CONDENSED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Cash flows from operating activities:
Net loss	$ (14,777)	$ (10,081)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	38	53
Amortization of premium (discount) on marketable securities	(9)	40
Amortization of debt discount	150
Non-cash stock compensation	828	1,015
Changes in operating assets and liabilities:
Prepaid expenses and other assets	449	294
Accounts payable and accrued expenses	(1,439)	1
Deferred revenue		(2,290)
Net cash used in operating activities	(14,760)	(10,968)
Cash flows from investing activities:
Purchases of marketable securities	(14,076)	(21,648)
Proceeds from sale or maturity of marketable securities	15,827	20,765
Additions to property and equipment		(15)
Net cash provided by (used in) investing activities	1,751	(898)
Cash flows from financing activities:
Proceeds from notes payable and warrants, net	14,624
Proceeds from stock offering, net		15,174
Proceeds from employee stock option exercises and employee stock purchase plan purchases	17	163
Net cash provided by financing activities	14,641	15,337
Net increase in cash and cash equivalents	1,632	3,471
Cash and cash equivalents, beginning of period	15,267	13,983
Cash and cash equivalents, end of period	$ 16,899	$ 17,454