Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Cash flows from operating activities:
Net loss	$ (6,883)	$ (11,859)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	884	1,330
Write-off of abandoned patents	50	331
Loss on sale of property and equipment	2	0
Compensation expense related to stock options and awards	1,534	2,451
Purchase of technology license with common stock	775	0
Changes in operating assets and liabilities:
Receivables and other assets	(1,403)	(99)
Accounts payable and accrued expenses	(1,885)	1,050
Deferred revenue	2,110	(113)
Deferred rent	(313)	(266)
Net cash used in operating activities	(5,129)	(7,175)
Cash flows from investing activities:
Maturities of marketable securities	15,030	8,000
Purchases of marketable securities	(11,500)	(20,437)
Purchases of property and equipment	(58)	(43)
Patent expenditures	(53)	(240)
Net cash provided by (used in) investing activities	3,419	(12,720)
Cash flows from financing activities:
Net proceeds from issuance of common stock	3	3,776
Payment of withholding taxes for net settlement of restricted stock units	(23)	(58)
Net cash (used in) provided by financing activities	(20)	3,718
Net decrease in cash and cash equivalents	(1,730)	(16,177)
Cash and cash equivalents at beginning of period	20,471	38,837
Cash and cash equivalents at end of period	$ 18,741	$ 22,660