Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 71,764	$ 14,031
Marketable securities	245,238	328,034
Collaboration receivable - related party	8,225	6,492
Tenant improvement and other receivables	3,374	2,334
Prepaid expenses and other current assets	8,728	4,814
Refundable income taxes		3,841
Total current assets	337,329	359,546
Marketable securities	58,905	125,382
Property and equipment, net	23,220	6,386
Other assets	611	590
Total assets	420,065	491,904
Current liabilities:
Accounts payable	14,748	11,067
Accrued expenses	15,996	14,020
Deferred revenue - related party	19,665	35,686
Deferred rent	2,479	310
Other current liabilities		6
Total current liabilities	52,888	61,089
Deferred revenue, net of current portion - related party	4,699	2,725
Deferred rent, net of current portion	$ 17,360	$ 3,724
Commitments and contingencies (Note 6)
Stockholders' equity:
Preferred stock, $0.001 par value; 25,000,000 shares authorized, no shares issued and outstanding at December 31, 2015 and 2014
Common stock, $0.001 par value; 125,000,000 shares authorized and 37,696,502 and 37,100,513 shares issued and outstanding at December 31, 2015 and 2014, respectively	$ 38	$ 37
Additional paid-in capital	630,078	591,334
Accumulated other comprehensive loss	(318)	(57)
Accumulated deficit	(284,680)	(166,948)
Total stockholders' equity	345,118	424,366
Total liabilities and stockholders' equity	$ 420,065	$ 491,904