Balance Sheets (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Current assets:
Cash and cash equivalents	$ 73,306	$ 104,713
Receivables	58	79
Prepaid expenses and other current assets	6,912	1,313
Total current assets	80,276	106,105
Property and equipment, net	1,994	1,141
Deposits	133	91
Other non current assets	1,001	771
Total assets	83,404	108,108
Current liabilities:
Accounts payable	1,509	1,727
Accrued expenses	16,516	10,821
Deferred revenue-current portion	800	800
Deferred rent-current portion	39	15
Total current liabilities	18,864	13,363
Deferred revenue	2,733	3,533
Deferred rent	400	180
Warrant liabilities	12,962	19,425
Total liabilities	34,959	36,501
Commitments and contingencies (note 8)
Stockholders' equity:
Common stock, $0.001 par value; 250,000,000 shares authorized; 83,236,840 and 69,206,044 shares issued and outstanding at December 31, 2012 and 2011, respectively	83	69
Preferred stock, $0.001 par value; 30,000,000 shares authorized and no shares issued and outstanding
Additional paid-in capital-common stock	325,177	246,519
Additional paid-in capital-warrants issued	6,909	12,611
Deficit accumulated during the development stage	(283,724)	(187,592)
Total stockholders' equity	48,445	71,607
Total liabilities and stockholders' equity	$ 83,404	$ 108,108