CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 120,320	$ 293,764
Marketable securities	459,256	0
Prepaid expenses and other current assets	13,832	7,273
Total current assets	593,408	301,037
Long-term investment—restricted	1,510	1,069
Property and equipment, net	6,333	13,453
Operating lease assets	21,158	0
Total assets	622,409	315,559
Current liabilities:
Accounts payable	19,575	8,308
Accrued expenses and other current liabilities	38,716	13,709
Operating lease liabilities	1,747	539
Notes payable to related party	0	187
Total current liabilities	60,038	22,743
Notes payable to related party, net of current portion	0	1,107
Operating lease liabilities, net of current portion	15,904	11,347
Other long-term liabilities	0	381
Total liabilities	75,942	35,578
Commitments and contingencies (Note 15)
Stockholders' equity:
Preferred stock, $0.01 par value per share; 5,000,000 shares authorized; no shares issued or outstanding	0	0
Common stock, $0.01 par value per share; 125,000,000 shares authorized; 51,617,639 shares and 37,676,760 shares issued and outstanding as of December 31, 2019 and 2018, respectively	516	377
Additional paid-in capital	1,033,819	575,327
Accumulated other comprehensive income (loss)	111	0
Accumulated deficit	(487,979)	(295,723)
Total stockholders' equity	546,467	279,981
Total liabilities and stockholders' equity	$ 622,409	$ 315,559