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Statements of Cash Flows - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Operating activities      
Net loss $ (33,281,068) $ (31,809,838) $ (12,769,498)
Adjustments to reconcile net loss to net cash used in operating activities:      
Depreciation 263,541 190,540 191,484
Loss on disposal of property and equipment     8,001
Stock-based compensation expense 3,760,802 2,964,767 2,323,603
Changes in assets and liabilities      
Prepaid expenses and other current assets (2,816,381) (36,693) 474,439
Prepaid research and development expenses 555,167 (63,000)  
Deposits   (52,320)  
Accounts payable 1,061,880 1,000,975 (309,792)
Accrued expenses and bonuses 724,797 (2,504,179) 1,693,238
Deferred rent (37,099) 578,483 146,635
Net cash used in operating activities (29,768,361) (29,731,265) (8,241,890)
Investing activities      
Purchases of property and equipment (294,107) (704,202) (268,594)
Net cash used in investing activities (294,107) (704,202) (268,594)
Financing activities      
Proceeds from issuance of common stock, net of issuance costs 113,572,189 23,601,955  
Proceeds from exercise of stock options 373,376 72,593 114,121
Net cash provided by financing activities 113,945,565 23,674,548 114,121
Net change in cash and cash equivalents 83,883,097 (6,760,919) (8,396,363)
Cash and cash equivalents, beginning of period 40,041,641 46,802,560 55,198,923
Cash and cash equivalents, end of period 123,924,738 40,041,641 $ 46,802,560
Non-cash investing activities and financing activities      
Property acquisition costs included in accounts payable and accrued expenses 20,000 $ 21,292  
Issuance costs associated with financing included in accounts payable and accrued expenses $ 25,005