Unaudited Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Operating activities
Net loss	$ (31,751)	$ (20,966)
Adjustments to reconcile net loss to cash used in operating activities:
Depreciation and amortization	673	300
Stock-based compensation expense	4,119	2,585
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(3,904)	(3,735)
Accounts payable	29	180
Accrued expenses and other liabilities	(1,587)	646
Cash used in operating activities	(32,421)	(20,990)
Investing activities
Purchases of equipment	(1,229)	(941)
Purchases of investments	(63,005)	(106,566)
Proceeds from sale or maturity of investments	80,975	42,619
Cash provided by (used in) investing activities	16,741	(64,888)
Financing activities
Proceeds from initial public offering, net of issuance costs	0	80,213
Proceeds from the exercise of stock options	310	18
Deferred offering costs	(231)	(1,047)
Cash provided by financing activities	79	79,184
Net change in cash, cash equivalents, and restricted cash	(15,601)	(6,694)
Cash, cash equivalents, and restricted cash at beginning of year	65,713	47,964
Cash, cash equivalents, and restricted cash at end of period	50,112	41,270
Supplemental disclosure of noncash financing activities
Conversion of redeemable convertible preferred stock to common stock	0	259,130
Accretion of redeemable preferred stock to redemption value	0	49,900
Property and equipment additions in accounts payable	745	893
Deferred offering costs included in accounts payable	$ 0	$ 309