CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (10,522,772)	$ (5,554,793)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	220,596	130,156
Stock-based compensation expense	380,120	27,680
Accretion on short-term investments	(123,258)
Change in fair value associated with convertible preferred stock tranche liability		876,000
Changes in operating assets and liabilities:
Prepaid expense and other current assets	(1,727,680)	(390,919)
Accounts payable	510,889	350,402
Accrued expenses and other liabilities	(763,948)	(69,114)
Deferred revenue	(156,622)
Deferred rent	1,138,834	105,753
Net cash used in operating activities	(11,043,841)	(4,524,835)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of short-term investments	(18,595,056)
Maturities of short-term investments	26,270,000
Purchases of property and equipment	(1,035,109)	(848,433)
Net cash provided by (used in) investing activities	6,639,835	(848,433)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock	69,263
Payment of deferred offering costs	(1,026,776)
Proceeds from issuance of Series A convertible preferred stock, net of issuance costs		20,479,488
Net cash (used in) provided by financing activities	(957,513)	20,479,488
(DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	(5,361,519)	15,106,220
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	51,574,932	11,392,207
CASH AND CASH EQUIVALENTS, END OF PERIOD	46,213,413	26,498,427
SUPPLEMENTAL DISCLOSURES OF NONCASH FINANCING ACTIVITIES:
Reclassification of liability for common stock vested	15,416	20,605
Property and equipment additions included in accounts payable	26,980	$ 214,734
Deferred offering costs included in accounts payable and accrued expenses	$ 2,045,782