CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash flows from operating activities:
Net loss	$ (12,941)	$ (10,667)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	40	56
Stock-based compensation expense	578	200
Amortization and accretion of investments	(195)
Changes in operating assets and liabilities:
Grants receivable	(22)	(722)
Prepaid expenses and other assets	(2,548)	(318)
Accounts payable	2,747	596
Accrued expenses and other current liabilities	1,917	(2,278)
Deferred rent	(175)	41
Net cash used in operating activities	(10,599)	(13,092)
Cash flows from investing activities:
Purchase of property and equipment	(37)	(137)
Purchase of short-term investments	(25,050)
Net cash used in investing activities	(25,087)	(137)
Cash flows from financing activities:
Proceeds from exercise of stock options	11
Payments of initial public offering costs	(150)	(1,706)
Net cash used in financing activities	(139)	(1,706)
Effect of exchange rate changes on cash and cash equivalents		(44)
Net decrease in cash and cash equivalents	(35,825)	(14,979)
Cash and cash equivalents at beginning of the period	49,360	55,101
Cash and cash equivalents at end of the period	$ 13,535	40,122
Supplemental disclosure of non-cash investing and financing activities:
Purchases of property and equipment included in accounts payable		110
Deferred offering costs included in accounts payable and accrued expenses		$ 413