Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Operating Activities
Net loss	$ (4,111)	$ (2,150)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation	132	18
Stock-based compensation expense	313	65
Decrease in
Prepaids and other current assets	(352)	(118)
Accounts payable	583	(104)
Accrued expenses and other current liabilities	153	(80)
Amortization of right-of-use asset	67
Net cash used in operating activities	(3,215)	(2,369)
Investing Activities
Purchases of property and equipment	(1,707)
Purchases of short-term investments	(512)
Purchases of other noncurrent assets	(263)
Net cash used in investing activities	(2,482)
Financing Activities
Issuance of common stock, net	40	6
Net cash provided by financing activities	40	6
Net decrease in cash and cash equivalents	(5,657)	(2,363)
Cash and cash equivalents at beginning of period	103,670	49,591
Cash and cash equivalents at end of period	98,013	47,228
Supplemental Disclosures of Non-Cash Investing and Financing Activities
Unpaid purchase of property and equipment	375
Unpaid deferred offering costs	155	$ 1
Initial recognition of right-of-use assets	$ 3,394