Condensed Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Cash flows from operating activities
Net loss	$ (12,043)	$ (9,569)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization	336	293
Net amortization of premiums and discounts on marketable securities	5	(236)
Stock-based compensation	758	383
Loss on sale of property and equipment	2	3
Gain on sales of marketable securities	(2)	(1)
Changes in assets and liabilities
Prepaid expenses and other assets	1,097	(151)
Right-of-use assets	229	256
Accounts payable	(36)	242
Accrued and other liabilities	384	(727)
Lease liabilities	(275)	(278)
Net cash used in operating activities	(9,545)	(9,785)
Cash flows from investing activities
Purchases of property and equipment, net		(606)
Purchases of marketable securities	(11,253)	(19,326)
Maturities of marketable securities	22,159	39,340
Sales of marketable securities		6,000
Net cash provided by investing activities	10,906	25,408
Cash flows from financing activities
Proceeds from exercise of common stock options	34	1
Payments of deferred offering costs		(827)
Net cash provided by (used in) financing activities	34	(826)
Net increase in cash, cash equivalents and restricted cash	1,395	14,797
Cash, cash equivalents and restricted cash, at beginning of period	34,173	20,611
Cash, cash equivalents and restricted cash, at end of period	35,568	35,408
Reconciliation of cash, cash equivalents and restricted cash
Cash and cash equivalents	35,462	35,302
Restricted cash	106	106
Cash, cash equivalents and restricted cash, at end of period	35,568	35,408
Supplemental disclosure of cash flow information:
Cash paid for income taxes	1	1
Cash paid for interest	21	24
Supplemental non-cash investing and financing activities:
Unpaid deferred offering costs		375
Vesting of early exercised options and restricted stock	16	74
Purchases of property and equipment in accounts payable and accrued liabilities	$ 33	$ 481