STATEMENTS OF CASH FLOWS (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Operating Activities:
Net loss	$ (7,336)	$ (7,453)
Reconciliation of net loss to net cash used in operating activities:
Change in fair value of warrant liability	(101)	67
Depreciation and amortization	404	389
Amortization of net premiums on short-term investments		150
Deferred rent	336	(83)
Non-cash stock-based compensation	587	432
Changes in operating assets and liabilities:
Accounts receivables	240	37
Unbilled receivables	427	(834)
Prepaid expenses and other assets	183	(95)
Accounts payable and accrued expenses	40	(1,651)
Lease incentives received	1,000
Net cash used in operating activities	(4,220)	(9,041)
Investing Activities:
Capital expenditures	(772)	(41)
Proceeds from maturities of short-term investments		8,525
Purchases of short-term investments	(2,498)	(499)
Net cash (used in) provided by investing activities	(3,270)	7,985
Financing Activities:
Principal payments of notes payable	(13)	(20)
Proceeds from notes payable	100
Restricted cash	(755)
Net proceeds from sales of common stock, net of offering costs of $0.1 million and less than $0.1 million, respectively	7,923	1,317
Proceeds from the exercise of stock options	4	56
Net cash provided by financing activities	7,259	1,353
Net (decrease) increase in cash and cash equivalents	(231)	297
Cash and cash equivalents at beginning of period	14,104	8,061
Cash and cash equivalents at end of period	13,873	8,358
Supplemental disclosure of non-cash activities:
Deposit included in other current assets at December 31, 2011 applied towards the purchase of laboratory equipment	500
Equipment purchases included in accounts payable and accrued expenses	$ 224