CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Cash flows from operating activities:
Net loss	$ (62,181)	$ (98,903)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	6,280	5,909
Noncash lease expense	869	686
Stock-based compensation expense	12,582	9,682
Accretion of discount (amortization of premium) on short-term investments	664	(197)
Loss on disposal of property and equipment	6	0
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(2,133)	938
Accounts payable	(1,123)	486
Accrued expenses and other liabilities	225	924
Deferred revenue	(32,609)	(590)
Operating lease liabilities	(1,901)	(1,655)
Net cash used in operating activities	(79,321)	(82,720)
Cash flows from investing activities:
Purchases of short-term investments	(97,391)	(19,991)
Maturities of short-term investments	5,000	228,620
Purchases of property and equipment	(2,058)	(2,778)
Net cash (used in) provided by investing activities	(94,449)	205,851
Cash flows from financing activities:
Proceeds from issuance of common stock in follow-on public offering, net of discounts and issuance costs	49,744
Proceeds from issuance of common stock pursuant to employee stock purchase plan	825	938
Proceeds from issuance of common stock from option exercises	145	160
Net cash provided by financing activities	52,168	1,098
Net change in cash, cash equivalents and restricted cash	(121,602)	124,229
Cash, cash equivalents and restricted cash, beginning of period	218,705	55,048
Cash, cash equivalents and restricted cash, end of period	97,103	179,277
Supplemental disclosures of noncash investing and financing activities:
Reclassification of liability for common stock vested	12	21
Property and equipment additions included in accounts payable	34	677
Property and equipment additions included in accrued expenses and other liabilities	0	126
Unrealized gain (loss) on available for sale securities, net	(5)	(183)
A T M
Cash flows from financing activities:
Proceeds from issuance of common stock in follow-on public offering, net of discounts and issuance costs	$ 1,454