Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Current assets
Cash and cash equivalents	$ 466,254	$ 246,122
Short-term investments	92,540	68,564
Prepaid expenses and other current assets	4,312	4,760
Total current assets	563,106	319,446
Property and equipment, net	5,592	5,138
Operating lease right-of-use assets	2,473
Long-term investments	69,524	80,148
Restricted cash and other	2,611	2,521
Total assets	643,306	407,253
Current liabilities
Accounts payable	5,194	2,946
Accrued liabilities	20,469	20,758
Prepayment from collaboration partner	3,099	12,973
Operating lease liabilities - current	2,529
Total current liabilities	31,291	36,677
Other long-term liabilities	73	9
Total liabilities	31,364	36,686
Commitments and contingencies (Note 7)
Stockholders’ equity
Preferred stock, $0.0001 par value; 5,000,000 shares authorized; none issued and outstanding
Common stock, $0.0001 par value, 150,000,000 shares authorized at March 31, 2019 and December 31, 2018; 46,001,775 and 40,288,949 shares issued and outstanding at March 31, 2019 and December 31, 2018, respectively	5	4
Additional paid-in capital	851,664	573,183
Accumulated other comprehensive income (loss)	296	(67)
Accumulated deficit	(240,023)	(202,553)
Total stockholders’ equity	611,942	370,567
Total liabilities and stockholders’ equity	$ 643,306	$ 407,253