CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating activities
Net loss	$ (74,064)	$ (33,709)	$ (25,872)
Adjustments to reconcile net loss to cash used in operating activities:
Non-cash research and development license expense	4,448
Stock-based compensation expense	6,390	2,278	1,168
Depreciation and amortization	442	223	144
Non-cash rent expense	61	(76)	11
Mark to market revaluation of series A investor instrument		1,863
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(6,286)	(1,889)	41
Deferred issuance costs		9	1,472
Tenant improvement allowance			376
Accounts payable, accrued expenses and other current liabilities	6,953	1,946	160
Deferred grant income			(249)
Due to related parties		(105)	(470)
Net cash used in operating activities	(62,056)	(29,460)	(23,219)
Investing activities
Purchases of short-term investments	(248,592)	(126,917)	(15,222)
Maturities of short-term investments	162,166	17,006	11,169
Purchases of property, plant and equipment	(722)	(133)	(1,057)
Net cash used in investing activities	(87,148)	(110,044)	(5,110)
Financing activities
Net proceeds from issuance of common stock	162,878	125,658
Proceeds from the exercise of stock options		40,842
Net proceeds from issuance of Series A Convertible Preferred Stock	1,808	700
Net cash provided by financing activities	164,686	167,200
Net increase in cash, cash equivalents and restricted cash	15,482	27,696	(28,329)
Cash, cash equivalents and restricted cash at beginning of period	34,461	6,765	35,094
Cash, cash equivalents and restricted cash at end of period	$ 49,943	$ 34,461	$ 6,765