Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Operating activities:
Net loss	$ (32,499)	$ (13,687)
Adjustments to reconcile net loss to net cash used in operating activities:
Amortization of discount on marketable securities	(347)	(526)
Depreciation	502	346
Stock-based compensation	4,389	2,205
Deferred rent	0	472
Non-cash lease expense	131	0
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(462)	471
Deferred revenue	0	(5,160)
Accounts payable	(227)	(35)
Accrued liabilities	1,461	(492)
Lease liability	(420)	0
Net cash used in operating activities	(27,472)	(16,406)
Investing activities:
Purchases of property and equipment	(178)	(1,376)
Maturities of marketable securities	84,761	34,410
Purchases of marketable securities	(37,745)	(36,536)
Net cash provided by (used in) investing activities	46,838	(3,502)
Financing activities:
Proceeds from issuances of common stock upon exercise of options and participation in employee stock purchase plan	524	7
Net cash provided by financing activities	524	7
Net increase (decrease) in cash, cash equivalents and restricted cash	19,890	(19,901)
Cash, cash equivalents and restricted cash at beginning of period	40,104	35,138
Cash, cash equivalents and restricted cash, at end of period	59,994	15,237
Supplemental disclosures of cash flow information
Non cash tenant improvement allowance used for leasehold improvements	0	8,324
Purchases of property and equipment accrued but not yet paid	$ 0	$ 293