BALANCE SHEETS (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Current assets:
Cash and cash equivalents	$ 128,008	$ 104,713
Collaboration receivable		79
Prepaid expenses and other current assets	10,803	1,313
Total current assets	138,811	106,105
Property and equipment, net	1,177	1,141
Deposits	91	91
Other non-current assets	794	771
Total assets	140,873	108,108
Current liabilities:
Accounts payable	1,984	1,727
Accrued expenses	11,534	10,821
Deferred revenue - current portion	800	800
Deferred rent - current portion	12	15
Total current liabilities	14,330	13,363
Deferred revenue	3,333	3,533
Deferred rent	316	180
Warrant liabilities	25,194	19,425
Total liabilities	43,173	36,501
Commitments and contingencies (note 7)
Stockholders' equity:
Preferred stock, $0.001 par value; 30,000,000 shares authorized and no shares issued and outstanding
Common stock, $0.001 par value; 250,000,000 shares authorized; 79,532,882 and 69,206,044 shares issued and outstanding at March 31, 2012 and December 31, 2011, respectively	80	69
Additional paid-in capital - common stock	302,603	246,519
Additional paid-in capital - warrants issued	7,079	12,611
Deficit accumulated during the development stage	(212,062)	(187,592)
Total stockholders' equity	97,700	71,607
Total liabilities and stockholders' equity	$ 140,873	$ 108,108