Condensed Balance Sheets (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
CURRENT ASSETS:
Cash and cash equivalents	$ 112,246	$ 53,613
Short-term investments	95,358	77,656
Receivables from collaborations	3,494	662
Prepaid expenses and other current assets	3,970	2,982
Total current assets	215,068	134,913
Property and equipment, net	2,516	2,861
Long-term investments	27,586	6,115
Other assets	220	2,112
TOTAL ASSETS	245,390	146,001
CURRENT LIABILITIES:
Accounts payable	4,676	4,840
Accrued compensation and employee benefits	1,673	1,860
Accrued and other liabilities	16,325	7,399
Deferred revenue – current portion	6,696	4,042
Convertible preferred stock warrant liability		683
Total current liabilities	29,370	18,824
Deferred revenue – noncurrent portion	337
Other long-term liabilities	1,087	1,466
TOTAL LIABILITIES	30,794	20,290
Commitments and contingencies
Convertible preferred stock, $0.001 par value. Zero and 243,258,300 shares authorized at June 30, 2013 and December 31, 2012; Zero shares and 24,026,797 shares issued and outstanding at June 30, 2013 and December 31, 2012		317,280
STOCKHOLDERS’ EQUITY (DEFICIT):
Preferred stock, $0.001 par value. 5,000,000 shares authorized at June 30, 2013; Zero issued and outstanding
Common stock, $0.001 par value. 100,000,000 and 300,000,000 shares authorized; 35,171,769 and 1,385,508 shares issued and outstanding at June 30, 2013 and December 31, 2012	35	1
Additional paid-in capital	456,657	10,717
Accumulated deficit	(242,060)	(202,320)
Accumulated other comprehensive income (loss)	(36)	33
Total stockholders’ equity (deficit)	214,596	(191,569)
TOTAL LIABILITIES, CONVERTIBLE PREFERRED STOCK, AND STOCKHOLDERS’ EQUITY	$ 245,390	$ 146,001