Condensed Balance Sheets (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
CURRENT ASSETS:
Cash and cash equivalents	$ 58,710	$ 53,613
Short-term investments	116,066	77,656
Receivables from collaborations	227	662
Prepaid expenses and other current assets	3,669	2,982
Total current assets	178,672	134,913
Property and equipment, net	2,428	2,861
Long-term investments	44,151	6,115
Other assets	116	2,112
TOTAL ASSETS	225,367	146,001
CURRENT LIABILITIES:
Accounts payable	1,517	4,840
Accrued compensation and employee benefits	1,863	1,860
Accrued and other liabilities	15,960	7,399
Deferred revenue – current portion	3,930	4,042
Convertible preferred stock warrant liability		683
Total current liabilities	23,270	18,824
Deferred revenue – noncurrent portion	3,337
Other long-term liabilities	811	1,466
TOTAL LIABILITIES	27,418	20,290
Commitments and contingencies
Convertible preferred stock, $0.001 par value. 0 and 243,258,300 shares authorized at September 30, 2013 and December 31, 2012; 0 shares and 24,026,797 shares issued and outstanding at September 30, 2013 and December 31, 2012		317,280
STOCKHOLDERS’ EQUITY (DEFICIT):
Preferred stock, $0.001 par value. 5,000,000 shares authorized at September 30, 2013; 0 issued and outstanding at September 30, 2013; 0 authorized, issued or outstanding at December 31, 2012
Common stock, $0.001 par value. 100,000,000 and 300,000,000 shares authorized; 35,229,352 and 1,385,508 shares issued and outstanding at September 30, 2013 and December 31, 2012	35	1
Additional paid-in capital	458,460	10,717
Accumulated deficit	(260,610)	(202,320)
Accumulated other comprehensive income (loss)	64	33
Total stockholders’ equity (deficit)	197,949	(191,569)
TOTAL LIABILITIES, CONVERTIBLE PREFERRED STOCK, AND STOCKHOLDERS’ EQUITY	$ 225,367	$ 146,001