CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Cash flows from operating activities:
Net loss	$ (31,573)	$ (70,671)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	4,181	3,899
Noncash lease expense	569	450
Stock-based compensation expense	8,356	6,336
Accretion of discount (amortization of premium) on short-term investments	414	(193)
Loss on disposal of property and equipment	6
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(2,511)	1,025
Accounts payable	991	4,920
Accrued expenses and other liabilities	(2,106)	1,888
Deferred revenue	(31,008)	(397)
Operating lease liabilities	(1,253)	(1,090)
Net cash used in operating activities	(53,934)	(53,833)
Cash flows from investing activities:
Purchases of short-term investments	(97,392)	(19,992)
Maturities of short-term investments		189,620
Purchases of property and equipment	(1,581)	(2,621)
Net cash (used in) provided by investing activities	(98,973)	167,007
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	446	537
Proceeds from issuance of common stock from option exercises	145	151
Net cash provided by financing activities	51,789	688
Net change in cash, cash equivalents and restricted cash	(101,118)	113,862
Cash, cash equivalents and restricted cash, beginning of period	218,705	55,048
Cash, cash equivalents and restricted cash, end of period	117,587	168,910
Supplemental disclosures of noncash investing and financing activities:
Reclassification of liability for common stock vested	8	16
Property and equipment additions included in accounts payable	94	30
Property and equipment additions included in accrued expenses and other liabilities		7
Unrealized gain (loss) on available for sale securities, net	2	$ (172)
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