Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Current assets
Cash and cash equivalents	$ 443,693	$ 246,122
Short-term investments	114,792	68,564
Prepaid expenses and other current assets	4,316	4,760
Total current assets	562,801	319,446
Property and equipment, net	5,435	5,138
Operating lease right-of-use assets	1,756
Long-term investments	43,952	80,148
Restricted cash and other	2,109	2,521
Total assets	616,053	407,253
Current liabilities
Accounts payable	3,037	2,946
Accrued liabilities	24,744	20,758
Prepayment from collaboration partner	2,256	12,973
Operating lease liabilities - current	1,831
Total current liabilities	31,868	36,677
Other long-term liabilities		9
Total liabilities	31,868	36,686
Commitments and contingencies (Note 7)
Stockholders’ equity
Preferred stock, $0.0001 par value; 5,000,000 shares authorized; none issued and outstanding	0	0
Common stock, $0.0001 par value, 150,000,000 shares authorized at June 30, 2019 and December 31, 2018; 46,098,059 and 40,288,949 shares issued and outstanding at June 30, 2019 and December 31, 2018, respectively	5	4
Additional paid-in capital	861,880	573,183
Accumulated other comprehensive income (loss)	497	(67)
Accumulated deficit	(278,197)	(202,553)
Total stockholders’ equity	584,185	370,567
Total liabilities and stockholders’ equity	$ 616,053	$ 407,253