STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities
Net loss	$ (46,672)	$ (46,939)	$ (55,663)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	743	847	842
Accretion related to marketable securities	(644)	(608)	(195)
Stock-based compensation	7,348	7,135	6,229
Changes in operating assets and liabilities:
Prepaid and other current assets	(370)	187	(44)
Operating lease right-of-use asset	599
Other assets	(50)	406
Accounts payable	450	(1,456)	1,554
Accrued and other liabilities	2,030	(458)	1,499
Operating lease liability	(767)
Other long-term liabilities	12	(102)	(434)
Net cash used in operating activities	(37,321)	(40,988)	(46,212)
Cash flows from investing activities
Purchases of marketable securities	(138,586)	(161,861)	(88,309)
Maturities of marketable securities	141,866	132,024	173,401
Purchases of property and equipment	(25)	(355)	(266)
Net cash provided by (used in) investing activities	3,255	(30,192)	84,826
Cash flows from financing activities
Proceeds from issuance of common stock, net (includes $30,850 in aggregate gross proceeds from related parties for the year ended December 31, 2018)		64,877	711
Proceeds from exercise of common stock options	24	393	731
Net cash provided by financing activities	24	65,270	1,442
Net decrease in cash and cash equivalents	(34,042)	(5,910)	40,056
Cash and cash equivalents at beginning of the period	39,196	45,106	5,050
Cash and cash equivalents at end of the period	$ 5,154	39,196	$ 45,106
Supplemental disclosures of cash flow information
Purchases of property and equipment incurred but not paid		$ 84