CONSOLIDATED BALANCE SHEETS (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Current assets:
Cash and cash equivalents	$ 51,405	$ 70,228
Marketable securities	71,407	76,174
Billed and unbilled collaboration receivables	104	1,468
Prepaid expenses and other current assets	2,540	4,158
Total current assets	125,456	152,028
Marketable securities	103,416	114,407
Investment in equity securities of Regulus Therapeutics Inc	38,748
Property and equipment, net	19,799	14,643
Other assets	101	839
Total assets	287,520	281,917
Current liabilities:
Accounts payable	4,420	5,800
Accrued expenses	11,558	12,340
Deferred rent	950	484
Deferred revenue	31,417	62,366
Total current liabilities	48,345	80,990
Deferred rent, net of current portion	4,248	3,727
Deferred revenue, net of current portion	100,874	78,487
Other long-term liabilities		716
Total liabilities	153,467	163,920
Commitments and contingencies (Note 6)
Stockholders' equity:
Preferred stock, $0.01 par value per share, 5,000,000 shares authorized and no shares issued and outstanding at December 31, 2012 and 2011
Common stock, $0.01 par value per share, 125,000,000 shares authorized; 52,489,936 shares issued and outstanding at December 31, 2012; 42,721,942 shares issued and outstanding at December 31, 2011	525	427
Additional paid-in capital	624,876	518,731
Accumulated other comprehensive income (loss)	15,662	(165)
Accumulated deficit	(507,010)	(400,996)
Total stockholders' equity	134,053	117,997
Total liabilities and stockholders' equity	$ 287,520	$ 281,917