STATEMENTS OF CASH FLOWS (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011
Operating Activities:
Net loss	$ (20,472)	$ (15,659)
Reconciliation of net loss to net cash used in operating activities:
Change in fair value of warrant liability	401	(1,973)
Depreciation and amortization	1,216	1,197
Amortization of net premiums on short-term investments		317
Impairment of property and equipment		60
Gain on disposal of property and equipment	(26)
Deferred rent	436	(254)
Non-cash stock-based compensation	1,668	1,677
Changes in operating assets and liabilities:
Accounts receivables	(504)	37
Unbilled receivables	111	(3,848)
Prepaid expenses and other assets	(271)	(1,127)
Accounts payable and accrued expenses	(304)	(3,687)
Deferred revenue	803	2,500
Lease incentives received	2,803
Net cash used in operating activities	(14,139)	(20,760)
Investing Activities:
Capital expenditures	(2,202)	(414)
Proceeds from disposal of property and equipment	318
Proceeds from maturities of short-term investments	2,500	20,235
Purchases of short-term investments	(15,763)	(2,082)
Net cash (used in) provided by investing activities	(15,147)	17,739
Financing Activities:
Principal payments of capital lease	(90)
Principal payments of notes payable	(20)	(60)
Proceeds from notes payable	650
Restricted cash	(1,594)
Net proceeds from sales of common stock, net of offering costs of $0.4 million and $0.2 million, respectively	26,925	9,182
Proceeds from the exercise of stock options	49	156
Net cash provided by financing activities	25,920	9,278
Net (decrease) increase in cash and cash equivalents	(3,366)	6,257
Cash and cash equivalents at beginning of period	14,104	8,061
Cash and cash equivalents at end of period	10,738	14,318
Supplemental disclosure of non-cash activities:
Equipment acquired under a capital lease	399
Deposit applied towards the purchase of laboratory equipment	500
Property and equipment purchases included in accounts payable and accrued expenses	1,365	168
Settlement of notes receivable		$ 1,522