CONSOLIDATED BALANCE SHEETS (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
ASSETS
Cash and cash equivalents	$ 111,524	$ 71,301
Short-term investments	159,957	224,939
Receivable from collaboration partner	40,142	35,458
Restricted cash	343	343
Prepaid expenses and other current assets	13,069	12,175
Total current assets	325,035	344,216
Property and equipment, net	16,472	13,262
Restricted cash, net of current	8,843	8,843
Other non-current assets	5,443	5,545
Total assets	355,793	371,866
LIABILITIES AND STOCKHOLDERS' EQUITY
Accounts payable	3,157	2,073
Accrued expenses and other current liabilities	54,927	48,951
Deferred revenue	33,862	33,862
Total current liabilities	91,946	84,886
Convertible Notes, net of unamortized discount of $59,802 and $62,743 at March 31, 2013 and December 31, 2012, respectively	198,948	196,007
Deferred revenue, net of current		8,465
Other non-current liabilities	8,323	8,863
Total liabilities	299,217	298,221
Commitments and contingencies (Note 11)
Stockholders' equity:
Preferred stock, $0.01 par value per share; 1,000,000 shares authorized; no shares issued and outstanding
Common stock, $0.01 par value per share; 170,000,000 shares authorized; 74,957,571 and 74,774,939 shares issued and outstanding at March 31, 2013 and December 31, 2012, respectively	750	748
Additional paid-in capital	374,528	364,412
Accumulated other comprehensive income	16	33
Accumulated deficit	(318,718)	(291,548)
Total stockholders' equity	56,576	73,645
Total liabilities and stockholders' equity	$ 355,793	$ 371,866