STATEMENTS OF CASH FLOWS (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Operating Activities:
Net loss	$ (9,996)	$ (7,336)
Reconciliation of net loss to net cash used in operating activities:
Change in fair value of warrant liability		(101)
Depreciation and amortization	449	404
Amortization of net premiums on investments	94
Gain on disposal of property and equipment	(17)
Deferred rent	240	336
Non-cash stock-based compensation	483	587
Changes in operating assets and liabilities:
Restricted cash	769
Accounts receivables	(1,591)	240
Unbilled receivables	(1)	427
Prepaid expenses and other assets	54	183
Accounts payable and accrued expenses	(1,590)	(517)
Deferred revenue	(180)
Lease incentives received	703	1,557
Net cash used in operating activities	(10,583)	(4,220)
Investing Activities:
Capital expenditures	(1,542)	(772)
Proceeds from disposal of property and equipment	51
Proceeds from maturities of investments	5,235
Purchases of investments	(8,718)	(2,498)
Net cash used in investing activities	(4,974)	(3,270)
Financing Activities:
Principal payments of capital lease	(14)
Principal payments of notes payable	(39)	(13)
Proceeds from notes payable	809	100
Restricted cash	(1)	(755)
Net proceeds from sales of common stock, net of offering costs of $0.2 million and $0.1 million, respectively	6,425	7,923
Proceeds from the exercise of stock options	1	4
Net cash provided by financing activities	7,181	7,259
Net decrease in cash and cash equivalents	(8,376)	(231)
Cash and cash equivalents at beginning of period	17,399	14,104
Cash and cash equivalents at end of period	9,023	13,873
Supplemental disclosure of non-cash activities:
Sale of common stock under the 2012 Sales Agreement not settled at quarter-end	376
Property and equipment purchases included in accounts payable and accrued expenses	422	224
Deposit applied towards the purchase of laboratory equipment		500
Supplemental disclosure of cash flow information:
Cash payments of interest	$ 20