Document and Entity Information - USD ($) $ in Millions	12 Months Ended
Document and Entity Information - USD ($) $ in Millions	Dec. 31, 2018	Mar. 15, 2019	Jun. 29, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2018
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	Non-accelerated Filer
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		31,271,913
Entity Public Float			$ 179.7

Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 10,837	$ 24,417
Short-term investments	128,186	63,694
Prepaid expenses and other current assets	6,835	4,450
Total current assets	145,858	92,561
Property and equipment, net	1,360	1,358
Other assets	2,219	1,546
Total assets	149,437	95,465
Current liabilities:
Accounts payable	4,621	3,705
Accrued liabilities	9,044	4,808
Deferred grant funding, current	1,744	2,759
Total current liabilities	15,409	11,272
Lease-related liabilities, noncurrent	331	476
Other long-term liabilities	476	255
Total liabilities	16,216	12,003
Commitments and Contingencies (Note 5)
Stockholders’ equity:
Convertible preferred stock, $0.0001 par value; nil and 16,215,944 shares authorized as of December 31, 2018 and 2017; nil and 16,215,896 shares issued and outstanding as of December 31, 2018 and 2017		149,397
Common stock, $0.0001 par value: 200,000,000 shares authorized; 31,079,150 and 6,751,157 shares issued and outstanding at December 31, 2018 and 2017	3	1
Additional paid-in capital	273,069	3,507
Accumulated other comprehensive loss	(82)	(44)
Accumulated deficit	(139,769)	(69,399)
Total stockholders’ equity	133,221	83,462
Total liabilities and stockholders’ equity	$ 149,437	$ 95,465

Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized		16,215,944
Preferred stock, shares issued		16,215,896
Preferred stock, shares outstanding		16,215,896
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	31,079,150	6,751,157
Common stock, shares outstanding	31,079,150	6,751,157

Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating expenses:
Research and development	$ 56,673	$ 37,174	$ 14,464
General and administrative	15,432	8,130	5,153
Total operating expenses	72,105	45,304	19,617
Loss from operations	(72,105)	(45,304)	(19,617)
Interest and other income, net	1,735	406	78
Net loss	(70,370)	(44,898)	(19,539)
Unrealized loss on available-for-sale securities	(38)	(44)
Comprehensive loss	$ (70,408)	$ (44,942)	$ (19,539)
Net loss per share, basic and diluted	$ (3.75)	$ (6.94)	$ (3.15)
Shares used in computing net loss per share, basic and diluted	18,768,868	6,468,634	6,197,195

Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Series A-1 Convertible Preferred Shares	Series A-2 Convertible Preferred Shares	Series B Convertible Preferred Shares	Convertible Preferred Stock	Convertible Preferred Stock Series A-1 Convertible Preferred Shares	Convertible Preferred Stock Series A-2 Convertible Preferred Shares	Convertible Preferred Stock Series B Convertible Preferred Shares	Common Stock	Additional Paid-in Capital	Other Comprehensive Loss	Accumulated Deficit
Beginning balance at Dec. 31, 2015	$ 26,733				$ 29,655				$ 1	$ 2,039		$ (4,962)
Beginning balance, shares at Dec. 31, 2015					3,845,145				5,427,087
Issuance of stock		$ 4,590				$ 4,590
Issuance of stock, shares						593,546
Issuance of common stock related to Stanford license agreement, shares									1,000,160
Issuance of common stock for exercise of stock options	203									203
Issuance of common stock for exercise of stock options, shares									216,127
Vesting of restricted common stock	2									2
Stock-based compensation	245									245
Net loss	(19,539)											(19,539)
Ending balance at Dec. 31, 2016	12,234				$ 34,245				$ 1	2,489		(24,501)
Ending balance, shares at Dec. 31, 2016					4,438,691				6,643,374
Issuance of stock			$ 40,377	$ 74,775			$ 40,377	$ 74,775
Issuance of stock, shares							4,187,682	7,589,523
Issuance of common stock for exercise of stock options	155									155
Issuance of common stock for exercise of stock options, shares									107,783
Vesting of restricted common stock	2									2
Vesting of early exercised stock options	137									137
Stock-based compensation	724									724
Net loss	(44,898)											(44,898)
Unrealized loss on available-for-sale securities	(44)										$ (44)
Ending balance at Dec. 31, 2017	83,462				$ 149,397				$ 1	3,507	(44)	(69,399)
Ending balance, shares at Dec. 31, 2017					16,215,896				6,751,157
Conversion of preferred stock into common stock uponcompletion 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 initialpublic offering, net of issuance costs	116,336								$ 1	116,335
Issuance of common stock in connection with the initialpublic offering, 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 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

Statements of Cash Flows - USD ($)	12 Months Ended
Statements of Cash Flows - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net loss	$ (70,370,000)	$ (44,898,000)	$ (19,539,000)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	3,431,000	724,000	245,000
Depreciation and amortization	401,000	371,000	134,000
Accretion of discounts on marketable securities	(744,000)	0	0
Change in fair value of embedded derivative	331,000	0	0
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(2,385,000)	(568,000)	(3,881,000)
Other assets	(673,000)	205,000	(1,691,000)
Accounts payable	916,000	1,221,000	1,995,000
Accrued liabilities	4,214,000	3,356,000	853,000
Deferred grant funding	(1,015,000)	2,759,000	0
Lease-related liabilities	(123,000)	(90,000)	53,000
Other long-term liabilities		(17,000)	16,000
Net cash used in operating activities	(66,017,000)	(36,937,000)	(21,815,000)
Cash flows from investing activities:
Purchases of property and equipment	(403,000)	(114,000)	(1,103,000)
Purchases of available-for-sale securities	(142,027,000)	(79,738,000)	(4,000,000)
Proceeds from maturities of available-for-sale securities	78,241,000	16,000,000	4,000,000
Net cash used in investing activities	(64,189,000)	(63,852,000)	(1,103,000)
Cash flows from financing activities:
Proceeds from issuance of convertible preferred stock, net of issuance costs		115,152,000	4,590,000
Proceeds from issuance of common stock upon exercise of stock options	291,000	312,000	436,000
Proceeds from initial public offering, net of issuance costs	116,336,000	0	0
Repurchase of fractional common stock upon reverse stock split	(1,000)	0	0
Net cash provided by financing activities	116,626,000	115,464,000	5,026,000
Net (decrease) increase in cash and cash equivalents	(13,580,000)	14,675,000	(17,892,000)
Cash and cash equivalents — beginning of period	24,417,000	9,742,000	27,634,000
Cash and cash equivalents — end of period	10,837,000	24,417,000	9,742,000
Supplemental disclosures of cash flow information:
Purchases of property and equipment through accounts payable and accrued liabilities		10,000	0
Conversion of convertible preferred stock to common stock at close of initial public offering	$ 149,397,000	$ 0	$ 0

Description of Business

12 Months Ended

Dec. 31, 2018

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, 5F9, dosed as a monotherapy and in combination with marketed cancer therapies, can transform the treatment of cancer.

Initial Public Offering

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.

Reverse Stock Split

In June 2018, the Company’s board of directors approved an amended and restated certificate of incorporation to effect a reverse split of shares of the Company’s common stock and convertible preferred stock on a 1-for-7.75 basis (the “Reverse Stock Split”). The par values of the common stock and convertible preferred stock were not adjusted as a result of the Reverse Stock Split. All references to common stock, options to purchase common stock, restricted stock, share data, per share data, convertible preferred stock and related information contained in the financial statements have been retroactively adjusted to reflect this Reverse Stock Split for all periods presented. The Reverse Stock Split was effected on June 14, 2018.

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 $139.0 million as of December 31, 2018. Since inception through December 31, 2018, the Company has incurred cumulative net losses of $139.8 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, 2018

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 expenses during the reporting period. Significant estimates and assumptions made in the accompanying financial statements include but are not limited to the fair value of common stock prior to the IPO, the fair value of stock options, income tax uncertainties, 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 realized gains or losses on investments for the years ended December 31, 2018, 2017 and 2016. 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.

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 2018, 2017 and 2016, 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 $281,000, $211,000 and $107,000 for the years ended December 31, 2018, 2017 and 2016.

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 foreign currency losses for the year ended December 31, 2018. Foreign currency transaction gains and losses were not material for the years ended December 31, 2017 and 2016.

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

In June 2018, the FASB issued Accounting Standards Update No. 2018-07, Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting (“ASU 2018-07”). ASU 2018-07 is intended to reduce the cost and complexity and to improve financial reporting for nonemployee share-based payments. ASU 2018-07 expands the scope of Topic 718, Compensation-Stock Compensation (which currently only includes share-based payments to employees) to include share-based payments issued to nonemployees for goods or services. Consequently, the accounting for share-based payments to nonemployees and employees will be substantially aligned. The Company adopted this guidance for the year ended December 31, 2018. The adoption of this guidance during the year ended December 31, 2018 did not have a material impact on the Company’s financial statements.

Recent Accounting Pronouncements Not Yet Adopted

In February 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update No. 2016-02, Leases (Topic 842). The principle of Topic 842 is that a lessee should recognize the assets and liabilities that arise from leases and provide enhanced disclosures. In July 2018, the FASB issued guidance to permit an alternative transition method for Topic 842, which allows transition to the new lease standard by recognizing a cumulative-effect adjustment to the opening balance of retained earnings in the period of adoption. The Company expects to adopt Topic 842 as of January 1, 2019 under this new alternative transition method. The Company will elect 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. In addition, as a practical expedient relating to its property lease, the Company will not separate lease components from nonlease components. The Company will not elect the hindsight practical expedient permitted under the transition guidance within the new lease standard. The Company has substantially completed an assessment of the new standard’s impact, and while the Company does not expect a material impact from adoption on its statements of operations or comprehensive loss, the Company does expect to record a material increase in its assets and liabilities on the balance sheet upon adoption of this standard. Upon adoption, the Company expects to recognize a right-of-use asset in the range of $2.2 million to $2.4 million and a lease liability in the range of $2.7 million to $2.9 million for the headquarters property lease.

Fair Value Measurements

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Fair Value Measurements

3.	Fair Value Measurements

The Company measures and reports its cash equivalents, short-term investments, and embedded derivative at fair value.

Money market funds 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, 2018 and 2017 had maturities of less than one year. There were no realized gains or losses on investments for the years ended December 31, 2018, 2017 and 2016. Any unrealized losses were deemed to be temporary. The Company does not intend to sell its securities that are in an unrealized loss position, 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, 2018 and 2017 are presented in the following tables:

	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
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
US government debt securities	Level 2		15,963		—		(1	)		15,962
Total cash equivalents and available-for-sale securities		$	136,227	$	—	$	(82	)	$	136,145

	As of December 31, 2017
	Fair Value Hierarchy	Amortized Cost		Unrealized Gains		Unrealized Losses			Market Value
	(in thousands)
Money market funds	Level 1	$	19,052	$	—	$	—		$	19,052
Commercial paper	Level 2		31,467		—		—			31,467
Corporate debt securities	Level 2		24,556		—		(35	)		24,521
Asset-backed securities	Level 2		7,717		—		(7	)		7,710
US government debt securities	Level 2		1,993		—		(2	)		1,991
Total cash equivalents and available-for-sale securities		$	84,785	$	—	$	(44	)	$	84,741

The Company’s contingent milestone payments in its agreement with the Leukemia & Lymphoma Society, Inc. were concluded to be an embedded derivative. The embedded derivative contains unobservable inputs that are supported by little or no market activity at the measurement date. Accordingly, the Company’s embedded derivative is measured at fair value on a recurring basis using unobservable inputs that are classified as level 3 inputs. Refer to Note 6 for the valuation techniques and assumptions used in estimating the fair value of the embedded derivative.

- 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, 2018

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,
	2018			2017
	(in thousands)
Furniture and fixtures	$	108		$	14
Laboratory equipment		1,297			988
Computer equipment and software		91			91
Leasehold improvements		770			770
		2,266			1,863
Less: Accumulated depreciation and amortization		(906	)		(505	)
Total property and equipment, net	$	1,360		$	1,358

Depreciation and amortization expense for property and equipment amounted to $401,000, $371,000 and $134,000 for the years ended December 31, 2018, 2017 and 2016.

Accrued Liabilities

Accrued liabilities consist of the following:

	December 31,
	2018		2017
	(in thousands)
Accrued research and development expenses	$	5,870	$	4,096
Accrued bonus		1,602		—
Lease-related liabilities, current		155		133
Other		1,417		579
Total accrued liabilities	$	9,044	$	4,808

Commitments and Contingencies

12 Months Ended

Dec. 31, 2018

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

5.	Commitments and Contingencies

Leases

In August 2016, the Company entered into an operating lease agreement for its headquarters in Menlo Park, California. The lease term is for 60 months. The lease rental payments are on a graduated scale; however, rent expense is recognized on a straight-line basis over the lease term. The landlord provided the Company with a tenant improvement allowance of up to $646,000. The allowance is amortized as an offset to rent expense over the lease term. Rent expense for the years ended December 31, 2018, 2017 and 2016 was $1,071,000, $993,000 and $587,000. At December 31, 2018 and 2017, $142,000 and $135,000 was accrued as deferred rent expense.

Effective September 2016, the Company entered into a sublease agreement to lease a portion of the Menlo Park facility to a tenant. Sublease income was $0, $124,000 and $62,000 for the years ended December 31, 2018, 2017 and 2016 and was recorded as an offset to rent expense.

In conjunction with the lease agreement, the Company paid a security deposit of $265,000 included in prepaid expenses and other current assets and other assets as of December 31, 2017. The security deposit was reduced to $176,000, included in prepaid expenses and other current assets and other assets as of December 31, 2018.

At December 31, 2018, future minimum payments are as follows (in thousands):

2019	$	1,134
2020		1,168
2021		794
Total future minimum lease payments	$	3,096

Manufacturing Commitment

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 5F9-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, 2018, future minimum payments under the Lonza development and manufacturing agreements are as follows (in thousands):

2019	$	14,447
2020		10,250
2021		5,150
Total future minimum payments	$	29,847

Research and License Agreements

12 Months Ended

Dec. 31, 2018

Research And License Agreements [Abstract]

Research and License Agreements

6.	Research and License Agreements

Stanford License Agreement

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.

California Institute of Regenerative Medicine (CIRM) Grants

In January 2017, the Company was awarded a research grant from 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. As of December 31, 2018 and 2017, the Company had received $7.2 million and $3.8 million under the award.

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. As of December 31, 2018 and 2017, the Company had received $2.4 million and $1.1 million under the award. 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 revenue sharing, 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 the remaining CIRM funds on hand.

Leukemia & Lymphoma Society Grant

In March 2017, the Company entered into an agreement with the Leukemia & Lymphoma Society, Inc. (“LLS”) and amended the agreement to include an additional study in June 2018. The LLS research grant stipulates various milestone-based payments with a total award of $4.2 million through December 2019. As of December 31, 2018 and 2017, the Company had received $3.9 million and $1.0 million under the award. 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 $15 million in total. The Company concluded that the contingent milestone payments were an embedded derivative and the Company recorded a liability for the derivative of approximately $0.3 million, as part of other long-term liabilities as of December 31, 2018. The liability for the derivative was not material to the financial statements as of December 31, 2017. The value of the embedded derivative was estimated using the probability-adjusted and discounted future milestone payments. The Company recorded the $0.3 million increase in the fair value of the embedded derivative in the statement of operations as Interest and other income, net.

The Company recognizes research grants as a reduction of research and development expense when the eligible costs are incurred. For the years ended December 31, 2018 and 2017, the Company recognized $8.0 million and $3.9 million as a reduction to research and development expenses for research grants.

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 5F9 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, 2018, the Company recorded a receivable of $0.6 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 year ended December 31, 2018, the Company recognized $1.2 million 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, 2018. 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, 2018, the Company recognized $3.0 million in research and development expense related to the Blink asset purchase agreement.

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 5F9 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 (approximately $46 million US Dollar based on the exchange rate at December 31, 2018) 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.

Convertible Preferred Stock

12 Months Ended

Dec. 31, 2018

Equity [Abstract]

Convertible Preferred Stock

7.	Convertible Preferred Stock

On the completion of the IPO (see Note 1) on July 2, 2018, all outstanding shares of convertible preferred stock were automatically converted into 16,215,896 shares of common stock and the related carrying value was reclassified to common stock and additional paid-in capital. Accordingly, there were no shares of convertible preferred stock outstanding as of December 31, 2018. The convertible preferred stock as of December 31, 2017 consisted of the following:

	December 31, 2017
	Shares Authorized		Shares Issued and Outstanding		Net Carrying Value
	(in thousands, except share data)
Series A-1		4,438,709		4,438,691	$	34,245
Series A-2		4,187,698		4,187,682		40,377
Series B		7,589,537		7,589,523		74,775
		16,215,944		16,215,896	$	149,397

Stock-Based Compensation

12 Months Ended

Dec. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation

8.	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, 2018, there were 3,291,597 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 has 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, 2018, there were 2,886,650 shares available for future grants 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, 2016		624,177		$	2.02		9.35	$	1,790
Options granted		1,756,056			5.08
Options exercised		(107,783	)		2.90
Options forfeited		(169,922	)		2.71
Balance, December 31, 2017		2,102,528		$	4.47		9.43	$	1,672
Options granted		1,426,038			9.44
Options exercised		(63,865	)		4.56
Options forfeited		(59,854	)		4.96
Balance outstanding December 31, 2018		3,404,847		$	6.55		8.82	$	31,388
Exercisable, December 31, 2018		2,167,142		$	6.18		8.75	$	20,771
Vested and expected to vest, December 31, 2018		3,404,847		$	6.55		8.82	$	31,388

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, 2018, 2017 and 2016 was $554,000, $226,000 and $0, respectively.

During the years ended December 31, 2018, 2017 and 2016, the estimated total grant-date fair value of the options vested was $2.3 million, $3.7 million and $1.1 million and the estimated weighted-average grant-date fair value of employee options granted was $6.14, $3.39 and $1.33 per share, respectively. As of December 31, 2018, there was $11.4 million of unrecognized stock-based compensation related to unvested stock options that is expected to be recognized over a weighted-average period of 3.3 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. 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,
	2018	2017	2016
Expected term (years)	5.48 - 6.44	6.0	6.0
Expected volatility	65.8% - 70.8%	75.5%	75%
Weighted average risk-free interest rate	2.51% - 3.10%	1.77% – 2.21%	1.25% – 2.08%
Dividend yield	0%	0%	0%

The fair value of the ESPP is estimated using the Black-Scholes option pricing model. For the year ended December 31, 2018, the weighted average grant date fair value per share for the ESPP was $6.95. Stock-based compensation expense for the ESPP was $423,000 for the year ended December 31, 2018.

For the year ended December 31, 2018, the fair value of ESPP was estimated using the following assumptions:

		Year Ended December 31,
		2018
Expected term (years)		0.58 - 2.08
Expected volatility		66% - 68%
Weighted average risk-free interest rate		2.14% - 2.53%
Dividend yield		0%

The ESPP commenced in June 2018. No shares of common stock were purchased pursuant to the ESPP in 2018. Cash received from payroll deductions pursuant to the ESPP in 2018 was $473,000.

Total stock-based compensation was as follows:

	Year Ended December 31,
	2018		2017		2016
	(in thousands)
Research and development	$	1,193	$	206	$	93
General and administrative		2,238		518		152
Total	$	3,431	$	724	$	245

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, 2018 and 2017, there was $144,000 and $255,000 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 for the years ended December 31, 2017 and 2018:

	Number of Restricted Shares Outstanding
Unvested shares—December 31, 2016		299,327
Early exercised options		32,258
Restricted shares vested		(174,596	)
Unvested shares—December 31, 2017		156,989
Early exercised options		11,418
Restricted shares vested		(77,420	)
Repurchased by the Company		(41,935	)
Unvested shares—December 31, 2018		49,052

Net Loss Per Share

12 Months Ended

Dec. 31, 2018

Earnings Per Share [Abstract]

Net Loss Per Share

9.	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,
	2018		2017		2016
Convertible preferred stock		—		16,215,896		4,438,691
Stock options to purchase common stock		3,404,847		2,102,528		624,177
Restricted stock subject to future vesting		49,052		156,988		299,327
Shares committed under ESPP		35,383		—		—
Total		3,489,282		18,475,412		5,362,195

Income Taxes

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Income Taxes

10.	Income Taxes

The provision for income taxes for the years ended December 31, 2018, 2017 and 2016 was an immaterial amount. 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,
	2018			2017			2016
	(in thousands)
Computed expected tax benefit	$	(14,778	)	$	(9,428	)	$	(6,540	)
State taxes (net of federal tax benefits)		(1,621	)		(3,119	)		(1,111	)
Increase in valuation allowance		17,992			10,185			7,685
Other		481			(65	)		89
R&D tax credits		(2,074	)		(326	)		(123	)
Federal rate change (pursuant to the Tax Act)		—			2,753			—
Total provision for income taxes	$	—		$	—		$	—

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

	December 31,
	2018			2017
	(in thousands)
Net operating loss carryforwards	$	19,748		$	4,309
Capitalized R&D		10,673			13,537
Stock-based compensation		583			122
Intangibles		2,891			896
Tax credits		2,711			637
Accruals and other		1,067			180
Total deferred tax assets		37,673			19,681
Less: valuation allowance		(37,673	)		(19,681	)
Net deferred tax assets	$	—		$	—

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 $18.0 million, $10.2 million and $7.7 million during the years ended December 31, 2018, 2017 and 2016.

The Company has net operating carryforwards for federal and California income tax purposes of approximately $76.4 million and $53.4 million, respectively, as of December 31, 2018. As of December 31, 2017, we had federal and California net operating loss carryforwards of approximately $16.5 million and $16.6 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 $2.1 million and $1.5 million, respectively, as of December 31, 2018. As of December 31, 2017, we had federal and California research credit carryforwards of approximately $0.4 million and $0.6 million, respectively. The Company also has Federal Orphan Drug credits of approximately $0.2 million as of December 31, 2018. 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,
	2018		2017		2016
	(in thousands)
Balance at beginning of year	$	283	$	93	$	—
Increases related to current year tax positions		553		190		93
Balance at end of year	$	836	$	283	$	93

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 2018 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 2018 remain open to examination by the domestic taxing jurisdictions.

In December 2017, the Tax Cuts and Jobs Act of 2017 (“Tax Act”) was signed into law. The Tax Act, among other changes, lowers the Company’s federal tax rate from 34% to 21%. Based on provisions of the Tax Act, the Company remeasured its deferred tax assets and liabilities as of December 31, 2017 to reflect the lower statutory tax rate. The Company completed its accounting for the income tax effects of the Tax Act in 2018, and no material adjustments were required to the provisional amounts initially recorded.

Selected Quarterly Financial Data (Unaudited)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

Selected Quarterly Financial Data (Unaudited)

12. Selected Quarterly Financial Data (Unaudited)

Selected quarterly results from operations for the years ended December 31, 2018 and 2017 are as follows:

	Three Months Ended,
	December 31,			September 30,			June 30,			March 31,
	(in thousands, except per share data)
2018
Loss from operations		(17,784	)		(22,367	)		(16,958	)		(14,996	)
Net loss		(17,214	)		(21,659	)		(16,722	)		(14,775	)
Net loss per share, basic and diluted	$	(0.56	)	$	(0.71	)	$	(2.52	)	$	(2.24	)
2017
Loss from operations		(12,583	)		(10,893	)		(10,886	)		(10,942	)
Net loss		(12,330	)		(10,833	)		(10,827	)		(10,908	)
Net loss per share, basic and diluted	$	(1.88	)	$	(1.67	)	$	(1.68	)	$	(1.71	)

Summary of Significant Accounting Policies (Policies)	12 Months Ended
Summary of Significant Accounting Policies (Policies)	Dec. 31, 2018
Accounting Policies [Abstract]
Basis of Presentation	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	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 expenses during the reporting period. Significant estimates and assumptions made in the accompanying financial statements include but are not limited to the fair value of common stock prior to the IPO, the fair value of stock options, income tax uncertainties, 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	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 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 realized gains or losses on investments for the years ended December 31, 2018, 2017 and 2016. Interest on marketable securities is included in interest income.
Concentration of Credit Risk and Other Risks and Uncertainties	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	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, 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	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.
Research and Development Expenditures	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	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	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 2018, 2017 and 2016, 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 $281,000, $211,000 and $107,000 for the years ended December 31, 2018, 2017 and 2016.
Income Taxes	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	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 foreign currency losses for the year ended December 31, 2018. Foreign currency transaction gains and losses were not material for the years ended December 31, 2017 and 2016.
Comprehensive Loss	Comprehensive Loss The Company’s comprehensive loss is comprised of changes in unrealized losses on available-for-sale securities.
Net Loss per Share	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	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	Recently Issued and Adopted Accounting Pronouncements In June 2018, the FASB issued Accounting Standards Update No. 2018-07, Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting (“ASU 2018-07”). ASU 2018-07 is intended to reduce the cost and complexity and to improve financial reporting for nonemployee share-based payments. ASU 2018-07 expands the scope of Topic 718, Compensation-Stock Compensation (which currently only includes share-based payments to employees) to include share-based payments issued to nonemployees for goods or services. Consequently, the accounting for share-based payments to nonemployees and employees will be substantially aligned. The Company adopted this guidance for the year ended December 31, 2018. The adoption of this guidance during the year ended December 31, 2018 did not have a material impact on the Company’s financial statements.
Recent Accounting Pronouncements Not Yet Adopted	Recent Accounting Pronouncements Not Yet Adopted In February 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update No. 2016-02, Leases (Topic 842). The principle of Topic 842 is that a lessee should recognize the assets and liabilities that arise from leases and provide enhanced disclosures. In July 2018, the FASB issued guidance to permit an alternative transition method for Topic 842, which allows transition to the new lease standard by recognizing a cumulative-effect adjustment to the opening balance of retained earnings in the period of adoption. The Company expects to adopt Topic 842 as of January 1, 2019 under this new alternative transition method. The Company will elect 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. In addition, as a practical expedient relating to its property lease, the Company will not separate lease components from nonlease components. The Company will not elect the hindsight practical expedient permitted under the transition guidance within the new lease standard. The Company has substantially completed an assessment of the new standard’s impact, and while the Company does not expect a material impact from adoption on its statements of operations or comprehensive loss, the Company does expect to record a material increase in its assets and liabilities on the balance sheet upon adoption of this standard. Upon adoption, the Company expects to recognize a right-of-use asset in the range of $2.2 million to $2.4 million and a lease liability in the range of $2.7 million to $2.9 million for the headquarters property lease.

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2018

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, 2018 and 2017 are presented in the following tables:

	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
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
US government debt securities	Level 2		15,963		—		(1	)		15,962
Total cash equivalents and available-for-sale securities		$	136,227	$	—	$	(82	)	$	136,145

	As of December 31, 2017
	Fair Value Hierarchy	Amortized Cost		Unrealized Gains		Unrealized Losses			Market Value
	(in thousands)
Money market funds	Level 1	$	19,052	$	—	$	—		$	19,052
Commercial paper	Level 2		31,467		—		—			31,467
Corporate debt securities	Level 2		24,556		—		(35	)		24,521
Asset-backed securities	Level 2		7,717		—		(7	)		7,710
US government debt securities	Level 2		1,993		—		(2	)		1,991
Total cash equivalents and available-for-sale securities		$	84,785	$	—	$	(44	)	$	84,741

Balance Sheet Components (Tables)

12 Months Ended

Dec. 31, 2018

Balance Sheet Related Disclosures [Abstract]

Schedule of Property and Equipment, Net

Property and equipment, net consists of the following:

	December 31,
	2018			2017
	(in thousands)
Furniture and fixtures	$	108		$	14
Laboratory equipment		1,297			988
Computer equipment and software		91			91
Leasehold improvements		770			770
		2,266			1,863
Less: Accumulated depreciation and amortization		(906	)		(505	)
Total property and equipment, net	$	1,360		$	1,358

Schedule of Accrued Liabilities

Accrued liabilities consist of the following:

	December 31,
	2018		2017
	(in thousands)
Accrued research and development expenses	$	5,870	$	4,096
Accrued bonus		1,602		—
Lease-related liabilities, current		155		133
Other		1,417		579
Total accrued liabilities	$	9,044	$	4,808

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2018

Schedule of Future Minimum Payments

At December 31, 2018, future minimum payments are as follows (in thousands):

2019	$	1,134
2020		1,168
2021		794
Total future minimum lease payments	$	3,096

Lonza Development and Manufacturing Agreement

Schedule of Future Minimum Payments

At December 31, 2018, future minimum payments under the Lonza development and manufacturing agreements are as follows (in thousands):

2019	$	14,447
2020		10,250
2021		5,150
Total future minimum payments	$	29,847

Convertible Preferred Stock (Tables)

12 Months Ended

Dec. 31, 2018

Equity [Abstract]

Schedule of Convertible Preferred Stock

The convertible preferred stock as of December 31, 2017 consisted of the following:

	December 31, 2017
	Shares Authorized		Shares Issued and Outstanding		Net Carrying Value
	(in thousands, except share data)
Series A-1		4,438,709		4,438,691	$	34,245
Series A-2		4,187,698		4,187,682		40,377
Series B		7,589,537		7,589,523		74,775
		16,215,944		16,215,896	$	149,397

Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

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, 2016		624,177		$	2.02		9.35	$	1,790
Options granted		1,756,056			5.08
Options exercised		(107,783	)		2.90
Options forfeited		(169,922	)		2.71
Balance, December 31, 2017		2,102,528		$	4.47		9.43	$	1,672
Options granted		1,426,038			9.44
Options exercised		(63,865	)		4.56
Options forfeited		(59,854	)		4.96
Balance outstanding December 31, 2018		3,404,847		$	6.55		8.82	$	31,388
Exercisable, December 31, 2018		2,167,142		$	6.18		8.75	$	20,771
Vested and expected to vest, December 31, 2018		3,404,847		$	6.55		8.82	$	31,388

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,
	2018	2017	2016
Expected term (years)	5.48 - 6.44	6.0	6.0
Expected volatility	65.8% - 70.8%	75.5%	75%
Weighted average risk-free interest rate	2.51% - 3.10%	1.77% – 2.21%	1.25% – 2.08%
Dividend yield	0%	0%	0%

Schedule of Estimated Fair Value of Employee Stock Purchase Plan

For the year ended December 31, 2018, the fair value of ESPP was estimated using the following assumptions:

		Year Ended December 31,
		2018
Expected term (years)		0.58 - 2.08
Expected volatility		66% - 68%
Weighted average risk-free interest rate		2.14% - 2.53%
Dividend yield		0%

Schedule of Total Stock-Based Compensation

Total stock-based compensation was as follows:

	Year Ended December 31,
	2018		2017		2016
	(in thousands)
Research and development	$	1,193	$	206	$	93
General and administrative		2,238		518		152
Total	$	3,431	$	724	$	245

Summary of Restricted Stock Activity

A summary of restricted stock activity follows for the years ended December 31, 2017 and 2018:

	Number of Restricted Shares Outstanding
Unvested shares—December 31, 2016		299,327
Early exercised options		32,258
Restricted shares vested		(174,596	)
Unvested shares—December 31, 2017		156,989
Early exercised options		11,418
Restricted shares vested		(77,420	)
Repurchased by the Company		(41,935	)
Unvested shares—December 31, 2018		49,052

Net Loss Per Share (Tables)

12 Months Ended

Dec. 31, 2018

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,
	2018		2017		2016
Convertible preferred stock		—		16,215,896		4,438,691
Stock options to purchase common stock		3,404,847		2,102,528		624,177
Restricted stock subject to future vesting		49,052		156,988		299,327
Shares committed under ESPP		35,383		—		—
Total		3,489,282		18,475,412		5,362,195

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2018

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,
	2018			2017			2016
	(in thousands)
Computed expected tax benefit	$	(14,778	)	$	(9,428	)	$	(6,540	)
State taxes (net of federal tax benefits)		(1,621	)		(3,119	)		(1,111	)
Increase in valuation allowance		17,992			10,185			7,685
Other		481			(65	)		89
R&D tax credits		(2,074	)		(326	)		(123	)
Federal rate change (pursuant to the Tax Act)		—			2,753			—
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,
	2018			2017
	(in thousands)
Net operating loss carryforwards	$	19,748		$	4,309
Capitalized R&D		10,673			13,537
Stock-based compensation		583			122
Intangibles		2,891			896
Tax credits		2,711			637
Accruals and other		1,067			180
Total deferred tax assets		37,673			19,681
Less: valuation allowance		(37,673	)		(19,681	)
Net deferred tax assets	$	—		$	—

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,
	2018		2017		2016
	(in thousands)
Balance at beginning of year	$	283	$	93	$	—
Increases related to current year tax positions		553		190		93
Balance at end of year	$	836	$	283	$	93

Selected Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

Summary of Selected Quarterly Results From Operations

Selected quarterly results from operations for the years ended December 31, 2018 and 2017 are as follows:

	Three Months Ended,
	December 31,			September 30,			June 30,			March 31,
	(in thousands, except per share data)
2018
Loss from operations		(17,784	)		(22,367	)		(16,958	)		(14,996	)
Net loss		(17,214	)		(21,659	)		(16,722	)		(14,775	)
Net loss per share, basic and diluted	$	(0.56	)	$	(0.71	)	$	(2.52	)	$	(2.24	)
2017
Loss from operations		(12,583	)		(10,893	)		(10,886	)		(10,942	)
Net loss		(12,330	)		(10,833	)		(10,827	)		(10,908	)
Net loss per share, basic and diluted	$	(1.88	)	$	(1.67	)	$	(1.68	)	$	(1.71	)

Description of Business - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands			1 Months Ended
	Jul. 27, 2018	Jul. 02, 2018	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Description Of Business [Line Items]
Reverse stock split			1-for-7.75
Cash, cash equivalents and short-term investments				$ 139,000
Cumulative net losses				$ 139,769	$ 69,399
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 price per share	$ 16.00
Proceeds from issuance of common stock	$ 116,300
Underwriting discounts and commissions, offering expenses	$ 13,200

Summary of Significant Accounting Policies - Additional Information (Details)	12 Months Ended
	Dec. 31, 2018 USD ($) Institution Segment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Realized gains or loss on investments	$ 0	$ 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)	401(k)
Percentage of maximum matching contributions on employee’s eligible compensation deferred	3.00%	3.00%	3.00%
Company recognized expense related to its contributions plan	$ 281,000	$ 211,000	$ 107,000
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.
Foreign currency transaction gain (loss)	$ (200,000)
Number of operating segments \| Segment	1
Minimum
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of property and equipment	3 years
Right-of-use asset	$ 2,200,000
Lease,liability	$ 2,700,000
Maximum
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of property and equipment	5 years
Right-of-use asset	$ 2,400,000
Lease,liability	$ 2,900,000

Fair Value Measurements - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
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	$ 0
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, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 136,227	$ 84,785
Unrealized Losses	(82)	(44)
Market Value	136,145	84,741
Level 1
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	7,959	19,052
Market Value	7,959	19,052
Commercial Paper \| Level 2
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	43,277	31,467
Market Value	43,277	31,467
Corporate Debt Securities \| Level 2
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	46,186	24,556
Unrealized Losses	(54)	(35)
Market Value	46,132	24,521
Asset-backed Securities \| Level 2
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	22,842	7,717
Unrealized Losses	(27)	(7)
Market Value	22,815	7,710
US Government Debt Securities \| Level 2
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	15,963	1,993
Unrealized Losses	(1)	(2)
Market Value	$ 15,962	$ 1,991

Balance Sheet Components - Schedule of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 2,266	$ 1,863
Less: Accumulated depreciation and amortization	(906)	(505)
Total property and equipment, net	1,360	1,358
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment, gross	108	14
Laboratory Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	1,297	988
Computer Equipment and Software
Property Plant And Equipment [Line Items]
Property and equipment, gross	91	91
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 770	$ 770

Balance Sheet Components - Schedule of Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accrued Liabilities Current [Abstract]
Accrued research and development expenses	$ 5,870	$ 4,096
Accrued bonus	1,602
Lease-related liabilities, current	155	133
Other	1,417	579
Total accrued liabilities	$ 9,044	$ 4,808

Commitments and Contingencies - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Aug. 31, 2016
Operating Leased Assets [Line Items]
Lease term				60 months
Rent expense	$ 1,071,000	$ 993,000	$ 587,000
Accrued deferred rent expense	142,000	135,000
Sublease income	0	124,000	$ 62,000
Security deposit	$ 176,000	$ 265,000
Maximum
Operating Leased Assets [Line Items]
Tenant improvement allowance				$ 646,000

Commitments and Contingencies - Schedule of Future Minimum Payments (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Operating Leased Assets [Line Items]
2019	$ 1,134
2020	1,168
2021	794
Total future minimum lease payments	3,096
Lonza Development and Manufacturing Agreement
Operating Leased Assets [Line Items]
2019	14,447
2020	10,250
2021	5,150
Total future minimum lease payments	$ 29,847

Research and License Agreements - Additional Information (Details) € in Millions	1 Months Ended					12 Months Ended
	Jul. 31, 2018 USD ($)	Jun. 30, 2018 USD ($)	Feb. 28, 2018 USD ($)	Nov. 30, 2017 USD ($)	Jan. 31, 2017 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2018 EUR (€)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Mar. 31, 2017 USD ($)	Nov. 30, 2015 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Increase in Fair Value of Derivatives						$ (331,000)		$ 0	$ 0
Research and development						56,673,000		37,174,000	$ 14,464,000
Technology License Agreement \| Leland Stanford Junior University
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payments			$ 75,000
Technology License Agreement \| Leland Stanford Junior University \| Research and Development
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payments						75,000
Technology License Agreement \| Leland Stanford Junior University \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payments to be paid upon achievement of development and regulatory approval milestones											$ 5,600,000
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 milestone payments receivable period					4 years
Proceeds from collaborators						7,200,000		3,800,000
Research Grant \| Leukemia & Lymphoma Society, Inc. ("LLS")
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaborative arrangement research grant receivable										$ 4,200,000
Proceeds from collaborators						3,900,000		1,000,000
Increase in Fair Value of Derivatives						300,000
Reduction to research and development expenses for research grants						8,000,000		3,900,000
Research Grant \| Leukemia & Lymphoma Society, Inc. ("LLS") \| Other Long-Term Liabilities
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Derivative Liability						300,000		0
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						15,000,000
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 milestone payments receivable period				5 years
Proceeds from collaborators						2,400,000		$ 1,100,000
Collaborative arrangement, amended research grant receivable						$ 3,200,000
Collaborative arrangement, amended milestone payments receivable period						5 years	5 years
Merck Collaboration Agreement \| Ares Trading S.A
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Reduction to research and development expenses for research grants						$ 1,200,000
Receivable for reimbursement of research and development costs incurred						600,000
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						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						$ 46,000,000	€ 40.0

Convertible Preferred Stock - Additional Information (Details) - shares	Dec. 31, 2018	Jul. 02, 2018	Dec. 31, 2017
Class Of Stock [Line Items]
Preferred Stock, Shares Outstanding			16,215,896
Intial Public Offering
Class Of Stock [Line Items]
Convertible preferred stock automatically converted into common stock		16,215,896
Preferred Stock, Shares Outstanding	0

Convertible Preferred Stock - Schedule of Convertible Preferred Stock (Details) $ in Thousands	Dec. 31, 2017 USD ($) shares
Class Of Stock [Line Items]
Shares Authorized	16,215,944
Shares Issued and Outstanding	16,215,896
Net Carrying Value \| $	$ 149,397
Series A-1
Class Of Stock [Line Items]
Shares Authorized	4,438,709
Shares Issued and Outstanding	4,438,691
Net Carrying Value \| $	$ 34,245
Series A-2
Class Of Stock [Line Items]
Shares Authorized	4,187,698
Shares Issued and Outstanding	4,187,682
Net Carrying Value \| $	$ 40,377
Series B
Class Of Stock [Line Items]
Shares Authorized	7,589,537
Shares Issued and Outstanding	7,589,523
Net Carrying Value \| $	$ 74,775

Stock-Based Compensation - Additional Information (Details)	12 Months Ended
	Dec. 31, 2018 USD ($) Period $ / shares shares	Dec. 31, 2017 USD ($) $ / shares	Dec. 31, 2016 USD ($) $ / shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 3,431,000	$ 724,000	$ 245,000
Number of common stock purchase to ESPP \| shares	0
Employee Stock Purchase Plan Commecement Period	2018-06
Proceeds from payroll deductions from employee stock purchase plan	$ 473,000
Common Stock \| Stock-Based Compensation
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield	0.00%
2015 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common stock reserved for future issuance \| shares	3,291,597
2015 Plan \| 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	$ 144,000	255,000
2018 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common stock reserved for future issuance \| shares	3,000,000
Number of shares available for future grants \| shares	2,886,650
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.
Percentage of increase in number of shares of common stock reserved for issuance	5.00%
Maximum period to increase common stock reserved for issuance	10 years
2015 and 2018 Stock Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Intrinsic value of options exercised	$ 554,000	226,000	0
Total grant-date fair value of options vested	$ 2,300,000	$ 3,700,000	$ 1,100,000
Weighted-average grant-date fair value of employee options granted \| $ / shares	$ 6.14	$ 3.39	$ 1.33
Unrecognized share-based compensation related to unvested stock options	$ 11,400,000
Unrecognized stock-based compensation, recognized over weighted-average period	3 years 3 months 18 days
Employees Share Purchase Plan (ESPP)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common stock reserved for future issuance \| shares	450,000
Employees 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.
Employees share purchase plan purchase price percentage	85.00%
Subsequent offering period	24 months
Number of purchase period \| Period	4
Employees Share Purchase Plan (ESPP) \| Black Scholes Option Pricing Model
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average grant-date fair value of employee options granted \| $ / shares	$ 6.95
Expected dividend yield	0.00%
Stock-based compensation expense	$ 423,000,000

Stock-Based Compensation - Summary of Option Activity (Details) - 2015 and 2018 Stock Plan - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares Issuable Under Options, Beginning balance	2,102,528	624,177
Shares Issuable Under Options, Options granted	1,426,038	1,756,056
Shares Issuable Under Options, Options exercised	(63,865)	(107,783)
Shares Issuable Under Options, Options forfeited	(59,854)	(169,922)
Shares Issuable Under Options, Ending balance	3,404,847	2,102,528	624,177
Shares Issuable Under Options, Exercisable	2,167,142
Shares Issuable Under Options, Vested and expected to vest	3,404,847
Weighted-Average Exercise Price, Beginning balance	$ 4.47	$ 2.02
Weighted-Average Exercise Price, Options granted	9.44	5.08
Weighted-Average Exercise Price, Options exercised	4.56	2.90
Weighted-Average Exercise Price, Options forfeited	4.96	2.71
Weighted-Average Exercise Price, Ending balance	6.55	$ 4.47	$ 2.02
Weighted-Average Exercise Price, Exercisable	6.18
Weighted-Average Exercise Price, Vested and expected to vest	$ 6.55
Weighted-Average Remaining Contract Term Exercisable	8 years 9 months 25 days	9 years 5 months 4 days	9 years 4 months 6 days
Weighted-Average Remaining Contract Term	8 years 9 months 25 days
Weighted-Average Remaining Contract Term Vested and expected to vest	8 years 9 months
Aggregate Intrinsic Value	$ 31,388	$ 1,672	$ 1,790
Aggregate Intrinsic Value Exercisable	20,771
Aggregate Intrinsic Value Vested and expected to vest	$ 31,388

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, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (years)		6 years	6 years
Expected volatility		75.50%	75.00%
Expected volatility, minimum	65.80%
Expected volatility, maximum	70.80%
Dividend yield	0.00%	0.00%	0.00%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (years)	5 years 5 months 23 days
Weighted average risk-free interest rate	2.51%	1.77%	1.25%
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (years)	6 years 5 months 8 days
Weighted average risk-free interest rate	3.10%	2.21%	2.08%

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

Stock-Based Compensation - Schedule of Total Stock-Based Compensation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	$ 3,431	$ 724	$ 245
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	1,193	206	93
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	$ 2,238	$ 518	$ 152

Stock-Based Compensation - Summary of Restricted Stock Activity (Details) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Restricted Shares Outstanding, Beginning balance	156,989	299,327
Number of Restricted Shares Outstanding, Early exercised options	11,418	32,258
Number of Restricted Shares Outstanding, Restricted shares vested	(77,420)	(174,596)
Number of Restricted Shares Outstanding, Repurchased by the Company	(41,935)
Number of Restricted Shares Outstanding, Ending balance	49,052	156,989

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, 2018	Dec. 31, 2017	Dec. 31, 2016
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Total	3,489,282	18,475,412	5,362,195
Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Total		16,215,896	4,438,691
Stock Options to Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Total	3,404,847	2,102,528	624,177
Restricted Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Total	49,052	156,988	299,327
Shares Committed under ESPP
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Total	35,383

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Line Items]
Federal tax rate	21.00%	34.00%
Increase in valuation allowance	$ 17,992,000	$ 10,185,000	$ 7,685,000
Operating loss carryforward, yearly utlization percentage	80.00%
Liability related to uncertain tax positions	$ 0
Federal
Income Tax Disclosure [Line Items]
Operating loss carryforwards	$ 76,400,000	16,500,000
Operating loss carryforwards, expiration year	2035
Tax credit carryforwards, expiration year	2035
Federal \| Research Credit
Income Tax Disclosure [Line Items]
Tax credit carryforwards	$ 2,100,000	400,000
Federal \| Orphan Drug Credits
Income Tax Disclosure [Line Items]
Tax credit carryforwards	200,000
California
Income Tax Disclosure [Line Items]
Operating loss carryforwards	$ 53,400,000	16,600,000
Operating loss carryforwards, expiration year	2035
California \| Research Credit
Income Tax Disclosure [Line Items]
Tax credit carryforwards	$ 1,500,000	$ 600,000

Income Taxes - Schedule of Reconciliation of Provision for Income Taxes Amount Computed by Federal Statutory Income tax (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Computed expected tax benefit	$ (14,778)	$ (9,428)	$ (6,540)
State taxes (net of federal tax benefits)	(1,621)	(3,119)	(1,111)
Increase in valuation allowance	17,992	10,185	7,685
Other	481	(65)	89
R&D tax credits	$ (2,074)	(326)	$ (123)
Federal rate change (pursuant to the Tax Act)		$ 2,753

Income Taxes - Schedule of Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Net operating loss carryforwards	$ 19,748	$ 4,309
Capitalized R&D	10,673	13,537
Stock-based compensation	583	122
Intangibles	2,891	896
Tax credits	2,711	637
Accruals and other	1,067	180
Total deferred tax assets	37,673	19,681
Less: valuation allowance	$ (37,673)	$ (19,681)

Selected Quarterly Financial Data (Unaudited) - Summary of Selected Quarterly Results From Operations (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement Of Financial Position [Abstract]
Loss from operations	$ (17,784)	$ (22,367)	$ (16,958)	$ (14,996)	$ (12,583)	$ (10,893)	$ (10,886)	$ (10,942)	$ (72,105)	$ (45,304)	$ (19,617)
Net loss	$ (17,214)	$ (21,659)	$ (16,722)	$ (14,775)	$ (12,330)	$ (10,833)	$ (10,827)	$ (10,908)	$ (70,370)	$ (44,898)	$ (19,539)
Net loss per share, basic and diluted	$ (0.56)	$ (0.71)	$ (2.52)	$ (2.24)	$ (1.88)	$ (1.67)	$ (1.68)	$ (1.71)	$ (3.75)	$ (6.94)	$ (3.15)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Statements of Stockholders' Equity (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Convertible preferred stock issuance costs	$ 4,047
Series A-1 Convertible Preferred Shares
Convertible preferred stock, price per share			$ 7.75
Convertible preferred stock issuance costs			$ 10
Series A-2 Convertible Preferred Shares
Convertible preferred stock, price per share		$ 9.6472
Convertible preferred stock issuance costs		$ 23
Series B Convertible Preferred Shares
Convertible preferred stock, price per share		$ 9.8820
Convertible preferred stock issuance costs		$ 225

Related-Party Relationship - Additional Information (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)
Related Party Transactions [Abstract]
Payments for royalty	$ 0