Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Current assets:
Cash and cash equivalents	$ 26,736	$ 33,971
Investments in marketable securities	58,015	65,151
Receivable due from GSK	1,308	3,225
Prepaid expenses and other current assets	1,753	2,270
Total current assets	87,812	104,617
Property and equipment, less accumulated depreciation and amortization of $8,501 and $8,872 at December 31, 2012 and March 31, 2013, respectively	4,962	5,029
Other non-current assets	442	442
Total Assets	93,216	110,088
Current liabilities:
Accounts payable and accrued expenses	6,427	8,845
Current portion of secured loan	398	398
Warrant liability	1,170
Total current liabilities	7,995	9,243
Deferred reimbursements	31,685	30,418
Warrant liability, non-current		908
Secured loan, less current portion	199	299
Commitments and contingencies
Stockholders' equity:
Common stock, $.01 par value, 125,000,000 shares authorized, 49,631,672 shares issued and outstanding at December 31, 2012, 125,000,000 shares authorized, 49,631,672 shares issued and outstanding at March 31, 2013	556	556
Additional paid-in capital	389,113	387,539
Accumulated other comprehensive income	15	14
Deficit accumulated during the development stage	(336,347)	(318,889)
Total stockholders' equity	53,337	69,220
Total Liabilities and Stockholders' Equity	$ 93,216	$ 110,088