CONDENSED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Cash flows from operating activities:
Net loss	$ (4,981)	$ (4,551)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	27	47
Amortization of premium on marketable securities	26	189
Amortization of deferred gain on sale leaseback		(140)
Non-cash stock compensation	594	818
Gain on sale of property and equipment		(228)
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(115)	525
Accounts payable and accrued expenses	(843)	(1,441)
Deferred revenue	(1,140)	(2,773)
Net cash used in operating activities	(6,432)	(7,554)
Cash flows from investing activities:
Purchases of marketable securities	(13,386)	(3,387)
Proceeds from sale or maturity of marketable securities	10,498	18,570
Additions to property and equipment		(52)
Proceeds from sale of property and equipment		99
Net cash provided by (used in) investing activities	(2,888)	15,230
Cash flows from financing activities:
Proceeds from stock offering, net	15,185
Proceeds from stock option exercises and employee stock plan purchases	114
Net cash provided by financing activities	15,299
Net increase in cash and cash equivalents	5,979	7,676
Cash and cash equivalents, beginning of period	13,983	12,525
Cash and cash equivalents, end of period	$ 19,962	$ 20,201