Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
CASH FLOWS FROM OPERATING ACTIVITIES
Net loss	$ (56,759)	$ (29,450)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	312	221
Purchase net discount on marketable securities	498	539
Net accretion of discount on marketable securities	(749)	(311)
Stock-based compensation	3,692	2,903
Research and development services settled with stock		107
Non-cash operating lease expense	337
Other, net		(30)
Changes in operating assets and liabilities:
Accounts receivable - grant		656
Prepaid expenses and other assets	(1,700)	(1,520)
Accounts payable	1,834	727
Deferred revenue		(20)
Operating lease liabilities	(165)
Accrued and other liabilities	3,302	1,185
Net cash used in operating activities	(49,398)	(24,993)
CASH FLOWS FROM INVESTING ACTIVITIES
Purchases of property and equipment	(318)	(219)
Purchases of marketable securities	(87,676)	(35,500)
Proceeds from maturities of marketable securities	75,929	22,620
Net cash used in investing activities	(12,065)	(13,099)
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from issuance of common stock and pre-funded warrants in public offering, net of offering costs	64,239	37,654
Proceeds from employee stock plan purchases and stock option exercises	826	2,153
Principal payments on finance lease obligation	(18)
Net cash provided by financing activities	65,047	39,807
NET INCREASE IN CASH, CASH EQUIVALENTS, AND RESTRICTED CASH	3,584	1,715
CASH, CASH EQUIVALENTS, AND RESTRICTED CASH
Beginning of period	22,461	12,817
End of period	26,045	14,532
Supplemental Disclosure of Non-Cash Investing and Financing Information:
Leased assets obtained in exchange for lease obligations	5,294
Unpaid amounts related to purchase of property and equipment	$ 26	$ 51