CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Cash flows from operating activities:
Net loss	$ (39,427)	$ (34,360)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	917	567
Gain or loss on sale of property and equipment	(8)
Equity-based compensation	6,120	3,057
Amortization/accretion of investment securities	(1,023)
Non-cash operating lease expense	742
Deferred payroll tax credit		199
Change in operating assets and liabilities:
Prepaid expenses and other current assets	(1,001)	(1,108)
Other assets	(85)
Accounts payable	(664)	294
Accrued expenses	984	543
Deferred rent		190
Operating lease liabilities	(625)
Deferred revenue	(12,919)
Other liabilities	(59)	95
Net cash used in operating activities	(47,048)	(30,523)
Cash flows from investing activities:
Purchase of property and equipment	(1,645)	(791)
Purchase of marketable securities	(194,608)
Proceeds from sale of property and equipment	8
Sales and maturities of marketable securities	112,800	1,499
Net cash (used in) provided by investing activities	(83,445)	708
Cash flows from financing activities:
Principal payments on loan payable	(365)	(500)
Proceeds from sale of common stock, net of issuance costs	48,348	77,839
Proceeds from stock option exercises	92	3
Other	(10)	(2)
Net cash provided by financing activities	48,065	77,340
Net (decrease) increase in cash and cash equivalents and restricted cash	(82,428)	47,525
Cash and cash equivalents and restricted cash, beginning of period	115,274	56,666
Cash and cash equivalents and restricted cash, end of period	32,846	104,191
Supplemental disclosure of non-cash items:
Property and equipment purchases in accounts payable and accrued expenses	832	177
Operating lease right-of-use asset obtained in exchange for operating lease obligation	5,444
Supplemental cash flow information:
Cash paid for interest	$ 7	$ 27