CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Cash flows from operating activities:
Net loss	$ (26,323)	$ (16,400)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	77	89
Stock-based compensation expense	1,177	473
Amortization and accretion of investments	(431)
Changes in operating assets and liabilities:
Grants receivable	492	(1,838)
Prepaid expenses and other assets	(3,403)	(1,405)
Accounts payable	(632)	678
Accrued expenses and other current liabilities	3,252	(845)
Deferred rent	(175)	82
Net cash used in operating activities	(25,966)	(19,166)
Cash flows from investing activities:
Purchase of property and equipment	(37)	(253)
Proceeds from maturities of short-term investments	7,000
Purchase of short-term investments	(25,050)
Net cash used in investing activities	(18,087)	(253)
Cash flows from financing activities:
Proceeds from exercise of stock options	40
Payments of initial public offering costs	(150)	(2,039)
Net cash used in financing activities	(110)	(2,039)
Net decrease in cash and cash equivalents	(44,163)	(21,458)
Cash and cash equivalents at beginning of the period	49,360	55,101
Cash and cash equivalents at end of the period	$ 5,197	33,643
Supplemental disclosure of non-cash investing and financing activities:
Deferred offering costs included in accounts payable and accrued expenses		$ 378