Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018		Sep. 30, 2017
Cash flow from operating activities
Net loss	$ (37,299)		$ (39,872)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	2,820		340
Depreciation and amortization	2,717		1,846
Share of loss from equity method investee	563		260
Gain on deemed sale from equity method investee	(1,229)
Accretion of discounts (amortization of premiums) on investments	(1,017)		82
Other non-operating income	(177)		(76)
Changes in operating assets and liabilities
Receivable from collaboration partner	(5,000)		(25,000)
Amounts owed by a related party	(89)		370
Prepaid expenses and other current assets	(1,188)		(1,288)
Accounts payable	(1,029)		(2,306)
Accrued liabilities	4,141		611
Other current liabilities	33		28
Deferred revenue	1,209		24,837
Deferred rent	(344)		(478)
Net cash used in operating activities	(35,889)		(40,646)
Cash flow from investing activities
Purchases of short-term and long-term investments	(222,067)		(37,718)
Proceeds from maturities of short-term and long-term investments	109,601		35,933
Purchases of property and equipment	(3,485)		(5,220)
Net cash used in investing activities	(115,951)		(7,005)
Cash flow from financing activities
Proceeds from initial public offering, net of issuance costs	125,111
Proceeds from issuance of common stock upon exercise of stock options, net of repurchases	3,327		797
Payment of preferred stock issuance costs	(135)
Net cash provided by financing activities	128,303		797
Net decrease in cash and cash equivalents	(23,537)		(46,854)
Cash and cash equivalents at beginning of period	98,426	[1]	65,160
Cash and cash equivalents at end of period	74,889		18,306
Non-cash investing and financing activities:
Unpaid financing cost included in accounts payable and accrued liabilities	0		33
Unpaid portion of property and equipment purchases included in accounts payable and accrued liabilities	103		23
Vesting of early exercised stock options and restricted stock	$ 982		$ 140

[1] The Condensed Consolidated Balance Sheet as of December 31, 2017 has been derived from the audited financial statements as of that date.