Condensed consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Cash flows from operating activities
Net loss	$ (24,740)	$ (14,959)
Adjustments to reconcile net loss to net cash (used in) provided by operating activities:
Depreciation	673	449
Net amortization of premiums and discounts on investments	(118)
Stock-based compensation	990	437
Change in deferred rent	(115)	473
Changes in operating assets and liabilities:
Accounts receivable	320	(173)
Prepaid expenses and other current assets	(1,791)	(437)
Other assets	47
Accounts payable	1,112	(749)
Accrued expenses	1,132	884
Deferred revenue	(9,193)	32,487
Net cash (used in) provided by operating activities	(31,683)	18,412
Cash flows from investing activities
Purchase of property and equipment	(433)	(1,338)
Purchase of marketable securities	(111,321)
Maturities of marketable securities	25,320
Net cash used in investing activities	(86,434)	(1,338)
Cash flows from financing activities
Proceeds from exercise of stock options	431	81
Net cash provided by financing activities	68,576	58,235
Increase (decrease) in cash and cash equivalents	(49,541)	75,309
Cash and cash equivalents, beginning of period	100,297	11,534
Cash and cash equivalents, end of period	50,756	86,843
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	82	18
Purchases of property and equipment reimbursed by landlord		356
Series B-1 convertible preferred stock
Cash flows from financing activities
Net proceeds from sale of convertible preferred stock		25,272
Series C-1 convertible preferred stock
Cash flows from financing activities
Net proceeds from sale of convertible preferred stock		$ 32,882
IPO
Cash flows from financing activities
Net proceeds from issuance of stock	67,420
Underwriter's option
Cash flows from financing activities
Net proceeds from issuance of stock	$ 725