CONDENSED BALANCE SHEETS (Unaudited) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Current assets:
Cash and cash equivalents	$ 14,974	$ 14,327
Marketable securities-short term	72,134	64,944
Prepaid expenses and other current assets	825	344
Total current assets	87,933	79,615
Marketable securities-long term	32,533	51,328
Property and equipment, net	1,757	1,992
Other assets	1,177	1,258
Total assets	123,400	134,193
Current liabilities:
Accounts payable and accrued expenses	7,482	10,163
Note payable	1,700	1,700
Current portion of deferred revenue	14,162	14,232
Current portion of deferred gain on sale leaseback	552	552
Total current liabilities	23,896	26,647
Deferred revenue, net of current portion	22,232	25,733
Deferred gain on sale leaseback, net of current portion	644	784
Total liabilities	46,772	53,164
Commitments and contingencies
Stockholders' equity:
Preferred stock, $0.01 par value; 1,000,000 shares authorized; no shares issued or outstanding
Common stock, $0.01 par value; 100,000,000 shares authorized; 62,623,782 and 62,399,827 shares issued and outstanding at March 31, 2013 and December 31, 2012, respectively	626	624
Additional paid-in capital	502,018	500,655
Accumulated other comprehensive income	111	102
Accumulated deficit	(426,127)	(420,352)
Total stockholders' equity	76,628	81,029
Total liabilities and stockholders' equity	$ 123,400	$ 134,193