CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities:
Net loss	$ (79,776)	$ (76,934)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	1,644	1,458
Non-cash lease expense	1,186	0
Net amortization of premiums and discounts on marketable securities	5	(4)
Stock-based compensation expense	7,340	6,229
Loss on disposal of property and equipment	39	25
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	268	(340)
Other long-term assets	(22)	125
Accounts payable	(2,311)	1,951
Accrued expenses and other liabilities	(716)	4,062
Lease liabilities	(1,069)	0
Net cash used in operating activities	(73,412)	(63,428)
Cash flows from investing activities:
Purchases of marketable securities	0	(72,939)
Sales and maturities of marketable securities	50,681	43,550
Purchases of property and equipment	(1,084)	(3,222)
Net cash provided by (used in) investing activities	49,597	(32,611)
Cash flows from financing activities:
Proceeds from initial public offering of common stock, net of commissions and underwriting discounts	0	93,000
Proceeds from equity offerings, net of issuance costs	334	0
Proceeds from exercise of stock options	115	404
Proceeds from the issuance of common stock under ESPP	63	0
Repurchase of unvested restricted common stock	(2)	(1)
Payment of initial public offering costs	0	(3,065)
Net cash provided by financing activities	510	90,338
Net decrease in cash, cash equivalents and restricted cash	(23,305)	(5,701)
Cash, cash equivalents and restricted cash, beginning of period	53,156	58,857
Cash, cash equivalents and restricted cash, end of period	29,851	53,156
Supplemental disclosure of non-cash items:
Accretion of redeemable convertible preferred stock to redemption value	0	6,371
Conversion of redeemable convertible preferred stock and contingently redeemable restricted common stock to common stock upon closing of the initial public offering	0	181,831
Purchases of property and equipment included in accounts payable and accrued expenses	$ 0	$ 497