Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Operating activities:
Net loss	$ (31,857)	$ (10,724)
Adjustments to reconcile net loss to net cash used in operating activities:
Accretion on marketable securities	(305)
Depreciation and amortization expense	55	3
Stock-based compensation expense	2,212	301
Change in fair value of tranche liability		(635)
Expense related to acquisition of intellectual property (see Note 6)		3,157
Changes in operating assets and liabilities:
Restricted cash	(84)
Prepaid expenses and other current assets	(769)	(214)
Accounts payable	2,807	1,094
Accrued liabilities	(386)	1,022
Funding advance	500
Other liabilities	22
Net cash used in operating activities	(27,805)	(5,996)
Investing activities:
Purchases of property and equipment	(333)	(39)
Purchases of marketable securities	(92,293)
Net cash used in investing activities	(92,626)	(39)
Financing activities:
Proceeds from issuance of Series A redeemable convertible preferred stock		10,000
Proceeds from initial public offering, net of issuance costs	89,938
Net cash provided by financing activities	89,938	10,000
Net (decrease) increase in cash and cash equivalents	(30,493)	3,965
Cash and cash equivalents at beginning of period	53,349
Cash and cash equivalents at end of period	22,856	$ 3,965
Supplemental disclosure of non-cash investing and financing activities:
Purchases of property and equipment included in accounts payable	2
Conversion of redeemable convertible preferred stock into common stock	$ 81,620