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Document And Entity Information - USD ($)	12 Months Ended
Document And Entity Information - USD ($)	Dec. 31, 2019	Mar. 16, 2020	Jun. 28, 2019
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Trading Symbol	FTSV
Entity Registrant Name	Forty Seven, Inc.
Entity Central Index Key	0001667633
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Common Stock, Shares Outstanding		48,156,251
Entity Public Float			$ 132,900
Entity Shell Company	false
Entity Current Reporting Status	Yes
Entity Incorporation, State or Country Code	DE
Entity File Number	001-38554
Entity Tax Identification Number	47-4065674
Entity Address, Address Line One	1490 O’Brien Drive
Entity Address, Address Line Two	Suite A
Entity Address, City or Town	Menlo Park
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94025
City Area Code	650
Local Phone Number	352-4150
Document Annual Report	true
Document Transition Report	false
Entity Interactive Data Current	Yes
Title of 12(b) Security	Common Stock, Par Value $0.0001 Per Share
Security Exchange Name	NASDAQ
Documents Incorporated by Reference	None.

Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 213,814	$ 10,837
Short-term investments	115,280	128,186
Prepaid expenses and other current assets	8,430	6,835
Total current assets	337,524	145,858
Property and equipment, net	1,223	1,360
Operating lease right-of-use assets	1,978
Other assets	2,868	2,219
Total assets	343,593	149,437
Current liabilities:
Accounts payable	8,629	4,621
Accrued liabilities	9,075	9,044
Lease liabilities, current	1,471
Deferred grant funding, current	2,761	1,744
Other current liabilities	859
Total current liabilities	22,795	15,409
Lease liabilities, noncurrent	844
Deferred rent, noncurrent		331
Other long-term liabilities	2,026	476
Total liabilities	25,665	16,216
Commitments and Contingencies (Note 6)
Stockholders’ equity:
Common stock, $0.0001 par value: 200,000,000 shares authorized; 47,983,366 and 31,079,150 shares issued and outstanding at December 31, 2019 and 2018, respectively	5	3
Additional paid-in capital	545,221	273,069
Accumulated other comprehensive gain (loss)	92	(82)
Accumulated deficit	(227,390)	(139,769)
Total stockholders’ equity	317,928	133,221
Total liabilities and stockholders’ equity	$ 343,593	$ 149,437

Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2019	Dec. 31, 2018
Statement Of Financial Position [Abstract]
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	47,983,366	31,079,150
Common stock, shares outstanding	47,983,366	31,079,150

Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income Statement [Abstract]
License revenue	$ 15,678
Revenue from Contract with Customer, Product and Service [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Operating expenses:
Research and development	$ 83,792	$ 56,673
General and administrative	20,418	15,432
Total operating expenses	104,210	72,105
Loss from operations	(88,532)	(72,105)
Other income (expenses):
Remeasurement loss on contingent repayment obligation	(2,554)	(331)
Interest and other income, net	3,465	2,066
Net loss	(87,621)	(70,370)
Unrealized gains (losses) on available-for-sale securities	174	(38)
Comprehensive loss	$ (87,447)	$ (70,408)
Net loss per share, basic and diluted	$ (2.40)	$ (3.75)
Shares used in computing net loss per share, basic and diluted	36,434,365	18,768,868

Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Convertible Preferred Stock	Common Stock	Additional Paid-in Capital	Other Comprehensive Gain (Loss)	Accumulated Deficit
Beginning balance at Dec. 31, 2017	$ 83,462	$ 149,397	$ 1	$ 3,507	$ (44)	$ (69,399)
Beginning balance, shares at Dec. 31, 2017		16,215,896	6,751,157
Conversion of preferred stock into common stock upon completion of the initial public offering		$ (149,397)	$ 1	149,396
Conversion of preferred stock into common stock upon completion of the initial public offering, Shares		(16,215,896)	16,215,896
Issuance of common stock in connection with the initial public offerings, net of issuance costs	116,336		$ 1	116,335
Issuance of common stock in connection with the initial public offerings, net of issuance costs, shares			8,090,250
Repurchase of fractional shares resulting from reverse stock split	(1)			(1)
Repurchase of fractional shares resulting from reverse stock split, shares			(83)
Issuance of common stock for exercise of stock options	191			191
Issuance of common stock for exercise of stock options, shares			63,865
Vesting of restricted common stock	1			1
Vesting of early exercised stock options	209			209
Repurchase of unvested common stock, shares			(41,935)
Stock-based compensation	3,431			3,431
Net loss	(70,370)					(70,370)
Unrealized gain (loss) on available-for-sale securities	(38)				(38)
Ending balance at Dec. 31, 2018	133,221		$ 3	273,069	(82)	(139,769)
Ending balance, shares at Dec. 31, 2018			31,079,150
Issuance of common stock in connection with the initial public offerings, net of issuance costs	264,060		$ 2	264,058
Issuance of common stock in connection with the initial public offerings, net of issuance costs, shares			16,370,250
Issuance of common stock for exercise of stock options	2,130			2,130
Issuance of common stock for exercise of stock options, shares			439,398
Issuance of common stock pursuant to the ESPP	859			859
Issuance of common stock pursuant to the ESPP, shares			94,568
Vesting of restricted common stock	1			1
Vesting of early exercised stock options	143			143
Stock-based compensation	4,961			4,961
Net loss	(87,621)					(87,621)
Unrealized gain (loss) on available-for-sale securities	174				174
Ending balance at Dec. 31, 2019	$ 317,928		$ 5	$ 545,221	$ 92	$ (227,390)
Ending balance, shares at Dec. 31, 2019			47,983,366

Statements of Stockholders' Equity (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Statement Of Stockholders Equity [Abstract]
Convertible preferred stock issuance costs	$ 1,073	$ 4,047

Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
Statements of Cash Flows - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities:
Net loss	$ (87,621)	$ (70,370)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	4,961	3,431
Depreciation and amortization	523	401
Amortization of right-of-use assets	1,063	0
Accretion of discounts on marketable securities	(1,047)	(744)
Realized gain on sale of available-for-sale securities	(6)	0
Change in fair value of contingent repayment obligation	2,554	331
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,595)	(2,385)
Other assets	(651)	(673)
Accounts payable	4,009	916
Accrued liabilities	(107)	4,214
Deferred grant funding	1,017	(1,015)
Lease related liabilities	(1,213)	(123)
Net cash used in operating activities	(78,113)	(66,017)
Cash flows from investing activities:
Purchases of property and equipment	(356)	(403)
Purchases of available-for-sale securities	(166,040)	(142,027)
Proceeds from sales of available-for-sale securities	3,996	0
Proceeds from maturities of available-for-sale securities	176,177	78,241
Net cash provided by (used in) investing activities	13,777	(64,189)
Cash flows from financing activities:
Proceeds from issuance of common stock upon ESPP purchase	859	0
Proceeds from issuance of common stock upon exercise of stock options	2,130	291
Proceeds from public offerings of common stock, net of issuance costs	264,324	116,336
Repurchase of fractional common stock upon reverse stock split	0	(1)
Net cash provided by financing activities	267,313	116,626
Net increase (decrease) in cash and cash equivalents	202,977	(13,580)
Cash and cash equivalents — beginning of period	10,837	24,417
Cash and cash equivalents — end of period	213,814	10,837
Supplemental disclosures of cash flow information:
Operating cash flows paid for operating leases	1,406	0
Noncash investing and financing activities:
Lease liability obtained in exchange for right-of-use asset	712	0
Public offering costs included in accrued liabilities	264	0
Property and equipment purchases included in accrued liabilities	30
Conversion of convertible preferred stock to common stock at close of initial public offering	$ 0	$ 149,397

Description of Business

12 Months Ended

Dec. 31, 2019

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Basis of Presentation

1.	Description of Business

The Company is a clinical-stage immuno-oncology company focused on developing novel checkpoint therapies to activate macrophages in the fight against cancer. Forty Seven was founded based on the insight that blocking CD47, a key signaling molecule that is over-expressed on cancer cells, renders tumors susceptible to macrophages and the innate immune system. By harnessing macrophages, the Company believes that its lead product candidate, magrolimab (formerly known as magrolimab), dosed as a monotherapy and in combination with marketed cancer therapies, can transform the treatment of cancer.

Public Offerings

On July 2, 2018, the Company completed its initial public offering (“IPO”) of 7,035,000 shares of common stock, and subsequently on July 27, 2018, the Company issued and sold an additional 1,055,250 shares upon the exercise of the underwriters’ over-allotment option. In connection with the IPO, including the over-allotment option, the Company issued and sold an aggregate of 8,090,250 shares of common stock at $16.00 per share, raising $116.3 million in proceeds, net of underwriting discounts and commissions, and offering expenses of $13.2 million. Upon the closing of the IPO, all outstanding shares of convertible preferred stock were automatically converted into 16,215,896 shares of common stock.

The Company completed an underwritten public offering in July 2019 of 10,781,250 shares of the Company’s common stock, including 1,406,250 shares sold pursuant to the underwriters’ exercise of their option to purchase additional shares at a public offering price of $8.00 per share. The net proceeds from the offering to the Company were $80.5 million.

The Company completed another underwritten public offering in December 2019 of 5,589,000 shares of the Company’s common stock, including 729,000 shares sold pursuant to the underwriters’ exercise of their option to purchase additional shares at a public offering price of $35.00 per share. The net proceeds from the offering to the Company were $183.6 million.

Liquidity

In the course of its development activities, the Company has sustained operating losses and expects to continue to generate operating losses for the foreseeable future. The Company’s ultimate success depends on the outcome of its research and development activities. The Company had cash, cash equivalents and short-term investments of $329.1 million as of December 31, 2019. Since inception through December 31, 2019, the Company has incurred cumulative net losses of $227.4 million. Management expects to incur additional losses in the future to conduct product research and development and recognizes the need to raise additional capital to fully implement its business plan.

The Company intends to raise such capital through the issuance of additional equity financing and/or third-party collaboration funding. However, if such financing is not available at adequate levels, the Company will need to reevaluate its operating plan and may be required to delay the development of its products. The Company expects that its cash, cash equivalents and short-term investments will be sufficient to fund operating expenses and capital expenditure requirements for a period of at least one year from the date the audited financial statements are filed with the Securities and Exchange Commission (“SEC”).

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2019

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2.	Summary of Significant Accounting Policies

Basis of Presentation

The accompanying financial statements have been prepared in conformity with U.S. generally accepted accounting principles (“U.S. GAAP”).

Use of Estimates

The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Significant estimates and assumptions made in the accompanying financial statements include but are not limited to revenue recognition, the fair value of common stock prior to the IPO, the fair value of stock options, the fair value of investments, income tax uncertainties, lease liability, and certain accruals. The Company evaluates its estimates and assumptions on an ongoing basis using historical experience and other factors and adjusts those estimates and assumptions when facts and circumstances dictate. Actual results could differ from those estimates.

Cash and Cash Equivalents

The Company considers all highly liquid investments purchased with original maturities of three months or less from the purchase date to be cash equivalents. Cash equivalents consist primarily of amounts invested in money market accounts.

Investments

Investments have been classified as available-for-sale and are carried at estimated fair value as determined based upon quoted market prices or pricing models for similar securities. Management determines the appropriate classification of its investments in debt securities at the time of purchase. Investments with original maturities beyond three months at the date of purchase and which mature at, or less than twelve months from the balance sheet date are classified as current.

Unrealized gains and losses are excluded from earnings and are reported as a component of comprehensive loss. The Company periodically evaluates whether declines in fair values of its marketable securities below their book value are other-than-temporary. This evaluation consists of several qualitative and quantitative factors regarding the severity and duration of the unrealized loss as well as the Company’s ability and intent to hold the marketable security until a forecasted recovery occurs. Additionally, the Company assesses whether it has plans to sell the security or it is more likely than not it will be required to sell any marketable securities before recovery of its amortized cost basis. Realized gains and losses and declines in fair value judged to be other than temporary, if any, on marketable securities are included in interest and other income, net. The cost of investments sold is based on the specific-identification method. There were no material realized gains or losses on investments for the years ended December 31, 2019 and 2018. Interest on marketable securities is included in interest income.

Concentration of Credit Risk and Other Risks and Uncertainties

Financial instruments that potentially subject the Company to concentrations of risk consist of cash, cash equivalents, and short-term investments. The Company’s cash, cash equivalents and short-term investments are held by one financial institution in the United States, which management believes to be of high credit quality. Deposits in this financial institution may at times exceed federally insured limits. The Company has not experienced any losses on its deposits of cash, cash equivalents, or short-term investments.

Fair Value Measurement

Assets and liabilities recorded at fair value on a recurring basis in the balance sheets are categorized based upon the level of judgment associated with the inputs used to measure their fair values. Fair value is defined as the exchange price that would be received for an asset or an exit price that would be paid to transfer a liability in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. The authoritative guidance on fair value measurements establishes a three-tier fair value hierarchy for disclosure of fair value measurements as follows:

Level 1—Observable inputs such as unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date;

Level 2—Inputs (other than quoted prices included in Level 1) are either directly or indirectly observable for the asset or liability. These include quoted prices for similar assets or liabilities in active markets and quoted prices for identical or similar assets or liabilities in markets that are not active;

Level 3— Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities.

Property and Equipment, Net

Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation is computed on a straight-line basis over the estimated useful lives of the related assets, generally three to five years. Leasehold improvements are amortized using the straight-line method over the shorter of the assets’ estimated useful lives or the remaining term of the lease. Maintenance and repairs are charged to operations as incurred. Upon sale or retirement of assets, the cost and related accumulated depreciation are removed from the balance sheet and the resulting gain or loss is reflected in operations.

Impairment of Long-Lived Assets

Long-lived assets are reviewed for indications of possible impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Recoverability is measured by comparison of the carrying amounts to the future undiscounted cash flows attributable to these assets. An impairment loss is recognized to the extent an asset group is not recoverable, and the carrying amount exceeds the projected discounted future cash flows arising from these assets. There were no impairments of long-lived assets for any of the periods presented.

Contingent repayment obligation

The Company’s research and license agreements may include contingent milestone payments that are based upon reaching certain development and regulatory approval milestones. With respect to the Company’s agreement with the Leukemia & Lymphoma Society, Inc. (“LLS”), as amended, the Company has concluded that contingent milestone payments result in a contingent repayment obligation. The Company estimates the fair value of the contingent repayment obligation using a valuation model designed to estimate the probability of the occurrence of such contingent milestone payments based on various assumptions and incorporating estimated success rates. Estimated payments are then discounted using present value techniques to arrive at an estimated fair value. The contingent repayment obligation is subject to revaluation at each balance sheet date, with revaluations recognized as remeasurement gain or loss on contingent repayment obligation in the Company’s statements of operations and comprehensive loss.

The Company has separately presented the change in the fair value of the contingent repayment obligation as a component of non-operating income and expense on the statement of operations and has reclassified the prior period to conform to the current period presentation.

Revenue Recognition

License and Collaboration Arrangements

The Company accounts for revenue recognition under Accounting Standards Codification (“ASC”), Revenue from Contracts with Customers (Topic 606) (“ASC 606”). Under ASC 606, the Company recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the Company expects to receive in exchange for those goods or services.

The Company enters into collaborative arrangements with partners that fall under the scope of ASC, Collaborative Arrangements (Topic 808) (“ASC 808”). The Company analyzes it collaboration arrangements to assess whether they are within the scope of ASC 808 to determine whether such arrangements involve joint operating activities performed by parties that are both active participants in the activities and exposed to significant risks and rewards dependent on the commercial success of such activities. This assessment is performed throughout the life of the arrangement based on changes in the responsibilities of all parties in the arrangement. The accounting for some of the activities under collaboration arrangements may be analogized to ASC 606 for distinct units of account that are reflective of a vendor-customer relationship. For other elements of collaboration arrangements, such as research and development performed under cost-sharing arrangements, the Company follows the illustrative examples in ASC 808 and generally records research and development reimbursements received as a reduction of research and development expenses.

In determining the appropriate amount of revenue to be recognized as the Company fulfills its obligations under each of its agreements, the Company performs 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 based on estimated selling prices; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation.

If a license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, the Company recognizes revenues attributed to the license when the license is transferred to the customer and the customer is able to use and benefit from the license.

At the inception of each arrangement that includes development milestone payments, the Company evaluates whether the milestones are considered probable of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the control of the Company or the licensee, such as regulatory approvals, are not generally considered probable of being achieved until those approvals are received. At the end of each reporting period, the Company re-evaluates the probability of achievement of any development milestones, and if necessary, adjusts its estimate of the transaction price. Any such adjustments would be recorded on a cumulative catch-up basis, which would affect revenues and earnings in the period of adjustment.

Research and Development Expenditures

Research and development expenses consist of costs incurred for the Company’s own and for sponsored and collaborative research and development activities. Research and development costs are expensed as incurred. Research and development costs consist of salaries and benefits, including associated stock-based compensation, and laboratory supplies and facility costs, as well as fees paid to other entities that conduct certain research and development activities on the Company’s behalf. The Company estimates preclinical study and clinical trial expenses based on the services performed pursuant to contracts with research institutions and contract research organizations, or CROs, and clinical manufacturing organizations, or CMOs, that conduct and manage preclinical studies and clinical trials on the Company’s behalf based on actual time and expenses incurred by them. Further, the Company accrues expenses related to clinical trials based on the level of patient enrollment and activity according to the related agreement. The Company monitors patient enrollment levels and related activity to the extent reasonably possible and adjust estimates accordingly. If the Company does not identify costs that have begun to be incurred or if the Company underestimates or overestimates the level of services performed or the costs of these services, actual expenses could differ from the Company’s estimates. To date, the Company has not experienced significant changes in its estimates of preclinical studies and clinical trial accruals.

The Company expenses payments for the acquisition and development of technology as research and development costs if, at the time of payment, the technology is under development; is not approved by the U.S. Food and Drug Administration or other regulatory agencies for marketing; has not reached technical feasibility; or otherwise has no foreseeable alternative future use. In addition, funding from research grants is offset against the related qualified research and development costs incurred.

Stock-Based Compensation

The Company measures its stock-based awards granted to employees and directors based on the estimated fair values of the awards and recognizes the compensation over the requisite service period. The Company uses the Black-Scholes option-pricing model to estimate the fair value of its stock-based awards and employee stock purchase plan (“ESPP”) purchases. Stock-based compensation is recognized using the straight-line method.

Defined Contribution Plan

The Company has a defined contribution retirement savings plan under Section 401(k) of the Internal Revenue Code (“IRC”). This plan allows eligible employees to defer a portion of their annual compensation on a pre-tax or after-tax basis. The Company may make discretionary matching contributions. During 2019 and 2018, the Company made matching contributions on up to 3% of an employee’s eligible compensation deferred. The Company recognized expense related to its contributions to the plan of $0.4 million and $0.3 million for the years ended December 31, 2019 and 2018, respectively.

Income Taxes

Income taxes are accounted for under the asset and liability method. Under this method, deferred tax assets and liabilities are determined based on the difference between the financial statement and tax bases of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to affect taxable income. Management makes an assessment of the likelihood that the resulting deferred tax assets will be realized. A valuation allowance is provided when it is more likely than not that some portion or all of a deferred tax asset will not be realized. Due to the Company’s historical operating performance and the recorded cumulative net losses in prior fiscal periods, the net deferred tax assets have been fully offset by a valuation allowance.

The Company recognizes uncertain income tax positions at the largest amount that is more likely than not to be sustained upon audit by the relevant taxing authority. An uncertain income tax position will not be recognized if it has less than a 50% likelihood of being sustained. Changes in recognition or measurement are reflected in the period in which judgment occurs. The Company’s policy is to recognize interest and penalties related to the underpayment of income taxes as a component of provision for income taxes.

Foreign Currency Transactions

Gains or losses from foreign currency transactions are included in interest and other income, net, in the statements of operations and comprehensive loss. The Company recognized $0.2 million in net foreign currency gains and $0.2 million in net foreign currency losses for the years ended December 31, 2019 and 2018, respectively.

Comprehensive Loss

The Company’s comprehensive loss is comprised of changes in unrealized losses on available-for-sale securities.

Net Loss per Share

Basic net loss per share is calculated by dividing the net loss by the weighted-average number of shares of common stock outstanding for the period, without consideration for potential dilutive shares of common stock. Since the Company was in a loss position for all periods presented, basic net loss per share is the same as diluted net loss per share since the effects of potentially dilutive securities are antidilutive. Shares of common stock subject to repurchase are excluded from the weighted-average shares.

Segment Reporting

The Company has one operating segment. The Company’s chief operating decision maker, its Chief Executive Officer, manages the Company’s operations on a consolidated basis for the purposes of allocating resources.

Recently Issued and Adopted Accounting Pronouncements

Leases

On January 1, 2019, the Company adopted the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update No. 2016-02, Leases (Topic 842) using the modified retrospective method. For its operating leases in excess of 12 months, the Company recognizes a right-of-use asset and a lease liability on its balance sheet. The lease liability is determined as the present value of future lease payments using an estimated rate of interest that the Company would pay to borrow equivalent funds on a collateralized basis at the adoption date for the existing lease and at lease commencement date for new leases. The right-of-use asset is based on the liability adjusted for any prepaid or deferred rent, and lease incentives, as applicable.

Rent expense for the operating leases is recognized on a straight-line basis over the lease term and is included in operating expenses on the statements of operations and comprehensive loss. Variable lease payments include lease operating expenses.

Under the modified prospective adoption method, the leases are presented under ASC 842 for the year ended December 31, 2019. The prior period continues to be reported in accordance with previous lease guidance, ASC 840, Leases. The Company elected the package of practical expedients permitted under the transition guidance within the new standard, which among other things, allows the Company to carry forward the historical lease classification of the leases in place as of January 1, 2019. As allowed under ASC 842, the Company has elected to not separate lease and nonlease components. The Company has also elected to not apply the recognition requirement of ASC 842 to leases with a term of 12 months or less.

The impact of the adoption of ASC 842 on the accompanying balance sheet as of January 1, 2019 was as follows:

	December 31, 2018		Adjustments due to the adoption of ASC 842			January 1, 2019
	(In thousands)
Assets
Operating lease right-of-use asset	$	—	$	2,328		$	2,328
Liabilities and stockholders' equity
Deferred rent classified as accrued liabilities	$	155	$	(155	)	$	—
Lease liability, current	$	—	$	968		$	968
Lease liability, noncurrent	$	—	$	1,847		$	1,847
Deferred rent, noncurrent	$	331	$	(331	)	$	—

The adjustments due to the adoption of ASC 842 related to the recognition of an operating lease right-of-use asset and lease liability for the Company's existing property operating lease and the derecognition of the deferred rent recognized under ASC 840. There was no impact on the Company’s statement of operations and comprehensive loss from the adoption and no cumulative-effect adjustment to the beginning accumulated deficit.

Recent Accounting Pronouncements Not Yet Adopted

Accounting Standards Update (“ASU”) No. 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments (“ASU 2016-13”). ASU 2016-13 requires an entity to utilize a new impairment model that requires measurement and recognition of expected credit losses for most financial assets and certain other instruments, including but not limited to available-for-sale debt securities. Credit losses relating to available-for-sale debt securities will be recorded through an allowance for credit losses rather than as a direct write-down to the security. The standard is effective for fiscal years, and interim periods within those years, beginning after December 15, 2019. In November 2019, the FASB issued ASU No. 2019-10, Financial Instruments—Credit Losses (Topic 326), Derivatives and Hedging (Topic 815) and Leases (Topic 842): Effective Dates, which defers the effective date of this ASU to fiscal years beginning after December 15, 2022 for all entities except SEC reporting companies that are not smaller reporting companies. As a smaller reporting company, the Company plans to adopt this ASU beginning January 1, 2023.

Fair Value Measurements

12 Months Ended

Dec. 31, 2019

Fair Value Disclosures [Abstract]

Fair Value Measurements

3.	Fair Value Measurements

The Company measures and reports its cash equivalents, short-term investments, and contingent repayment obligation at fair value.

Money market funds and US government debt securities are measured at fair value on a recurring basis using quoted prices and are classified as a Level 1 input. Short-term investments are measured at fair value based on inputs other than quoted prices that are derived from observable market data and are classified as Level 2 inputs. There were no transfers between Levels 1, 2 or 3 for any of the periods presented. All of the investments held as of December 31, 2019 had maturities of less than one year. There were no material realized gains or losses on investments for the years ended December 31, 2019 and 2018. Any unrealized losses were deemed to be temporary. The Company does not intend to sell its securities that are in an unrealized loss position, if any, and it is unlikely that the Company will be required to sell its securities before recovery of their amortized cost basis, which may be maturity.

The fair value and amortized cost of cash equivalents and available-for-sale securities by major security type as of December 31, 2019 and 2018 are presented in the following tables:

	As of December 31, 2019
	Fair Value Hierarchy	Amortized Cost		Unrealized Gains		Unrealized Losses		Market Value
	(in thousands)
Money market funds	Level 1	$	203,902	$	—	$	—	$	203,902
US government debt securities	Level 1		24,039		24		—		24,063
Commercial paper	Level 2		17,962		—		—		17,962
Corporate debt securities	Level 2		52,111		48		—		52,159
Asset-backed securities	Level 2		25,070		20		—		25,090
Total cash equivalents and available-for-sale securities		$	323,084	$	92	$	—	$	323,176

	As of December 31, 2018
	Fair Value Hierarchy	Amortized Cost		Unrealized Gains		Unrealized Losses			Market Value
	(in thousands)
Money market funds	Level 1	$	7,959	$	—	$	—		$	7,959
US government debt securities	Level 1		15,963		—		(1	)		15,962
Commercial paper	Level 2		43,277		—		—			43,277
Corporate debt securities	Level 2		46,186		—		(54	)		46,132
Asset-backed securities	Level 2		22,842		—		(27	)		22,815
Total cash equivalents and available-for-sale securities		$	136,227	$	—	$	(82	)	$	136,145

The interest earned from the investments in securities is $3.2 million and $2.2 million for the years ended December 2019 and 2018, respectively, and is reported as part of interest and other income, net in the statements of operations and comprehensive loss.

The Company’s contingent milestone payments in its agreement with LLS, as amended, result in a contingent repayment obligation. The contingent repayment obligation contains unobservable inputs that are supported by little or no market activity at the measurement date. Accordingly, the Company’s contingent repayment obligation is measured at fair value on a recurring basis using unobservable inputs that are classified as Level 3 inputs. As of December 31, 2019, the liability for the contingent repayment obligation was $2.9 million, with $0.9 million recorded as part of other current liabilities and $2.0 million recorded as part of other long-term liabilities. As of December 31, 2018, the liability for the contingent repayment obligation of $0.3 million was recorded as part of other long-term liabilities. Refer to Note 7 for the valuation techniques and assumptions used in estimating the fair value of the contingent repayment obligation.

The change in fair value of the contingent repayment obligation is presented in the following tables and is reported as remeasurement loss on contingent repayment obligation in the statements of operations and comprehensive loss:

	December 31,
	2019		2018
	(in thousands)
Beginning balance	$	331	$	—
Initial value of contingent repayment obligation	—			331
Change in fair value of contingent repayment obligation		2,554	—
Ending balance	$	2,885	$	331

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Balance Sheet Components

12 Months Ended

Dec. 31, 2019

Balance Sheet Related Disclosures [Abstract]

Balance Sheet Components

4.	Balance Sheet Components

Property and Equipment, Net

Property and equipment, net consists of the following:

	December 31,
	2019			2018
	(in thousands)
Furniture and fixtures	$	236		$	108
Laboratory equipment		1,257			1,297
Computer equipment and software		164			91
Leasehold improvements		908			770
		2,565			2,266
Less: Accumulated depreciation and amortization		(1,342	)		(906	)
Total property and equipment, net	$	1,223		$	1,360

Depreciation and amortization expense for property and equipment amounted to $0.5 million and $0.4 million for the year ended December 31, 2019 and 2018, respectively.

Accrued Liabilities

Accrued liabilities consist of the following:

	December 31,
	2019		2018
	(in thousands)
Accrued research and development expenses	$	4,143	$	5,870
Accrued bonus		2,788		1,602
Deferred rent, current		—		155
Other		2,144		1,417
Total accrued liabilities	$	9,075	$	9,044

Leases

12 Months Ended

Dec. 31, 2019

Leases [Abstract]

Leases

The Company leases office property and laboratory space at its headquarters in Menlo Park (the “Menlo Park Lease”) through August 2021. The lease requires monthly lease payments subject to annual increases throughout the lease term and includes a renewal option at the election of the Company to extend the lease for an additional five years. The landlord provided the Company with a tenant improvement allowance of $0.6 million.

In April 2019, the Company entered into a sublease (the “Sublease”) to obtain 6,230 rentable square feet to expand its current headquarters through February 2021. The lease requires monthly lease payments subject to annual increases throughout the lease term.

In April 2019, the Company also entered into an amendment of the Menlo Park Lease to add additional space to its headquarter lease upon the expiration of the Sublease in February 2021. Under the terms of the lease amendment, the lease for the additional space will commence on March 1, 2021 and the base rent for the additional space will be approximately $36,000 per month.

As of December 31, 2019, the weighted average remaining lease term was 1.6 years and the weighted average incremental borrowing rate used to determine the operating lease liabilities was 7.0%.

The undiscounted future non-cancellable lease payments under the Company's operating leases are as follows:

	December 31, 2019
	(In thousands)
Years
2020	$	1,585
2021		864
Total undiscounted lease payments		2,449
Present value adjustment for minimum lease commitments		(134	)
Lease liabilities	$	2,315

As of December 31, 2019, the current and noncurrent portion of the lease liabilities was $1.5 million and $0.8 million. Operating lease cost for the operating leases was $1.3 million for the year ended December 31, 2019. Variable lease payments for operating expenses were $0.6 million for the year ended December 31, 2019.

As of December 31, 2018, our leases continue to be presented in accordance with Topic 840. The tenant improvement allowance was amortized as an offset to rent expense over the lease term. For the year ended December 31, 2018 rent expense was $1.1 million and variable lease payments for operating expenses were $0.5 million. As of December 31, 2018, $0.1 million was accrued as deferred rent expense.

In conjunction with the lease agreement, the Company paid a security deposit, which was included in prepaid expenses and other current assets and other assets as of December 31, 2019 and 2018 in the amount of $0.2 million.

Commitments and Contingencies

12 Months Ended

Dec. 31, 2019

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

6.	Commitments and Contingencies

In August 2016, the Company entered into a development and manufacturing agreement with Lonza Sales AG and, in December 2017, the Company entered into a second manufacturing agreement with Lonza Biologics Tuas Pte Ltd, each relating to the manufacturing of magrolimab-related products.

The August 2016 agreement was amended by the Company in November 2017 as relating to the manufacturing of the Company’s other preclinical program related products, and in November and December 2018 as relating to the scope of manufacturing services. Under the agreements, the Company is required to pay Lonza fixed fees based on manufacturing services performed on the Company’s behalf. Payments were due beginning in January 2018 through to the expiration of the agreements in December 2021. The fees payable under the August 2016 agreement and as amended in November 2017 and November and December 2018, are specified in Great British Pounds and are converted into U.S. Dollars based on current exchange rates.

At December 31, 2019, future minimum payments under the Lonza development and manufacturing agreements are as follows:

	December 31, 2019
	(In thousands)
Years
2020	$	10,250
2021		5,150
Total future minimum payments		15,400

Research and License Agreements

12 Months Ended

Dec. 31, 2019

Research And License Agreements [Abstract]

Research and License Agreements

7.	Research and License Agreements

California Institute of Regenerative Medicine Grants

In January 2017, the Company was awarded a research grant from the California Institute of Regenerative Medicine (“CIRM”). The CIRM grant stipulates various milestone-based payments to the Company with the total award of $10.2 million over a period of four years. During the year ended December 31, 2019, the Company received $2.0 million related to this grant. As of December 31, 2019 and 2018, the Company had received $9.2 million and $7.2 million under the award, respectively.

In November 2017, the Company was awarded a second research grant from CIRM for a separate clinical trial study. The total amount of the research grant awarded was $5.0 million in various milestone-based payments over a period of five years. During 2018, the award was amended to $3.2 million in various-milestone payments over a period of five years, as was provided for under the terms of the original award because the Company opted not to expand the patient population participating in the study. In July 2019, the award was further amended to include the patient population expansion that the Company previously opted out of in 2018. In connection with the same, the award was reinstated to $5.0 million consistent with the original award amount. During the year ended December 31, 2019, the Company received $1.2 million related to this grant with an additional $0.7 million receivable as of December 31, 2019 that was recorded within prepaid expenses and other current assets. As of December 31, 2019 and 2018, the Company had received $3.6 million and $2.4 million under the award, respectively. Under the terms of the CIRM grants, the Company is obligated to pay royalties and licensing fees based on a low single digit royalty percentage on net sales of CIRM-funded product candidates or CIRM-funded technology. The Company has the option to decline any and all amounts awarded by CIRM. As an alternative to royalty payments, the Company has the option to convert the award to a loan. No such election has been made as of the date of the issuance of these financial statements. In the event that the Company terminates a CIRM-funded clinical trial, it will be obligated to repay any remaining unspent CIRM funds.

Leukemia & Lymphoma Society Grants

In March 2017, the Company entered into an agreement with the LLS, as amended in June 2018 to include an additional study. The LLS research grant stipulates various milestone-based payments with a total award of $4.2 million through December 2019. During the year ended December 31, 2019, the Company received $0.3 million related to this grant. As of December 31, 2019 and 2018, the Company had received $4.2 million and $3.9 million under the award, respectively. The Company could be required in the future to pay amounts to LLS upon reaching certain development and regulatory approval milestones as well as a low single digit percentage royalty rate on net sales, up to a maximum of $13.7 million in total.

In July 2019, the Company entered into an amendment to its agreement with LLS to further advance the treatment of myelodysplastic syndromes (“MDS”). Under the amendment, the Company is eligible for up to $3.0 million in additional grant funding based on milestone payments from LLS upon the achievement of certain clinical or regulatory milestones in addition to the $4.2 million award that the Company has received pursuant to the March 2017 agreement, as amended. As of December 31, 2019, the Company had not received any funding related to this grant. Pursuant to the amendment, the Company could be required in the future to pay amounts to LLS upon reaching certain development and regulatory approval milestones on the additional funding, up to a maximum of $6.0 million in the aggregate.

The Company concluded that the contingent milestone payments in its agreement with LLS, as amended, represent a contingent repayment obligation and the Company recorded a liability for the contingent payment obligation of approximately $2.9 million and $0.3 million as of December 31, 2019 and 2018. The value of the contingent repayment obligation was estimated using a valuation model designed to estimate the probability of the occurrence of such contingent milestone payments and incorporating estimated success rates. Estimated payments were then discounted using present value techniques. The resulting fair value measurement of the contingent repayment obligation is based on significant inputs not observable in the market and represents a Level 3 measurement. Significant inputs to the valuation include the discount rate, changes in the assumed achievement or timing of development milestones, changes in the probability of certain clinical events, and changes in the probability of regulatory approval.

The Company recognizes research grants as a reduction of research and development expense when the eligible costs are incurred. Under both CIRM and LLS grants, the Company recognized a total of $3.1 million and $8.0 million as a reduction to research and development expenses for research grants for the years ended December 31, 2019 and 2018, respectively.

Master Combination Study Agreement with Genentech, Inc.

In November 2017, the Company entered into a master clinical trial collaboration agreement with Genentech to evaluate the safety, tolerability and preliminary efficacy of magrolimab combined with Genentech’s cancer immunotherapy, atezolizumab, a fully humanized monoclonal antibody targeting PD-L1, in two separate Phase 1b clinical trials (in patients with bladder cancer and AML, respectively). Pursuant to the agreement, the Company will supply magrolimab for the studies and will partially reimburse Genentech for its costs in connection with the bladder cancer study, and Genentech will supply atezolizumab for the studies and be solely responsible for all of its costs in connection with the AML study.

Merck Collaboration Agreement

In January 2018, the Company entered into a clinical trial collaboration and supply agreement with Ares Trading S.A, a subsidiary of Merck KGaA (“Merck”), to evaluate magrolimab combined with Merck’s cancer immunotherapy, avelumab, in a Phase 1b clinical trial in patients with ovarian cancer. Pursuant to the agreement, the parties will jointly pay for the cost of the study. As of December 31, 2019, the Company recorded a receivable of $0.3 million from Merck for reimbursement of research and development costs incurred. Reimbursement under this collaboration agreement is recorded as a reduction to research and development expense. For the years ended December 31, 2019 and 2018, the Company recognized $1.7 million and $1.2 million, respectively, as a reduction to research and development expenses under this collaboration agreement.

BliNK Purchase Agreement

In June 2018, the Company entered into an asset purchase agreement with BliNK Biomedical SAS (“Blink”), under which Blink transferred its patents, intellectual property rights and know-how, and materials related to its CD47 antibody program to the Company. Under the agreement, the Company paid an initial upfront fee of $2.5 million in June 2018, including $0.5 million upon the completion of the transfer of intellectual property rights and know-how. An additional $0.5 million was paid in July 2018 upon completion of material transfers related to its CD47 antibody program to the Company. Additionally, the Company is required to make annual payments of $0.3 million until it receives marketing approval for the first product. The Company could also be required to pay up to $43.0 million in milestone payments in aggregate per product based on the achievement of certain development and regulatory approval milestones. No such milestone payments have been made as of December 31, 2019. In addition, the Company could be required to pay Blink a royalty of single digit percentage on net sales of approved products, which is subject to buy-out provisions for a one-time payment that can be exercised by the Company prior to certain development milestones being achieved. During the year ended December 31, 2019 and 2018, the Company recognized $0.3 million and $3.0 million in research and development expense related to the Blink asset purchase agreement, respectively.

Synthon License Agreement

In July 2018, the Company entered into a settlement and license agreement with Synthon Biopharmaceuticals B.V. (“Synthon”). Under the agreement, the Company agreed to discontinue its ongoing oppositions and challenges at the European Patent Office (“EPO”) and the U.S. Patent and Trademark Office (“USPTO”) directed towards certain patents licensed by Synthon from Stichting Sanquin Bloedvoorziening (“SSB”) that relate to the use of anti-CD47 products in combination with other antibodies to treat cancer. The Company also agreed to request the withdrawal of such proceedings with the USPTO and EPO. In return Synthon agreed to grant the Company a non-exclusive, worldwide sublicense to certain patents Synthon have licensed from SSB, including the SSB patents the Company was opposing at the USPTO and EPO to commercialize a single anti-CD47 product (such as magrolimab or an alternate anti-CD47 product) to treat cancer in combination with other antibodies.

In exchange, for these sublicenses and option rights, the Company agreed to pay Synthon an aggregate of up to approximately 40.0 million Euros comprising an upfront payment upon grant of sublicense and the achievement of future regulatory and commercial milestones which comprise the significant majority of the aggregate payments. The Company also has an option to expand its rights to cover a follow-on anti-CD47 product in exchange for a specified option exercise fee. If the Company exercises its option right, the Company will pay Synthon additional amounts upon the achievement of certain regulatory and commercial milestones related to such follow-on anti-CD47 product. In addition, the Company will be required to pay Synthon an annual license fee and a royalty of a tiered, low single digit percentage on net sales of any approved licensed products. The Company has the right to buy out its royalty obligations for each licensed product in full by paying Synthon specified lump sum amounts prior to the occurrence of certain defined events.

Ono License and Collaboration Agreement

In July 2019, the Company entered into an exclusive license and collaboration agreement with Ono Pharmaceutical Co., Ltd., (“Ono”). Under the agreement, the Company granted Ono an exclusive license to develop, manufacture and commercialize magrolimab, the Company’s monoclonal antibody against CD47, as well as other anti-CD47 antibodies controlled by the Company in Japan, South Korea, Taiwan and the ASEAN countries (the “Ono Territory”). The Company retains all rights to magrolimab and other licensed antibodies outside of the Ono Territory.

Under the agreement, the parties will collaborate on the development, manufacturing and commercialization of magrolimab and other licensed antibodies. Each party will be responsible for conducting development and commercialization of licensed antibodies in its respective territory at its own cost. Further, each party will have the right to participate, at its cost, in global clinical studies of magrolimab and other licensed antibodies conducted by the other party.

The Company received a one-time upfront nonrefundable payment from Ono of 1.7 billion Japanese Yen ($15.7 million US Dollars based on the exchange rate as of the date of the agreement) and is eligible to receive up to an additional 11.2 billion Japanese Yen if specified future development and commercial milestones are achieved by Ono. The Company is also eligible to receive tiered percentage royalties spanning from the mid-teens to the low-twenties on future net sales of magrolimab and other licensed antibodies in the Ono Territory, subject to certain offsets.

The Company accounts for the Ono Agreement as a collaboration arrangement under ASC 808. The Company concluded that the license delivered to Ono is a distinct unit of account for which there is a vendor-customer relationship. For the license, the Company analogized to ASC 606 for the recognition, measurement and reporting of this unit of account. The up-front payment, future milestones and royalties are attributed to the license component of the Ono Agreement. For the cost-sharing and reimbursement-based activities, the Company follows the presentation and disclosure guidance of ASC 808 and the reimbursement under this collaboration agreement is recorded as a reduction to research and development expense. For the year ended December 31, 2019, the Company recognized $0.9 million as a reduction to research and development expenses under this collaboration agreement.

The transaction price at inception included upfront fixed nonrefundable consideration of $15.7 million. All potential future milestones were considered constrained at the inception of the Ono Agreement. Revenue for the license of $15.7 million was recognized upon delivery of the license in July 2019.

bluebird bio, Inc. Research Collaboration Agreement

In September 2019, the Company entered into a research collaboration with bluebird bio, Inc. (“bluebird”) to pursue clinical proof-of-concept for the Company’s novel antibody-based conditioning regimen, FSI-174 (anti-cKIT antibody) plus magrolimab (anti-CD47 antibody), with bluebird’s ex vivo lentiviral vector hematopoietic stem cell (LVV HSC) gene therapy platform. This collaboration will focus initially on diseases that have the potential to be corrected with transplantation of autologous gene-modified blood-forming stem cells. As of December 31, 2019, the Company recorded a receivable of $0.7 million from bluebird for reimbursement of research and development costs incurred. Reimbursement under this collaboration agreement is recorded as a reduction to research and development expense. For the year ended December 31, 2019, the Company recognized $1.2 million as a reduction to research and development expenses under this collaboration agreement.

A summary of the reduction of research and development expenses related to grant funding and collaboration agreements for the years ended December 31, 2019 and 2018 is as follows:

	Year Ended December 31,
	2019		2018
	(in thousands)
CIRM	$	2,756	$	5,722
LLS		347		2,258
Merck		1,693		1,199
bluebird		1,202	—
Ono		866	—
Total grant funding and cost share reimbursement	$	6,864	$	9,179

Convertible Preferred Stock

12 Months Ended

Dec. 31, 2019

Equity [Abstract]

Convertible Preferred Stock

8.	Convertible Preferred Stock

Through December 31, 2017, the Company’s convertible preferred stock was classified in permanent equity as redemption of the convertible preferred stock was in the control of the Company. In February 2018, the Company appointed an additional director which increased the influence of the convertible preferred stockholders on the board of directors. This change to the Company’s board composition resulted in the convertible preferred stock being reclassified outside of stockholders’ deficit because, in the event of certain “liquidation events” that were not solely within the Company’s control (including merger, acquisition, or sale of all or substantially all of the Company’s assets), the shares could become redeemable at the option of the holders. Upon the closing of the initial public offering on July 2, 2018, all outstanding shares of convertible preferred stock were automatically converted into 16,215,896 shares of common stock. Accordingly, there were no shares of convertible preferred stock outstanding as of December 31, 2019 and 2018.

Stock-Based Compensation

12 Months Ended

Dec. 31, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation

9.	Stock-Based Compensation

2015 and 2018 Equity Incentive Plans

In November 2015, the Company adopted the 2015 Equity Incentive Plan (“2015 Plan”). The 2015 Plan provides for the Company to sell or issue common stock or restricted common stock, or to grant incentive stock options or nonqualified stock options for the purchase of common stock, to employees, members of the board of directors and consultants of the Company under terms and provisions established by the board of directors. As of December 31, 2019, there were 2,736,742 shares of the Company’s common stock reserved for future issuance under the 2015 Plan upon the exercise of outstanding stock options.

In June 2018, the Company adopted the 2018 Equity Incentive Plan (“2018 Plan”), which became effective upon the execution of the underwriting agreement related to the IPO. As a result, the Company will not grant any additional awards under the 2015 Plan. The terms of the 2015 Plan and applicable award agreements will continue to govern any outstanding awards thereunder. The Company initially reserved 3,000,000 shares of common stock for issuance under the 2018 Plan. In addition, the number of shares of common stock reserved for issuance under the 2018 Plan will automatically increase on the first day of January for a period of up to ten years, commencing on January 1, 2019, in an amount equal to 5% of the total number of shares of the Company’s capital stock outstanding on the last day of the preceding year, or a lesser number of shares determined by the Company’s board of directors. As of December 31, 2019, there were 2,997,556 shares available for future issuance under the 2018 Plan.

A summary of option activity under the 2015 Plan and 2018 Plan is as follows:

	Shares Issuable Under Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contract Term		Aggregate Intrinsic Value
						(in years)		(in thousands)
Balance, December 31, 2018		3,404,847		$	6.55		8.82	$	31,388
Options granted		1,469,768			30.91
Options exercised		(439,554	)		4.84
Options forfeited		(149,636	)		8.22
Balance outstanding December 31, 2019		4,285,425		$	15.02		8.54	$	104,366
Exercisable, December 31, 2019		2,138,837		$	6.56		7.83	$	70,176

The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying stock options and the fair value of the Company's common stock for stock options that were in-the-money.

The intrinsic value of options exercised for the years ended December 31, 2019 and 2018 was $6.6 million and $0.6 million, respectively.

During the years ended December 31, 2019 and 2018, the estimated total grant-date fair value of the options vested was $3.8 million and $2.3 million, the estimated weighted-average grant-date fair value of employee options granted was $19.03 and $6.14 per share, respectively. As of December 31, 2019, there was $34.5 million of unrecognized stock-based compensation related to unvested stock options that is expected to be recognized over a weighted-average period of 2.9 years.

Employees Share Purchase Plan (ESPP)

In June 2018, the Company adopted the 2018 Employee Stock Purchase Plan (“ESPP”), which became effective upon the execution of the underwriting agreement related to the IPO. The Company has initially reserved 450,000 shares of common stock for purchase under the ESPP. The initial offering period began June 27, 2018 and will end on August 15, 2020 with purchase dates of February 15, 2019, August 15, 2019, February 15, 2020, and August 15, 2020. Each subsequent offering will be approximately 24 months long and will consist of four purchase periods with purchase dates occurring on February 15 and August 15 of each year. On each purchase date, ESPP participants will purchase shares of common stock at a price per share equal to 85% of the lesser of (1) the fair market value per share of the common stock on the offering date or (2) the fair market value of the common stock on the purchase date.

Stock-Based Compensation

The Company uses the grant date fair value of its common stock to value options when granted. The Company measures its stock-based awards granted to employees and directors based on the estimated fair values of the awards and recognizes the compensation over the requisite service period. The Company recognizes forfeitures as they occur. In determining the fair value of the stock-based awards, the Company uses the Black-Scholes option-pricing model and assumptions discussed below. Each of these inputs is subjective and its determination generally requires significant judgment.

Expected term—The expected term represents the period that stock-based awards are expected to be outstanding. The expected term for option grants is determined using the simplified method, as we do not have sufficient historical data to use any other method to estimate expected term. The simplified method deems the term to be the average of the time-to-vesting and the contractual life of the stock-based awards.

Expected volatility—Since the Company has limited trading history for its common stock due to its short trading history, the expected volatility was estimated based on the average volatility for comparable publicly traded biotechnology companies over a period equal to the expected term of the stock option grants. The comparable companies were chosen based on their similar size, stage in the life cycle or area of specialty.

Risk-free interest rate—The risk-free interest rate is based on the U.S. Treasury zero coupon issues in effect at the time of grant for periods corresponding with the expected term of the option.

Dividend yield—The Company has never paid dividends on its common stock and has no plans to pay dividends on its common stock. Therefore, the Company used an expected dividend yield of zero.

The fair value of employee and director stock option awards was estimated at the date of grant using a Black-Scholes option-pricing model with the following assumptions:

	Year Ended December 31,
	2019	2018
Expected term (years)	5.50 - 6.08	5.48 - 6.44
Expected volatility	63.6% - 68.2%	65.8% - 70.8%
Weighted average risk-free interest rate	1.43% - 2.58%	2.51% - 3.10%
Dividend yield	0%	0%

Total stock-based compensation for stock awards was as follows:

	Year Ended December 31,
	2019		2018
	(in thousands)
Research and development	$	1,874	$	1,193
General and administrative		3,087		2,238
Total	$	4,961	$	3,431

The fair value of the ESPP was estimated using the Black-Scholes option pricing model using the following assumptions:

	Year Ended December 31,
	2019	2018
Expected term (years)	0.50 - 2.00	0.58 - 2.08
Expected volatility	57.5% - 69.1%	65.7% - 67.6%
Weighted average risk-free interest rate	1.48% - 2.55%	2.14% - 2.53%
Dividend yield	0%	0%

Stock-based compensation expense for the ESPP was $0.8 million and $0.4 million for the year ended December 31, 2019 and 2018, respectively.

The ESPP commenced in June 2018. A total of 94,568 shares of common stock were purchased pursuant to the ESPP during the year ended December 31, 2019 with a weighted-average purchase price per share of $9.08 for total proceeds of $0.9 million. No shares of common stock were purchased pursuant to the ESPP in 2018. The liability related to cash received from payroll deductions pursuant to the ESPP as of December 31, 2019 and 2018 was $0.4 million and $0.5 million, respectively.

Restricted Stock

The Company typically allows its employees and directors to exercise options granted under the 2015 Plan prior to vesting. The Company also has issued restricted stock purchase awards to employees and directors under the 2015 Plan. The shares related to early exercised stock options and restricted stock purchase awards are subject to the Company’s lapsing repurchase right upon termination of employment at the original purchase price. In order to vest, the holders are required to provide continued service to the Company. The proceeds are initially recorded in other long-term liabilities and are reclassified to common stock and paid-in capital as the repurchase right lapses. As of December 31, 2019, all early exercised stock options and restricted stock purchase awards were fully vested. As of December 31, 2018, there was $0.1 million recorded in other long-term liabilities related to shares held by employees and directors that were subject to repurchase.

A summary of restricted stock activity follows is as follows:

	Number of Restricted Shares Outstanding
Unvested shares—December 31, 2018		49,052
Restricted shares vested		(49,052	)
Unvested shares—December 31, 2019	—

Net Loss Per Share

12 Months Ended

Dec. 31, 2019

Earnings Per Share [Abstract]

Net Loss Per Share

10.Net Loss Per Share

The following outstanding potentially dilutive shares have been excluded from the calculation of diluted net loss per share for the periods presented due to their anti-dilutive effect:

	December 31,
	2019		2018
Stock options to purchase common stock		4,285,425		3,404,847
Restricted stock subject to future vesting	—			49,052
Shares committed under ESPP		71,181		35,383
Total		4,356,606		3,489,282

Income Taxes

12 Months Ended

Dec. 31, 2019

Income Tax Disclosure [Abstract]

Income Taxes

11.	Income Taxes

The Company did not record a provision for income taxes for the years ended December 31, 2019 and 2018. The Company has incurred net operating losses for all the periods presented. The Company has not reflected any benefit of such net operating loss carryforwards in the financial statements. The Company has established a full valuation allowance against its deferred tax assets due to the uncertainty surrounding the realization of such assets.

A reconciliation of total provision for income taxes and the amount computed by applying the federal statutory income tax rate of 21% to loss before provision from income taxes is as follows:

	Year Ended December 31,
	2019			2018
	(in thousands)
Computed expected tax benefit	$	(18,406	)	$	(14,778	)
State taxes (net of federal tax benefits)		(827	)		(1,621	)
Increase in valuation allowance		24,637			17,992
Other		(1,155	)		481
R&D tax credits		(4,249	)		(2,074	)
Total provision for income taxes	$	—		$	—

The components of the deferred tax assets and liabilities are as follows:

	December 31,
	2019			2018
	(in thousands)
Deferred tax assets:
Net operating loss carryforwards	$	39,561		$	19,748
Capitalized R&D		8,908			10,673
Stock-based compensation		1,025			583
Intangibles		2,800			2,891
Tax credits		7,880			2,711
Accruals and other		2,568			1,067
Total deferred tax assets		62,742			37,673

Deferred tax liabilities:
Right of use asset		(476	)	—
Total deferred tax liabilities		(476	)		—

Less: valuation allowance		62,266			37,673
Net deferred tax assets and liabilities	$	—		$	—

Realization of deferred tax assets is dependent upon future earnings, if any, the timing and amount of which are uncertain. Due to the lack of earnings history, the net deferred tax assets have been fully offset by a valuation allowance. The valuation allowance increased by approximately $24.6 million and $18.0 million during the years ended December 31, 2019 and 2018.

The Company has net operating carryforwards for federal and California income tax purposes of approximately $163.2 million and $63.9 million, respectively, as of December 31, 2019. As of December 31, 2018, we had federal and California net operating loss carryforwards of approximately $76.4 million and $53.4 million, respectively. The federal net operating loss carryforwards, if not utilized, will expire beginning in 2035. The state net operating loss carryforwards, if not utilized, will expire beginning in 2035. Under the U.S. Tax Cuts & Jobs Act, passed into law in December 2017, effective January 1, 2018, net operating losses generated after December 31, 2017 will be carried forward indefinitely with the yearly net operating loss utilization limited to 80 percent of taxable income.

The Company has federal and California research credit carryforwards of approximately $5.1 million and $2.8 million, respectively, as of December 31, 2019. As of December 31, 2018, we had federal and California research credit carryforwards of approximately $2.1 million and $1.5 million, respectively. The Company also has Federal Orphan Drug credits of approximately $2.6 and $0.2 million as of December 31, 2019 and 2018, respectively. The federal credits will begin to expire in 2035 while the California research credits have no expiration date.

Federal and California tax laws impose significant restrictions on the utilization of net operating loss carryforwards in the event of a change in ownership of the Company, as defined by Internal Revenue Code Section 382 (“Section 382”). A formal Section 382 study has not been completed to determine if an ownership change has occurred and if its net operating losses are subject to an annual limitation. Such annual limitations could affect the utilization of NOL and tax credit carryforwards in the future.

No liability related to uncertain tax positions is recorded on the financial statements related to uncertain tax positions. It is the Company’s policy to include penalties and interest expense related to income taxes as a component of interest and other income, net, as necessary.

A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows:

	Year Ended December 31,
	2019		2018
	(in thousands)
Balance at beginning of year	$	836	$	283
Increases related to prior year tax positions		210	—
Increases related to current year tax positions		1,117		553
Balance at end of year	$	2,163	$	836

Although it is reasonably possible that certain unrecognized tax benefits may increase or decrease within the next twelve months due to tax examination changes, settlement activities, expirations of statute of limitations, or the impact on recognition and measurement considerations related to the results of published tax cases or other similar activities, the Company does not anticipate any significant changes to unrecognized tax benefits over the next 12 months.

Income tax returns are filed in the U.S. and California. The years 2015 through 2019 remain open to examination by the domestic taxing jurisdictions to which the Company is subject. Net operating losses generated on a tax return basis by the Company for 2015 through 2019 remain open to examination by the domestic taxing jurisdictions.

Related-Party Relationships

12 Months Ended

Dec. 31, 2019

Related-Party Relationships

12.	Related-Party Relationships

In November 2015, the Company entered into a technology license agreement with The Board of Trustees of the Leland Stanford Junior University (“Stanford”) under which Stanford granted to the Company exclusive licenses under certain patents and other intellectual property rights relating to the Company’s current product candidates and non-exclusive licenses under certain other patents and intellectual property rights to develop, manufacture and commercialize products for use in certain licensed fields, the scope of which would include the application of the licensed intellectual property in oncology. With respect to these licenses, the Company could be required to pay Stanford up to $5.6 million in milestone payments based on the achievement of certain development and regulatory approval milestones. The first such milestone payment of $75,000 was paid to Stanford in February 2018 and included in research and development expense for the year ended December 31, 2018. In addition, the Company is required to pay Stanford a minimum annual fee and a royalty of single digit percentage on net sales of licensed products, reimburse patent-related expenses, share any non-royalty sublicensing income received related to the licensed technology, and pay a change of control fee. As a result of the $15.7 million license payment received under the Ono License and Collaboration Agreement (see Note 7), the Company recorded an estimated sublicensing fee liability of $0.3 million due to Stanford under the agreement as of December 31, 2019. However, the determination of the sublicensing fee is subject to judgment and the actual sublicensing fee may be higher when settled.

Dr. Weissman and Dr. Majeti, co-founders and members of the Company’s board of directors, are professors at Stanford. While employed by Stanford, Dr. Weissman and Dr. Majeti were co-inventors of some of the patents that the Company licenses under the Stanford License Agreement. Under Stanford's policies, as co-inventors Dr. Weissman and Dr. Majeti are entitled to receive a share of any royalties that the Company pays to Stanford under the agreement with respect to such patents. No royalty payments have been made to date. In addition, under Stanford’s policies, as co-inventors Dr. Weissman and Dr. Majeti are entitled to receive a share of the annual license fees that the Company pays to Stanford with respect to such patents.

Subsequent Event

12 Months Ended

Dec. 31, 2019

Subsequent Events [Abstract]

Subsequent Event

13.	Subsequent Event

On March 1, 2020, the Company entered into an Agreement and Plan of Merger (the “Merger Agreement”) with Gilead Sciences, Inc. (“Gilead”) and Toro Merger Sub, Inc. (“Toro”), a wholly-owned subsidiary of Gilead. Pursuant to the Merger Agreement, Gilead has agreed to cause Toro to commence a tender offer (“Offer”) to purchase all of the outstanding shares of common stock of the Company at a price of $95.50 per share.

Following the consummation of the Offer and subject to the terms and conditions of the Merger Agreement, Toro will be merged with and into the Company, with the Company continuing as the surviving corporation in the merger.

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2019

Accounting Policies [Abstract]

Basis of Presentation

The accompanying financial statements have been prepared in conformity with U.S. generally accepted accounting principles (“U.S. GAAP”).

Use of Estimates

Cash and Cash Equivalents

Investments

Concentration of Credit Risk and Other Risks and Uncertainties

Fair Value Measurement

Level 1—Observable inputs such as unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date;

Level 3— Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities.

Property and Equipment, Net

Impairment of Long-Lived Assets

Contingent Repayment Obligation

Contingent repayment obligation

Revenue Recognition

License and Collaboration Arrangements

Research and Development Expenditures

Stock-Based Compensation

Defined Contribution Plan

Income Taxes

Foreign Currency Transactions

Comprehensive Loss

The Company’s comprehensive loss is comprised of changes in unrealized losses on available-for-sale securities.

Net Loss per Share

Segment Reporting

Recently Issued and Adopted Accounting Pronouncements

Leases

The impact of the adoption of ASC 842 on the accompanying balance sheet as of January 1, 2019 was as follows:

	December 31, 2018		Adjustments due to the adoption of ASC 842			January 1, 2019
	(In thousands)
Assets
Operating lease right-of-use asset	$	—	$	2,328		$	2,328
Liabilities and stockholders' equity
Deferred rent classified as accrued liabilities	$	155	$	(155	)	$	—
Lease liability, current	$	—	$	968		$	968
Lease liability, noncurrent	$	—	$	1,847		$	1,847
Deferred rent, noncurrent	$	331	$	(331	)	$	—

Recent Accounting Pronouncements Not Yet Adopted

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2019

Accounting Policies [Abstract]

Schedule of Impact of Adoption of ASC 842 on Accompanying Balance Sheet

The impact of the adoption of ASC 842 on the accompanying balance sheet as of January 1, 2019 was as follows:

	December 31, 2018		Adjustments due to the adoption of ASC 842			January 1, 2019
	(In thousands)
Assets
Operating lease right-of-use asset	$	—	$	2,328		$	2,328
Liabilities and stockholders' equity
Deferred rent classified as accrued liabilities	$	155	$	(155	)	$	—
Lease liability, current	$	—	$	968		$	968
Lease liability, noncurrent	$	—	$	1,847		$	1,847
Deferred rent, noncurrent	$	331	$	(331	)	$	—

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2019

Fair Value Disclosures [Abstract]

Fair Value, Assets Measured on Recurring and Nonrecurring Basis

The fair value and amortized cost of cash equivalents and available-for-sale securities by major security type as of December 31, 2019 and 2018 are presented in the following tables:

	As of December 31, 2019
	Fair Value Hierarchy	Amortized Cost		Unrealized Gains		Unrealized Losses		Market Value
	(in thousands)
Money market funds	Level 1	$	203,902	$	—	$	—	$	203,902
US government debt securities	Level 1		24,039		24		—		24,063
Commercial paper	Level 2		17,962		—		—		17,962
Corporate debt securities	Level 2		52,111		48		—		52,159
Asset-backed securities	Level 2		25,070		20		—		25,090
Total cash equivalents and available-for-sale securities		$	323,084	$	92	$	—	$	323,176

	As of December 31, 2018
	Fair Value Hierarchy	Amortized Cost		Unrealized Gains		Unrealized Losses			Market Value
	(in thousands)
Money market funds	Level 1	$	7,959	$	—	$	—		$	7,959
US government debt securities	Level 1		15,963		—		(1	)		15,962
Commercial paper	Level 2		43,277		—		—			43,277
Corporate debt securities	Level 2		46,186		—		(54	)		46,132
Asset-backed securities	Level 2		22,842		—		(27	)		22,815
Total cash equivalents and available-for-sale securities		$	136,227	$	—	$	(82	)	$	136,145

Schedule of Change in Fair Value of Contingent Repayment Obligation

	December 31,
	2019		2018
	(in thousands)
Beginning balance	$	331	$	—
Initial value of contingent repayment obligation	—			331
Change in fair value of contingent repayment obligation		2,554	—
Ending balance	$	2,885	$	331

Balance Sheet Components (Tables)

12 Months Ended

Dec. 31, 2019

Balance Sheet Related Disclosures [Abstract]

Schedule of Property and Equipment, Net

Property and equipment, net consists of the following:

	December 31,
	2019			2018
	(in thousands)
Furniture and fixtures	$	236		$	108
Laboratory equipment		1,257			1,297
Computer equipment and software		164			91
Leasehold improvements		908			770
		2,565			2,266
Less: Accumulated depreciation and amortization		(1,342	)		(906	)
Total property and equipment, net	$	1,223		$	1,360

Schedule of Accrued Liabilities

Accrued liabilities consist of the following:

	December 31,
	2019		2018
	(in thousands)
Accrued research and development expenses	$	4,143	$	5,870
Accrued bonus		2,788		1,602
Deferred rent, current		—		155
Other		2,144		1,417
Total accrued liabilities	$	9,075	$	9,044

Leases (Tables)

12 Months Ended

Dec. 31, 2019

Leases [Abstract]

Schedule of Undiscounted Future Non-Cancellable Lease Payments under Company's Operating Leases

The undiscounted future non-cancellable lease payments under the Company's operating leases are as follows:

	December 31, 2019
	(In thousands)
Years
2020	$	1,585
2021		864
Total undiscounted lease payments		2,449
Present value adjustment for minimum lease commitments		(134	)
Lease liabilities	$	2,315

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2019

Schedule of Undiscounted Future Non-Cancellable Lease Payments under Company's Operating Leases

The undiscounted future non-cancellable lease payments under the Company's operating leases are as follows:

	December 31, 2019
	(In thousands)
Years
2020	$	1,585
2021		864
Total undiscounted lease payments		2,449
Present value adjustment for minimum lease commitments		(134	)
Lease liabilities	$	2,315

Lonza Development and Manufacturing Agreement

Schedule of Undiscounted Future Non-Cancellable Lease Payments under Company's Operating Leases

At December 31, 2019, future minimum payments under the Lonza development and manufacturing agreements are as follows:

	December 31, 2019
	(In thousands)
Years
2020	$	10,250
2021		5,150
Total future minimum payments		15,400

Research and License Agreements (Tables)

12 Months Ended

Dec. 31, 2019

Research And License Agreements [Abstract]

Summary of Reduction of Research and Development Expenses Related to Grant Funding and Collaboration Agreements

A summary of the reduction of research and development expenses related to grant funding and collaboration agreements for the years ended December 31, 2019 and 2018 is as follows:

	Year Ended December 31,
	2019		2018
	(in thousands)
CIRM	$	2,756	$	5,722
LLS		347		2,258
Merck		1,693		1,199
bluebird		1,202	—
Ono		866	—
Total grant funding and cost share reimbursement	$	6,864	$	9,179

Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Option Activity

A summary of option activity under the 2015 Plan and 2018 Plan is as follows:

	Shares Issuable Under Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contract Term		Aggregate Intrinsic Value
						(in years)		(in thousands)
Balance, December 31, 2018		3,404,847		$	6.55		8.82	$	31,388
Options granted		1,469,768			30.91
Options exercised		(439,554	)		4.84
Options forfeited		(149,636	)		8.22
Balance outstanding December 31, 2019		4,285,425		$	15.02		8.54	$	104,366
Exercisable, December 31, 2019		2,138,837		$	6.56		7.83	$	70,176

Schedule of Estimated Fair Value of Employee and Director Stock Options Awards Using Black-Scholes Option-Pricing Model

The fair value of employee and director stock option awards was estimated at the date of grant using a Black-Scholes option-pricing model with the following assumptions:

	Year Ended December 31,
	2019	2018
Expected term (years)	5.50 - 6.08	5.48 - 6.44
Expected volatility	63.6% - 68.2%	65.8% - 70.8%
Weighted average risk-free interest rate	1.43% - 2.58%	2.51% - 3.10%
Dividend yield	0%	0%

Schedule of Total Stock-Based Compensation

Total stock-based compensation for stock awards was as follows:

	Year Ended December 31,
	2019		2018
	(in thousands)
Research and development	$	1,874	$	1,193
General and administrative		3,087		2,238
Total	$	4,961	$	3,431

Schedule of Estimated Fair Value of Employee Stock Purchase Plan Using Black-Scholes Option Pricing Model

The fair value of the ESPP was estimated using the Black-Scholes option pricing model using the following assumptions:

	Year Ended December 31,
	2019	2018
Expected term (years)	0.50 - 2.00	0.58 - 2.08
Expected volatility	57.5% - 69.1%	65.7% - 67.6%
Weighted average risk-free interest rate	1.48% - 2.55%	2.14% - 2.53%
Dividend yield	0%	0%

Summary of Restricted Stock Activity

A summary of restricted stock activity follows is as follows:

	Number of Restricted Shares Outstanding
Unvested shares—December 31, 2018		49,052
Restricted shares vested		(49,052	)
Unvested shares—December 31, 2019	—

Net Loss Per Share (Tables)

12 Months Ended

Dec. 31, 2019

Earnings Per Share [Abstract]

Summary of Outstanding Potentially Dilutive Shares Excluded from Calculation of Diluted Net Loss Per Share

The following outstanding potentially dilutive shares have been excluded from the calculation of diluted net loss per share for the periods presented due to their anti-dilutive effect:

	December 31,
	2019		2018
Stock options to purchase common stock		4,285,425		3,404,847
Restricted stock subject to future vesting	—			49,052
Shares committed under ESPP		71,181		35,383
Total		4,356,606		3,489,282

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2019

Income Tax Disclosure [Abstract]

Schedule of Reconciliation of Provision for Income Taxes Amount Computed by Federal Statutory Income tax

A reconciliation of total provision for income taxes and the amount computed by applying the federal statutory income tax rate of 21% to loss before provision from income taxes is as follows:

	Year Ended December 31,
	2019			2018
	(in thousands)
Computed expected tax benefit	$	(18,406	)	$	(14,778	)
State taxes (net of federal tax benefits)		(827	)		(1,621	)
Increase in valuation allowance		24,637			17,992
Other		(1,155	)		481
R&D tax credits		(4,249	)		(2,074	)
Total provision for income taxes	$	—		$	—

Schedule of Components of Deferred Tax Assets and Liabilities

The components of the deferred tax assets and liabilities are as follows:

	December 31,
	2019			2018
	(in thousands)
Deferred tax assets:
Net operating loss carryforwards	$	39,561		$	19,748
Capitalized R&D		8,908			10,673
Stock-based compensation		1,025			583
Intangibles		2,800			2,891
Tax credits		7,880			2,711
Accruals and other		2,568			1,067
Total deferred tax assets		62,742			37,673

Deferred tax liabilities:
Right of use asset		(476	)	—
Total deferred tax liabilities		(476	)		—

Less: valuation allowance		62,266			37,673
Net deferred tax assets and liabilities	$	—		$	—

Schedule of Reconciliation of Unrecognized Tax Benefits

A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows:

	Year Ended December 31,
	2019		2018
	(in thousands)
Balance at beginning of year	$	836	$	283
Increases related to prior year tax positions		210	—
Increases related to current year tax positions		1,117		553
Balance at end of year	$	2,163	$	836

Description of Business - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands			1 Months Ended		12 Months Ended
	Jul. 27, 2018	Jul. 02, 2018	Dec. 31, 2019	Jul. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Description Of Business [Line Items]
Proceeds from issuance of common stock					$ 859	$ 0
Net proceeds from offering					264,324	116,336
Cash, cash equivalents and short-term investments			$ 329,100		329,100
Cumulative net losses			$ 227,390		$ 227,390	$ 139,769
IPO
Description Of Business [Line Items]
Common stock shares, issued		7,035,000
Convertible preferred stock automatically converted into common stock		16,215,896
Over-Allotment Option
Description Of Business [Line Items]
Common stock shares, issued	1,055,250
IPO and Over-Allotment
Description Of Business [Line Items]
Common stock shares, issued	8,090,250
Common stock issued price per share	$ 16.00
Proceeds from issuance of common stock	$ 116,300
Underwriting discounts and commissions, offering expenses	$ 13,200
Underwritten Public Offering
Description Of Business [Line Items]
Common stock shares, issued			5,589,000	10,781,250
Common stock issued price per share			$ 35.00	$ 8.00	$ 35.00
Shares issued pursuant to exercise of underwriters' option to purchase			729,000	1,406,250
Net proceeds from offering			$ 183,600	$ 80,500

Summary of Significant Accounting Policies - Additional Information (Details)	12 Months Ended
	Dec. 31, 2019 USD ($) Institution Segment	Dec. 31, 2018 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Realized gains or loss on investments	$ 0	$ 0
Number of financial institution which held Company’s cash, cash equivalents and short-term investments \| Institution	1
Impairments of long-lived assets	$ 0
Defined contribution retirement savings plan, plan name	401(k)	401(k)
Percentage of maximum matching contributions on employee’s eligible compensation deferred	3.00%	3.00%
Company recognized expense related to its contributions plan	$ 400,000	$ 300,000
Description of uncertain income tax position	An uncertain income tax position will not be recognized if it has less than a 50% likelihood of being sustained.
Net foreign currency transaction gain (loss)	$ 200,000	$ (200,000)
Number of operating segments \| Segment	1
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of property and equipment	3 years
Maximum
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of property and equipment	5 years

Summary of Significant Accounting Policies - Schedule of Impact of Adoption of ASC 842 on Accompanying Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
Assets
Operating lease right-of-use asset	$ 1,978
Liabilities and stockholders' equity
Deferred rent classified as accrued liabilities			$ 155
Lease liability, current	1,471
Lease liability, noncurrent	$ 844
Deferred rent, noncurrent			$ 331
Accounting Standards Update 2016-02
Assets
Operating lease right-of-use asset		$ 2,328
Liabilities and stockholders' equity
Lease liability, current		968
Lease liability, noncurrent		1,847
Accounting Standards Update 2016-02 \| Adjustments due to the adoption of ASC 842
Assets
Operating lease right-of-use asset		2,328
Liabilities and stockholders' equity
Deferred rent classified as accrued liabilities		(155)
Lease liability, current		968
Lease liability, noncurrent		1,847
Deferred rent, noncurrent		$ (331)

Fair Value Measurements - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Transfers between level 1 to Level 2 and Level 3 amount	$ 0
Realized gains (losses) on investments	0	$ 0
Interest earned from the investments in securities	3,200,000	2,200,000
Leukemia & Lymphoma Society, Inc.
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone contingent repayment obligation	2,900,000	300,000
Leukemia & Lymphoma Society, Inc. \| Other Current Liabilities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone contingent repayment obligation	900,000
Leukemia & Lymphoma Society, Inc. \| Other Long-term Liabilities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone contingent repayment obligation	$ 2,000,000	$ 300,000
Maximum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Maturity of investments	1 year

Fair Value Measurements - Fair Value and Amortized Cost of Cash Equivalents and Available for Sale Securities by Major Security Type (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 323,084	$ 136,227
Unrealized Gains	92
Unrealized Losses		(82)
Market Value	323,176	136,145
Level 1
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	203,902	7,959
Market Value	203,902	7,959
Commercial Paper \| Level 2
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	17,962	43,277
Market Value	17,962	43,277
Corporate Debt Securities \| Level 2
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	52,111	46,186
Unrealized Gains	48
Unrealized Losses		(54)
Market Value	52,159	46,132
Asset-backed Securities \| Level 2
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	25,070	22,842
Unrealized Gains	20
Unrealized Losses		(27)
Market Value	25,090	22,815
US Government Debt Securities \| Level 1
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	24,039	15,963
Unrealized Gains	24
Unrealized Losses		(1)
Market Value	$ 24,063	$ 15,962

Fair Value Measurements - Schedule of Change in Fair Value of Contingent Repayment Obligation (Details) - Embedded Derivative - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value of embedded derivative, Beginning balance	$ 331
Initial value of contingent repayment obligation		$ 331
Change in fair value of contingent repayment obligation	2,554
Fair value of embedded derivative, Ending balance	$ 2,885	$ 331

Balance Sheet Components - Schedule of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 2,565	$ 2,266
Less: Accumulated depreciation and amortization	(1,342)	(906)
Total property and equipment, net	1,223	1,360
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment, gross	236	108
Laboratory Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	1,257	1,297
Computer Equipment and Software
Property Plant And Equipment [Line Items]
Property and equipment, gross	164	91
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 908	$ 770

Balnce Sheet Components - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Property Plant And Equipment [Abstract]
Depreciation and amortization	$ 523	$ 401

Balance Sheet Components - Schedule of Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Accrued Liabilities Current [Abstract]
Accrued research and development expenses	$ 4,143	$ 5,870
Accrued bonus	2,788	1,602
Deferred rent, current		155
Other	2,144	1,417
Total accrued liabilities	$ 9,075	$ 9,044

Leases - Additional Information (Details)	1 Months Ended	12 Months Ended
Leases - Additional Information (Details)	Apr. 30, 2019 USD ($) ft²	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Leases [Abstract]
Lease expiration date		Aug. 31, 2021
Renewal term		5 years
Tenant improvement allowance		$ 600,000
Sublease agreement date	2019-04
Additional area rentable \| ft²	6,230
Sublease expiration date	2021-02
Sublease amendment agreement date	2019-04
Additional space sublease commencement date	Mar. 01, 2021
Base rent per month	$ 36,000
Weighted average remaining lease term		1 year 7 months 6 days
Operating lease liabilities, weighted average incremental borrowing rate		7.00%
Operating lease, liabilities, current		$ 1,471,000
Operating lease, liabilities, noncurrent		844,000
Operating lease cost		1,300,000
Variable lease payments for operating expenses		600,000	$ 500,000
Rent expense for operating leases			1,100,000
Accrued deferred rent expense			100,000
Security deposit		$ 200,000	$ 200,000

Leases - Schedule of Undiscounted Future Non-Cancellable Lease Payments under Company's Operating Leases (Details) $ in Thousands	Dec. 31, 2019 USD ($)
Operating Leases
2020	$ 1,585
2021	864
Total undiscounted lease payments	2,449
Present value adjustment for minimum lease commitments	(134)
Lease liabilities	$ 2,315

Commitments and Contingencies - Schedule of Future Minimum Payments (Details) $ in Thousands	Dec. 31, 2019 USD ($)
Operating Leased Assets [Line Items]
2020	$ 1,585
2021	864
Total future minimum payments	2,449
Lonza Development and Manufacturing Agreement
Operating Leased Assets [Line Items]
2020	10,250
2021	5,150
Total future minimum payments	$ 15,400

Research and License Agreements - Additional Information (Details) € in Millions, ¥ in Billions	1 Months Ended					12 Months Ended
	Jul. 31, 2019 USD ($)	Jul. 31, 2018 USD ($)	Jun. 30, 2018 USD ($)	Nov. 30, 2017 USD ($)	Jan. 31, 2017 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2019 EUR (€)	Dec. 31, 2019 JPY (¥)	Dec. 31, 2018 USD ($)	Jul. 10, 2019 JPY (¥)	Jul. 31, 2017 USD ($)	Mar. 31, 2017 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Reduction to research and development expenses for research grants						$ 6,864,000			$ 9,179,000
Research and development						83,792,000			56,673,000
California Institute of Regenerative Medicine (CIRM)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Reduction to research and development expenses for research grants						2,756,000			5,722,000
Leukemia & Lymphoma Society, Inc. ("LLS")
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaborative arrangement research grant receivable						0
Milestone contingent repayment obligation						2,900,000			300,000
Reduction to research and development expenses for research grants						347,000			2,258,000
Leukemia & Lymphoma Society, Inc. ("LLS") \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional grant funding based on milestone payments	$ 3,000,000
Potential future milestone payments related to certain development and regulatory approval milestones	6,000,000
Ono Pharmaceutical Co., Ltd.
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Reduction to research and development expenses for research grants						866,000
Bluebird Bio, Inc.
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Reduction to research and development expenses for research grants						1,202,000
Research Grant \| California Institute of Regenerative Medicine (CIRM)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaborative arrangement research grant receivable					$ 10,200,000
Collaborative arrangement research grant received						2,000,000
Collaborative arrangement milestone payments receivable period					4 years
Proceeds from collaborators						9,200,000			7,200,000
Research Grant \| Leukemia & Lymphoma Society, Inc. ("LLS")
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaborative arrangement research grant receivable												$ 4,200,000
Collaborative arrangement research grant received						300,000
Proceeds from collaborators						4,200,000			3,900,000
Research Grant \| Leukemia & Lymphoma Society, Inc. ("LLS") \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payments to be paid upon achievement of development and regulatory approval milestones						13,700,000
Research Grant \| California Institute of Regenerative Medicine (CIRM) and Leukemia & Lymphoma Society, Inc. ("LLS")
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Reduction to research and development expenses for research grants						3,100,000			8,000,000
Second Research Grant \| California Institute of Regenerative Medicine (CIRM)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaborative arrangement research grant receivable				$ 5,000,000
Collaborative arrangement research grant received						1,200,000
Collaborative arrangement milestone payments receivable period				5 years
Proceeds from collaborators						3,600,000			2,400,000
Collaborative arrangement, reinstated research grant receivable consistent with original award amount											$ 5,000,000
Collaborative arrangement, amended research grant receivable									$ 3,200,000
Collaborative arrangement, amended milestone payments receivable period									5 years
Second Research Grant \| California Institute of Regenerative Medicine (CIRM) \| Prepaid Expenses and Other Current Assets
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaborative arrangement research grant receivable						700,000
Amended March 2017 Agreement \| Leukemia & Lymphoma Society, Inc. ("LLS")
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional milestone payments received upon achievement of certain clinical or regulatory milestones						4,200,000
Merck Collaboration Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Reduction to research and development expenses for research grants						1,693,000			$ 1,199,000
Merck Collaboration Agreement \| Ares Trading S.A
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Reduction to research and development expenses for research grants						1,700,000			1,200,000
Receivable for reimbursement of research and development costs incurred						300,000
BliNK Purchase Agreement \| Blink
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payments						0
Initial upfront fee payment		$ 500,000	$ 2,500,000
Additional annual payment to receive marketing approval for product						300,000
Research and development						300,000			$ 3,000,000
BliNK Purchase Agreement \| Blink \| Intellectual Property Rights and Know-How
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Initial upfront fee payment			$ 500,000
BliNK Purchase Agreement \| Blink \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payments to be paid upon achievement of development and regulatory approval milestones						43,000,000
License and Settlement Agreement \| Synthon Biopharmaceuticals B.V.
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Payments comprising upfront payment, achievement of future regulatory and commercial milestones \| €							€ 40.0
License and Collaboration Agreement \| Ono Pharmaceutical Co., Ltd.
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Reduction to research and development expenses for research grants						900,000
One time upfront nonrefundable payment received						15,700,000		¥ 1.7
Additional upfront payment receivable upon achievement of specified future development and commercial milestones \| ¥										¥ 11.2
Collaboration arrangement, upfront fixed nonrefundable consideration	15,700,000
Collaboration arrangement, revenue recognized	$ 15,700,000
Research And Collaboration Agreements \| Bluebird Bio, Inc.
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Reduction to research and development expenses for research grants						1,200,000
Receivable for reimbursement of research and development costs incurred						$ 700,000

Research and License Agreements - Summary of Reduction of Research and Development Expenses Related to Grant Funding and Collaboration Agreements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total grant funding and cost share reimbursement	$ 6,864	$ 9,179
Merck
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total grant funding and cost share reimbursement	1,693	1,199
CIRM
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total grant funding and cost share reimbursement	2,756	5,722
Leukemia & Lymphoma Society, Inc.
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total grant funding and cost share reimbursement	347	$ 2,258
Bluebird
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total grant funding and cost share reimbursement	1,202
Ono
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total grant funding and cost share reimbursement	$ 866

Convertible Preferred Stock - Additional Information (Details) - Initial Public Offering - shares	Dec. 31, 2019	Dec. 31, 2018	Jul. 02, 2018
Class Of Stock [Line Items]
Convertible preferred stock automatically converted into common stock			16,215,896
Preferred stock shares outstanding	0	0

Stock-Based Compensation - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 4,961,000	$ 3,431,000
Proceeds from issuance of common stock upon exercise of stock options	$ 2,130,000	291,000
Restricted Stock \| Other Long-term Liabilities
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares held by employees and directors subject to repurchase		100,000
2015 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common stock reserved for future issuance	2,736,742
2018 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common stock reserved for future issuance	3,000,000
Maximum period to increase common stock reserved for issuance	10 years
Percentage of increase in number of shares of common stock reserved for issuance	5.00%
Number of shares available for future issuance	2,997,556
Increase in number of shares of common stock reserved for issuance, description	Number of shares of common stock reserved for issuance under the 2018 Plan will automatically increase on the first day of January for a period of up to ten years, commencing on January 1, 2019, in an amount equal to 5% of the total number of shares of the Company’s capital stock outstanding on the last day of the preceding year, or a lesser number of shares determined by the Company’s board of directors.
2015 and 2018 Stock Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Intrinsic value of options exercised	$ 6,600,000	600,000
Total grant-date fair value of options vested	$ 3,800,000	$ 2,300,000
Weighted-average grant-date fair value of employee options granted	$ 19.03	$ 6.14
Unrecognized share-based compensation related to unvested stock options	$ 34,500,000
Unrecognized stock-based compensation, recognized over weighted-average period	2 years 10 months 24 days
Employee Share Purchase Plan (ESPP)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common stock reserved for future issuance	450,000
Employee share purchase plan description	The initial offering period began June 27, 2018 and will end on August 15, 2020 with purchase dates of February 15, 2019, August 15, 2019, February 15, 2020, and August 15, 2020. Each subsequent offering will be approximately 24 months long and will consist of four purchase periods with purchase dates occurring on February 15 and August 15 of each year. On each purchase date, ESPP participants will purchase shares of common stock at a price per share equal to 85% of the lesser of (1) the fair market value per share of the common stock on the offering date or (2) the fair market value of the common stock on the purchase date.
Employee share purchase plan purchase price percentage	170.00%
Number of common stock purchase to ESPP	94,568	0
Weighted average purchase price per share	$ 9.08
Proceeds from issuance of common stock upon exercise of stock options	$ 900,000
Proceeds from payroll deductions from employee stock purchase plan	$ 400,000	$ 500,000
Employee Stock Purchase Plan Commecement Period	2018-06
Employees Share Purchase Plan (ESPP) \| Black Scholes Option Pricing Model
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield	0.00%	0.00%
Stock-based compensation expense	$ 800,000	$ 400,000

Stock-Based Compensation - Summary of Option Activity (Details) - 2015 and 2018 Stock Plan - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares Issuable Under Options, Beginning balance	3,404,847
Shares Issuable Under Options, Options granted	1,469,768
Shares Issuable Under Options, Options exercised	(439,554)
Shares Issuable Under Options, Options forfeited	(149,636)
Shares Issuable Under Options, Ending balance	4,285,425	3,404,847
Shares Issuable Under Options, Exercisable	2,138,837
Weighted-Average Exercise Price, Beginning balance	$ 6.55
Weighted-Average Exercise Price, Options granted	30.91
Weighted-Average Exercise Price, Options exercised	4.84
Weighted-Average Exercise Price, Options forfeited	8.22
Weighted-Average Exercise Price, Ending balance	15.02	$ 6.55
Weighted-Average Exercise Price, Exercisable	$ 6.56
Weighted-Average Remaining Contract Term	8 years 6 months 14 days	8 years 9 months 25 days
Weighted-Average Remaining Contract Term Exercisable	7 years 9 months 29 days
Aggregate Intrinsic Value	$ 104,366	$ 31,388
Aggregate Intrinsic Value Exercisable	$ 70,176

Stock-Based Compensation - Schedule of Estimated Fair Value of Employee and Director Stock Options Awards Using Black-Scholes Option-Pricing Model (Details) - Employee and Director Stock Option Awards - Black Scholes Option Pricing Model	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility, minimum	63.60%	65.80%
Expected volatility, maximum	68.20%	70.80%
Weighted average risk-free interest rate, minimum	1.43%	2.51%
Weighted average risk-free interest rate, maximum	2.58%	3.10%
Dividend yield	0.00%	0.00%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (years)	5 years 6 months	5 years 5 months 23 days
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (years)	6 years 29 days	6 years 5 months 8 days

Stock-Based Compensation - Schedule of Total Stock-Based Compensation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	$ 4,961	$ 3,431
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	1,874	1,193
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	$ 3,087	$ 2,238

Stock-Based Compensation - Schedule of Estimated Fair Value of Employee Stock Purchase Plan Using Black-Scholes Option Pricing Model (Details) - Employees Share Purchase Plan (ESPP) - Black Scholes Option Pricing Model	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility, minimum	57.50%	66.00%
Expected volatility, maximum	69.10%	68.00%
Weighted average risk-free interest rate, minimum	1.48%	2.14%
Weighted average risk-free interest rate, maximum	2.55%	2.53%
Dividend yield	0.00%	0.00%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (years)	6 months	6 months 29 days
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (years)	2 years	2 years 29 days

Stock-Based Compensation - Summary of Restricted Stock Activity (Details)	12 Months Ended
	Dec. 31, 2019 shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Restricted Shares Outstanding, Beginning balance	49,052
Number of Restricted Shares Outstanding, Restricted shares vested	(49,052)

Net Loss Per Share - Summary of Outstanding Potentially Dilutive Shares Excluded from Calculation of Diluted Net Loss Per Share (Details) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Total	4,356,606	3,489,282
Stock Options to Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Total	4,285,425	3,404,847
Restricted Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Total		49,052
Shares Committed under ESPP
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Total	71,181	35,383

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Line Items]
Provision for income taxes	$ 0	$ 0
Federal tax rate	21.00%
Increase in valuation allowance	$ 24,637,000	17,992,000
Operating loss carryforward, yearly utlization percentage	80.00%
Liability related to uncertain tax positions	$ 0
Federal
Income Tax Disclosure [Line Items]
Operating loss carryforwards	$ 163,200,000	76,400,000
Operating loss carryforwards, expiration year	2035
Tax credit carryforwards, expiration year	2035
Federal \| Research Credit
Income Tax Disclosure [Line Items]
Tax credit carryforwards	$ 5,100,000	2,100,000
Federal \| Orphan Drug Credits
Income Tax Disclosure [Line Items]
Tax credit carryforwards	2,600,000	200,000
California
Income Tax Disclosure [Line Items]
Operating loss carryforwards	$ 63,900,000	53,400,000
Operating loss carryforwards, expiration year	2035
California \| Research Credit
Income Tax Disclosure [Line Items]
Tax credit carryforwards	$ 2,800,000	$ 1,500,000

Income Taxes - Schedule of Reconciliation of Provision for Income Taxes Amount Computed by Federal Statutory Income tax (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Computed expected tax benefit	$ (18,406,000)	$ (14,778,000)
State taxes (net of federal tax benefits)	(827,000)	(1,621,000)
Increase in valuation allowance	24,637,000	17,992,000
Other	(1,155,000)	481,000
R&D tax credits	(4,249,000)	(2,074,000)
Total provision for income taxes	$ 0	$ 0

Income Taxes - Schedule of Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Net operating loss carryforwards	$ 39,561	$ 19,748
Capitalized R&D	8,908	10,673
Stock-based compensation	1,025	583
Intangibles	2,800	2,891
Tax credits	7,880	2,711
Accruals and other	2,568	1,067
Total deferred tax assets	62,742	37,673
Deferred tax liabilities:
Right of use asset	(476)
Total deferred tax liabilities	(476)
Less: valuation allowance	$ 62,266	$ 37,673

Income Taxes - Schedule of Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Balance at beginning of year	$ 836	$ 283
Increases related to prior year tax positions	210
Increases related to current year tax positions	1,117	553
Balance at end of year	$ 2,163	$ 836

Related-Party Relationships - Additional Information (Details) - USD ($)	1 Months Ended	12 Months Ended
	Feb. 28, 2018	Dec. 31, 2019	Dec. 31, 2018	Nov. 30, 2015
Related Party Transaction [Line Items]
Payments for royalty		$ 0
License and Collaboration Agreement \| Ono Pharmaceutical Co., Ltd.
Related Party Transaction [Line Items]
License payment received		15,700,000
Technology License Agreement \| Leland Stanford Junior University
Related Party Transaction [Line Items]
Estimated sublicensing fee liability		$ 300,000
Milestone payments	$ 75,000
Technology License Agreement \| Leland Stanford Junior University \| Research and Development
Related Party Transaction [Line Items]
Milestone payments			$ 75,000
Technology License Agreement \| Leland Stanford Junior University \| Maximum
Related Party Transaction [Line Items]
Milestone payments to be paid upon achievement of development and regulatory approval milestones				$ 5,600,000

Subsequent Event - Additional Information (Details)	Mar. 01, 2020 $ / shares
Forty Seven, Inc. \| Gilead Science, Inc. and Toro Merger Sub, Inc. \| Merger Agreement
Business Acquisition [Line Items]
Share outstanding purchase price	$ 95.50