Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
CASH FLOWS FROM OPERATING ACTIVITIES
Net loss	$ (29,450)	$ (20,753)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	221	183
Purchase net discount on marketable securities	539	2
Net (accretion of discount) amortization of premium on marketable securities	(311)	82
Stock-based compensation	2,903	1,794
Research and development services settled with stock	107	15
Other, net	(30)	(15)
Changes in operating assets and liabilities:
Accounts receivable-grant	656	(1,237)
Prepaid expenses and other assets	(1,520)	275
Accounts payable	727	447
Deferred revenue	(20)	(21)
Accrued and other liabilities	1,185	486
Net cash used in operating activities	(24,993)	(18,742)
CASH FLOWS FROM INVESTING ACTIVITIES
Purchases of property and equipment	(219)	(428)
Purchases of marketable securities	(35,500)	(62,115)
Proceeds from maturities of marketable securities	22,620	35,170
Net cash used in investing activities	(13,099)	(27,373)
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from issuance of common stock in public offering, net of offering costs	37,654	11,380
Proceeds from employee stock plan purchases and stock option exercises	2,153	149
Net cash provided by financing activities	39,807	11,529
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	1,715	(34,586)
CASH AND CASH EQUIVALENTS
Beginning of period	12,817	47,748
End of period	14,532	$ 13,162
Supplemental Disclosure of Non-Cash Investing and Financing Information:
Unpaid amounts related to purchase of property and equipment	$ 51