Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
CASH FLOWS FROM OPERATING ACTIVITIES
Net loss		$ (28,144)	$ (20,242)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense		1,100	34
Shares issued in connection with Amgen Agreement			1,353
Acquisition of technology in connection with Amgen Agreement			5,000
Issuance date fair-value of anti-dilution liability			1,639
Change in fair value of preferred stock tranche liability	$ 8,400		8,400
Change in fair value of anti-dilution right liability	1,021		1,032
Changes in operating assets and liabilities:
Prepaid expenses and other assets		(1,097)	(563)
Accounts payable		(1,280)	459
Accrued expenses and other current liabilities		5,443	338
Net cash used in operating activities		(23,978)	(2,550)
CASH FLOWS FROM INVESTING ACTIVITIES
Acquisition of technology in connection with Amgen Agreement			(5,000)
Net cash used in investing activities			(5,000)
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs			14,784
Proceeds from issuance of common stock in initial public offering, net of issuance costs and underwriting fees		95,452
Proceeds from the early exercise of stock options in exchange for restricted common stock		321
Proceeds from the exercise of stock options		100
Net cash provided by financing activities		95,873	14,784
Net increase in cash, cash equivalents and restricted cash		71,895	7,234
Cash and restricted cash at beginning of period		76,000	618
Cash, cash equivalents and restricted cash at end of period	7,852	147,895	7,852
SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING INFORMATION:
Conversion of convertible preferred stock into common stock		$ 124,728
Accretion of redeemable convertible preferred stock to redemption value	$ 251		321
Issuance date fair value of preferred stock tranche obligation			7,350
Deferred offering costs included in accounts payable and accrued expenses and other current liabilities			60
Common stock issued in connection with Amgen Agreement			$ 1,353