Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018		Jun. 30, 2017
Cash flow from operating activities
Net loss	$ (26,488)		$ (16,750)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	1,772		203
Depreciation and amortization	1,743		1,114
Share of loss from equity method investee	377		141
Gain on deemed sale from equity method investee	(1,229)
Other non-operating income	(177)		(53)
Changes in operating assets and liabilities
Amounts owed by a related party	(168)		384
Prepaid expenses and other current assets	(409)		(366)
Accounts payable	1,347		(1,610)
Accrued liabilities	1,812		821
Other current liabilities	20		20
Deferred revenue	(2,500)
Deferred rent	(224)		(316)
Net cash used in operating activities	(24,124)		(16,412)
Cash flow from investing activities
Purchases of short-term and long-term investments	(90,671)		(31,440)
Proceeds from maturities of short-term investments	60,415		16,085
Purchases of property and equipment	(2,358)		(4,381)
Net cash used in investing activities	(32,614)		(19,736)
Cash flow from financing activities
Proceeds from initial public offering, net of issuance costs	125,114
Proceeds from issuance of common stock upon exercise of stock options, net of repurchases	3,331		608
Payment of preferred stock issuance costs	(135)
Net cash provided by financing activities	128,310		608
Net decrease in cash and cash equivalents	71,572		(35,540)
Cash and cash equivalents at beginning of period	98,426	[1]	65,160
Cash and cash equivalents at end of period	169,998		29,620
Non-cash investing and financing activities:
Unpaid portion of property and equipment purchases included in accounts payable and accrued liabilities	1,007		495
Vesting of early exercised stock options and restricted stock	$ 647		$ 86

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