Unaudited Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Operating activities:
Net loss	$ (20,966)	$ (10,173)
Adjustments to reconcile net loss to cash used in operating activities:
Depreciation and amortization	300	188
Stock-based compensation expense	2,585	633
Loss on fixed asset disposal	0	2
Accretion on investments	(252)	(76)
Changes in operating assets and liabilities:
Interest receivable on investments	(325)	(78)
Prepaid expenses and other current assets	(3,158)	153
Accounts payable	180	1,165
Accrued expenses and other liabilities	646	493
Cash used in operating activities	(20,990)	(7,693)
Investing activities:
Purchase of investments	(106,566)	(32,343)
Proceeds from sale or maturity of investments	42,619	5,530
Purchases of equipment	(941)	(772)
Cash used in investing activities	(64,888)	(27,585)
Financing activities:
Proceeds from the sale of Series B preferred stock	0	125,000
Proceeds from initial public offering, net of issuance costs	80,213	0
Proceeds from the exercise of stock options	18	219
Deferred offering costs	(1,047)	(57)
Payment of issuance costs	0	(150)
Cash provided by financing activities	79,184	125,012
Net change in cash, cash equivalents, and restricted cash	(6,694)	89,734
Cash, cash equivalents, and restricted cash at beginning of year	47,964	20,101
Cash, cash equivalents, and restricted cash at end of period	41,270	109,835
Supplemental disclosure of noncash investing and financing activities:
Conversion of redeemable convertible preferred stock to common stock	259,130	0
Accretion of redeemable preferred stock to redemption value	49,900	21,978
Deferred offering costs included in accounts payable	309	254
Property and equipment additions in accounts payable	893	19
Reclassification of early exercise liability upon vesting of options	$ 26	$ 26