CONDENSED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
Cash flows from operating activities:
Net loss	$ (10,081)	$ (8,568)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	53	91
Amortization of premium on marketable securities	40	322
Amortization of deferred gain on sale leaseback		(232)
Non-cash stock compensation	1,015	1,240
Gain on sale of property and equipment		(282)
Changes in operating assets and liabilities:
Prepaid expenses and other assets	294	1,196
Accounts payable and accrued expenses	1	(1,210)
Deferred revenue	(2,290)	(5,513)
Net cash used in operating activities	(10,968)	(12,956)
Cash flows from investing activities:
Purchases of marketable securities	(21,648)	(11,807)
Proceeds from sale or maturity of marketable securities	20,765	26,339
Additions to property and equipment	(15)	(280)
Proceeds from sale of property and equipment		228
Net cash provided by (used in) investing activities	(898)	14,480
Cash flows from financing activities:
Proceeds from stock offering, net	15,174
Proceeds from employee stock option exercises and employee stock purchase plan purchases	163	62
Net cash provided by financing activities	15,337	62
Net increase in cash and cash equivalents	3,471	1,586
Cash and cash equivalents, beginning of period	13,983	12,525
Cash and cash equivalents, end of period	$ 17,454	$ 14,111