Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 237,677	$ 144,052
Short-term investments	193,692	147,260
Collaboration receivables (all amounts are with a related party)	8,547	7,039
Prepaid expenses and other current assets	10,000	7,662
Total current assets	449,916	306,013
Property and equipment, net	6,812	7,106
Operating lease - right of use asset, net	23,908
Long-term investments	22,477	0
Other assets	1,793	1,702
Total assets	504,906	314,821
Current liabilities:
Accounts payable	2,295	419
Accrued expenses	24,895	18,209
Deferred rent		284
Operating lease liability - right of use	6,183
Total current liabilities	33,373	18,912
Deferred rent, net of current portion		2,381
Warrants to purchase common stock	1,856	1,491
Operating lease liability - right of use, net of current portion	20,201
Total liabilities	55,430	22,784
Commitments and contingencies (Note 8)	0	0
Stockholders' equity:
Undesignated preferred stock, $0.001 par value: 25,000,000 shares authorized and no shares issued or outstanding	0	0
Common stock, $0.001 par value: 175,000,000 shares authorized; 53,123,567 and 46,260,747 shares issued and outstanding at December 31, 2019 and 2018, respectively	53	47
Additional paid-in capital	1,160,807	879,099
Accumulated deficit	(711,407)	(586,549)
Accumulated other comprehensive income (loss)	23	(560)
Total stockholders' equity	449,476	292,037
Total liabilities and stockholders' equity	$ 504,906	$ 314,821