STATEMENTS OF CASH FLOWS (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Operating Activities:
Net loss	$ (22,630)	$ (13,256)
Reconciliation of net loss to net cash used in operating activities:
Change in fair value of warrant liability	(267)
Depreciation and amortization	961	810
Amortization of net premiums on investments	232
Gain on disposal of property and equipment	(43)	(19)
Deferred rent	458	251
Non-cash stock-based compensation	1,121	1,176
Changes in operating assets and liabilities:
Restricted cash	986
Accounts receivables	104	467
Unbilled receivables	(245)	(2,395)
Prepaid expenses and other assets	702	58
Accounts payable and accrued expenses	(40)	461
Deferred revenue	(258)
Lease incentives received	703	2,101
Net cash used in operating activities	(18,216)	(10,346)
Investing Activities:
Capital expenditures	(3,157)	(1,076)
Proceeds from disposal of property and equipment	81	167
Proceeds from maturities of investments	9,649	2,500
Purchases of investments	(14,754)	(9,389)
Net cash used in investing activities	(8,181)	(7,798)
Financing Activities:
Principal payments of capital lease	(29)
Principal payments of notes payable	(136)	(20)
Proceeds from notes payable	1,450	100
Restricted cash	(1)	(755)
Net proceeds from sales of common stock, net of offering costs of $0.3 million and $0.3 million, respectively	10,018	20,023
Proceeds from the exercise of stock options	270	26
Net cash provided by financing activities	11,572	19,374
Net (decrease) increase in cash and cash equivalents	(14,825)	1,230
Cash and cash equivalents at beginning of period	17,399	14,104
Cash and cash equivalents at end of period	2,574	15,334
Supplemental disclosure of non-cash activities:
Property and equipment purchases included in accounts payable and accrued expenses	1,026	278
Deposit applied towards the purchase of laboratory equipment		500
Supplemental disclosure of cash flow information:
Cash payments of interest	$ 61