Four [Member]
Class Of Warrant Or Right [Line Items]
Number Outstanding	2,978
Exercise Price Per Share | $ / shares	$ 50.37
Expiration Date	2024-06
Warrant Five [Member]
Class Of Warrant Or Right [Line Items]
Number Outstanding	2,886
Exercise Price Per Share | $ / shares	$ 51.98
Expiration Date	2024-12

Stockholders' Equity - Summar_4

Stockholders' Equity - Summary of Common Stock Reserved for Future Issuance (Detail)	Dec. 31, 2020shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance	1,055,093
Common Stock Warrants [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance	13,904
Common Stock Options and Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance	584,211
Shares Available Under the 2015 Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance	381,663
2015 Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance	75,315

Income Tax - Schedule of Pretax

Income Tax - Schedule of Pretax Losses for Domestic and Foreign Operations (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
United States	$ (15,950)	$ (23,315)	$ (34,021)
Foreign	(280)	(448)	(494)
Loss before income taxes	$ (16,230)	$ (23,763)	$ (34,515)

Income Tax - Additional Informa

Income Tax - Additional Information (Detail) - USD ($)	Jun. 29, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax [Line Items]
Income tax (benefit) provision		$ 0	$ 0	$ 0
Valuation allowance		$ 74,646,000	71,702,000
Period of change in ownership		3 years
Percentage of change in ownership		50.00%
Unrecognized tax benefits, income tax penalties and interest accrued		$ 0	$ 0	$ 0
Change in unrecognized tax benefits		0
Federal [Member]
Income Tax [Line Items]
NOLs carryforwards		177,800,000
NOLs carryforwards not subject to expiration		65,300,000
NOLs carryforwards subject to expiration		$ 112,500,000
NOLs carryforwards expiration year		2025
Research and development credit carryforward		$ 5,500,000
Research and development credit carryforward expiration year		2026
Federal [Member] | Orphan Drug Credits [Member]
Income Tax [Line Items]
Research and development credit carryforward		$ 12,500,000
Research and development credit carryforward expiration year		2035
State [Member]
Income Tax [Line Items]
NOLs carryforwards subject to expiration		$ 164,100,000
NOLs carryforwards expiration year		2021
Research and development credit carryforward		$ 4,500,000
Foreign [Member]
Income Tax [Line Items]
NOLs carryforwards not subject to expiration		$ 8,300,000
CALIFORNIA
Income Tax [Line Items]
Maximum amount of tax offset by business credit	$ 5,000,000

Income Tax - Schedule of Reconc

Income Tax - Schedule of Reconciliations of the Expected Statutory Federal Income Tax (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Expected income taxes benefit at federal statutory rate	$ (3,408,000)	$ (4,990,000)	$ (7,248,000)
State income taxes, net of federal benefit	(12,000)	(19,000)	(14,000)
Permanent items and other	169,000	49,000	(80,000)
Stock compensation	804,000	701,000	850,000
Research credits	(835,000)	(817,000)	(1,222,000)
Unrecognized tax benefits	334,000	327,000	489,000
Foreign rate differential	13,000	20,000	22,000
Change in tax rate	(7,000)	(49,000)	(11,000)
Change in valuation allowance	2,942,000	4,778,000	7,214,000
Income tax (benefit) expense	$ 0	$ 0	$ 0

Income Tax - Schedule of Signif

Income Tax - Schedule of Significant Components of Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Net operating loss carryforwards	$ 41,203	$ 38,180
Capitalized research and development expenses	17,007	16,900
Research credits and other state credits	12,954	12,452
Intangible assets	1,655	1,841
Reserve and accruals	544	481
Share-based compensation expense	1,253	1,829
Lease liability	472	631
Valuation allowance	(74,646)	(71,702)
Total deferred tax assets	442	612
Deferred tax liabilities:
Right of use lease assets	(442)	(612)
Total deferred tax liabilities	$ (442)	$ (612)

Income Tax - Schedule of Activi

Income Tax - Schedule of Activity Related to Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Balance as of beginning of year	$ 21,302	$ 19,643	$ 16,558
Increase related to prior year tax positions	3		2
Increase related to current year tax positions	402	1,659	3,083
Balance as of end of year	$ 21,707	$ 21,302	$ 19,643

Employee Benefits - Additional

Employee Benefits - Additional Information (Detail) - 401 (k) Plan [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Contribution Plan Disclosure [Line Items]
Policy approved month and year	2015-04
Date of Policy beginning	Jun. 1, 2015
Percentage of match employee contribution	50.00%
Percentage of participants's annual salary	6.00%
Discretionary contribution	$ 0.2	$ 0.2	$ 0.2

Subsequent Events - Additional

Subsequent Events - Additional Information (Detail) - Subsequent Event [Member] - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 22, 2021	Mar. 23, 2021
ATM Offering Program [Member] | Common Stock [Member]
Subsequent Event [Line Items]
Number of shares issued and sold	1,988,254
Shares issued, price per share	$ 4.99
Net proceeds from issuance of common stock	$ 9.9
Purchase Agreement [Member] | Common Stock [Member] | Asprire Capital [Member]
Subsequent Event [Line Items]
Number of shares issued and sold	3,000,000
Shares issued, price per share	$ 5.09
Net proceeds from issuance of common stock	$ 15.3
ATM Offering Program [Member] | Jones Trading [Member] | Sale Agreement [Member]
Subsequent Event [Line Items]
Commission rate equal to gross proceeds		3.00%
ATM Offering Program [Member] | Common Stock [Member] | Jones Trading [Member] | Sale Agreement [Member] | Maximum [Member]
Subsequent Event [Line Items]
Agreed upon value of sale of common stock per transaction		$ 25