Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Cash flows from operating activities
Net loss	$ (41,825)	$ (24,740)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	907	673
Loss on disposal of fixed assets	20
Net amortization of premiums and discounts on investments	(288)	(118)
Stock-based compensation	2,756	990
Change in deferred rent	77	(115)
Changes in operating assets and liabilities:
Accounts receivable	198	320
Prepaid expenses and other current assets	(1,379)	(1,791)
Other assets	(1,433)	47
Accounts payable	3,619	1,112
Accrued expenses	3,568	1,132
Deferred revenue	(5,532)	(9,193)
Net cash used in operating activities	(39,312)	(31,683)
Cash flows from investing activities
Maturities of marketable securities	74,565	25,320
Purchase of property and equipment	(1,093)	(433)
Purchase of marketable securities		(111,321)
Net cash provided by (used in) investing activities	73,472	(86,434)
Cash flows from financing activities
Proceeds from exercise of stock options	853	431
Net cash provided by financing activities	853	68,576
Increase (decrease) in cash, cash equivalents and restricted cash	35,013	(49,541)
Cash, cash equivalents and restricted cash, beginning of period	26,962	100,668
Cash, cash equivalents and restricted cash, end of period	61,975	51,127
Supplemental disclosures of non-cash activities:
Conversion of preferred stock to common stock upon closing of initial public offering		94,450
Purchases of property and equipment included in accounts payable and accrued expenses	75	82
Adjustment to accumulated deficit and deferred revenue upon adoption of Topic 606	$ 2,031
IPO
Cash flows from financing activities
Net proceeds from issuance of stock		67,420
Over allotment option
Cash flows from financing activities
Net proceeds from issuance of stock		$ 725