Balance Sheets (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Current assets:
Cash and cash equivalents	$ 60,385	$ 68,204
Receivables	27	145
Prepaid expenses and other current assets	1,589	1,948
Total current assets	62,001	70,297
Property and equipment, net	679	801
Deposits	128	128
Other non-current assets	517	528
Total assets	63,325	71,754
Current liabilities:
Accounts payable	578	422
Accrued expenses	5,752	6,357
Deferred revenue - current portion	800	800
Deferred rent - current portion	224	212
Total current liabilities	7,354	7,791
Deferred revenue	1,733	1,933
Deferred rent	792	851
Warrant liabilities	11,694	11,776
Other long- term liabilities	20	20
Total liabilities	21,593	22,371
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; 100,600,666 and 100,159,618 shares issued and outstanding at March 31, 2014 and December 31, 2013, respectively	101	100
Additional paid-in capital - common stock	388,570	386,511
Additional paid-in capital - warrants issued	3,603	3,603
Deficit accumulated during the development stage	(350,542)	(340,831)
Total stockholders' equity	41,732	49,383
Total liabilities and stockholders' equity	$ 63,325	$ 71,754