Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
CASH FLOWS FROM OPERATING ACTIVITIES
Net loss	$ (8,119)	$ (6,247)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	70	52
Purchase net premium on marketable securities		(55)
Net (accretion of discount) amortization of premium on marketable securities	(14)	49
Stock-based compensation	835	401
Research and development services settled with stock	30
Other, net	(11)	(5)
Changes in operating assets and liabilities:
Accounts receivable-grant	(295)	234
Prepaid expenses and other assets	(102)	151
Accounts payable	396	95
Deferred revenue	(20)	(20)
Accrued and other liabilities	(217)	(87)
Net cash used in operating activities	(7,447)	(5,432)
CASH FLOWS FROM INVESTING ACTIVITIES
Purchases of property and equipment	(83)	(271)
Purchases of marketable securities		(32,780)
Proceeds from maturities of marketable securities	4,785	7,440
Net cash provided by (used in) investing activities	4,702	(25,611)
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from employee stock plan purchases and stock option exercises	688	96
Net cash provided by financing activities	688	96
NET DECREASE IN CASH AND CASH EQUIVALENTS	(2,057)	(30,947)
CASH AND CASH EQUIVALENTS
Beginning of period	12,817	47,748
End of period	$ 10,760	16,801
Supplemental Disclosure of Non-Cash Investing and Financing Information:
Unpaid amounts related to purchase of property and equipment		$ 4