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CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($)
 3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Operating activities    
Net loss $ (8,268,395) $ (9,592,461)
Adjustments to reconcile net loss to net cash used in operating activities:    
Depreciation expense 91,367 30,813
Stock-based compensation expense 1,188,752 1,518,161
Amortization and accretion on investments 2,454 46,129
Changes in operating assets and liabilities:    
Accounts receivable (6,881) 0
Inventory 39,810 (112,569)
Prepaid expenses and other current assets (659,732) (1,083,871)
Other assets 0 (64,800)
Accounts payable (366,289) 594,639
Accrued expenses and other current liabilities (276,168) (517,446)
Deferred revenue (7,707) 0
Deferred rent 37,197 7,781
Net cash used in operating activities (8,225,592) (9,173,624)
Investing activities    
Purchases of marketable securities (9,607,422) (12,013,945)
Proceeds from maturities and sales of marketable securities 16,510,076 8,600,195
Purchases of property and equipment (22,150) (107,286)
Proceeds from sales of property and equipment 732 0
Net cash provided by (used in) investing activities 6,881,236 (3,521,036)
Financing activities    
Proceeds from exercise of common stock 0 7,594
Net cash provided by financing activities 0 7,594
Net decrease in cash and cash equivalents (1,344,356) (12,687,066)
Cash and cash equivalents at beginning of period 22,416,040 66,686,695
Cash and cash equivalents at end of period 21,071,684 53,999,629
Supplemental cash flow information    
Property and equipment purchases included in accounts payable and accrued expense at December 31, 2016 and 2015 7,100 106,680
Property and equipment purchases included in accounts payable and accrued expense at March 31, 2016 $ 0 $ 226,309