STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Cash flows from operating activities
Net loss	$ (12,935)	$ (11,643)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	184	186
Accretion related to marketable securities	(64)	(170)
Stock-based compensation	1,845	1,953
Changes in operating assets and liabilities:
Prepaid and other current assets	226	(205)
Operating lease right-of-use asset	164	146
Other assets		(51)
Accounts payable	908	266
Accrued and other liabilities	530	714
Operating lease liability	(211)	(186)
Net cash used in operating activities	(9,353)	(8,990)
Cash flows from investing activities
Purchases of marketable securities	(11,411)	(29,903)
Sales of marketable securities	1,009
Maturities of marketable securities	38,099	38,688
Purchases of property and equipment		(16)
Net cash provided by investing activities	27,697	8,769
Cash flows from financing activities
Proceeds from exercise of common stock options		1
Net cash provided by financing activities		1
Net increase (decrease) in cash and cash equivalents	18,344	(220)
Cash and cash equivalents at beginning of the period	5,154	39,196
Cash and cash equivalents at end of the period	$ 23,498	38,976
Supplemental disclosures of cash flow information
Purchases of property and equipment incurred but not paid		$ 16