CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (USD $) In Thousands, unless otherwise specified	6 Months Ended
Jun. 30, 2014	Jun. 30, 2013
Cash flows from operating activities:
Net loss	$ (15,248)	$ (11,393)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	206	402
Stock-based compensation	2,522	1,849
Amortization of intangible assets	500	500
Gain on sale of assets		(630)
Other non-cash items		5
Changes in assets and liabilities:
Accounts receivable	(159)	206
Prepaid expenses and other assets	(226)	(101)
Inventories	127	(125)
Deferred revenue	(67)	(240)
Accounts payable	286	27
Accrued expenses and other liabilities	135	(1,961)
Net cash used in operating activities	(11,924)	(11,461)
Cash flows from investing activities:
Purchases of property and equipment	(104)	(84)
Net proceeds from sale of assets		630
Proceeds from sales and maturities of marketable securities	22,807	31,018
Purchases of marketable securities	(13,792)	(43,671)
Net cash provided by (used in) investing activities	8,911	(12,107)
Cash flows from financing activities:
Proceeds from issuance of common stock	1,689	1,321
Proceeds from sale of common stock, net of issuance costs		27,890
Net cash provided by financing activities	1,689	29,211
Net increase (decrease) in cash and cash equivalents	(1,324)	5,643
Cash and cash equivalents at beginning of period	4,364	2,529
Cash and cash equivalents at end of period	$ 3,040	$ 8,172

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The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.