Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Current Assets
Cash and cash equivalents	$ 118,783	$ 201,153
Prepaid expenses	4,267	4,152
Other current assets	326
Total current assets	123,376	205,305
Restricted cash	2,382
Property and equipment, net	861	265
Build-to-suit lease asset	8,952
Total assets	135,571	205,570
Current liabilities
Accounts payable	4,233	1,357
Accrued liabilities	36,670	12,899
Income taxes payable		247
Upfront payment from collaboration partner (Note 5)	40,000	40,000
Total current liabilities	80,903	54,503
Build-to-suit lease obligation	5,428
Total liabilities	86,331	54,503
Commitments and contingencies (Note 6)
Stockholders' equity
Common stock, $0.0001 par value, 100,000,000 and 50,000,000 shares authorized at September 30, 2017 and December 31, 2016, respectively; 35,809,311 and 34,843,885 shares issued and outstanding at September 30, 2017 and December 31, 2016, respectively	4	3
Additional paid-in capital	454,596	440,667
Accumulated other comprehensive loss		(350)
Accumulated deficit	(405,360)	(289,253)
Total stockholders' equity	49,240	151,067
Total liabilities and stockholders’ equity	$ 135,571	$ 205,570