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Condensed Statement of Changes in Stockholders' Equity - USD ($) | Total | Common Stock [Member] | Additional Paid-in Capital [Member] | Accumulated Deficit [Member]
--- | --- | --- | --- | ---
Beginning balance at Dec. 31, 2018 | $ 18,370,435 | $ 265 | $ 243,111,741 | $ (224,741,571)
Beginning balance (in shares) at Dec. 31, 2018 |  | 26,526,303 |  |
Stock-based compensation - restricted stock units ("RSUs") | 3,083,567 |  | 3,083,567 |
Stock-based compensation - employee stock purchase plan ("ESPP") | 87,825 |  | 87,825 |
Issuance of shares for RSUs |  | $ 4 | (4) |
Issuance of shares for RSUs (in shares) |  | 434,522 |  |
Shares withheld for payroll tax on RSUs | (10,207) |  | (10,207) |
Shares withheld for payroll tax on RSUs (in shares) |  | (1,329) |  |
Shares withheld for payroll tax on performance share units ("PSUs") | (329,159) |  | (329,159) |
Shares withheld for payroll tax on performance share units ("PSUs") (in shares) |  | (44,481) |  |
Exercise of stock options | 400,103 | $ 1 | 400,102 |
Exercise of stock options (in shares) |  | 80,201 |  |
Proceeds from contributions to the ESPP | 173,167 |  | 173,167 |
Issuance of shares and warrants in a private placement, net of issuance costs | 23,319,156 | $ 33 | 23,319,123 |
Issuance of shares and warrants in a private placements, net of issuance costs (in shares) |  | 3,333,333 |  |
Net loss | (11,019,468) |  |  | (11,019,468)
Ending balance at Mar. 31, 2019 | 34,075,419 | $ 303 | 269,836,155 | (235,761,039)
Ending balance (in shares) at Mar. 31, 2019 |  | 30,328,549 |  |
Beginning balance at Dec. 31, 2018 | 18,370,435 | $ 265 | 243,111,741 | (224,741,571)
Beginning balance (in shares) at Dec. 31, 2018 |  | 26,526,303 |  |
Net loss | (20,823,464) |  |  |
Ending balance at Jun. 30, 2019 | 27,214,778 | $ 306 | 272,779,507 | (245,565,035)
Ending balance (in shares) at Jun. 30, 2019 |  | 30,603,534 |  |
Beginning balance at Mar. 31, 2019 | 34,075,419 | $ 303 | 269,836,155 | (235,761,039)
Beginning balance (in shares) at Mar. 31, 2019 |  | 30,328,549 |  |
Stock-based compensation - restricted stock units ("RSUs") | 2,675,184 |  | 2,675,184 |
Stock-based compensation - employee stock purchase plan ("ESPP") | 122,749 |  | 122,749 |
Issuance of shares for RSUs |  | $ 2 | (2) |
Issuance of shares for RSUs (in shares) |  | 189,220 |  |
Proceeds from contributions to the ESPP | 145,422 | $ 1 | 145,421 |
Proceeds from contributions to the ESPP (in shares) |  | 85,765 |  |
Net loss | (9,803,996) |  |  | (9,803,996)
Ending balance at Jun. 30, 2019 | 27,214,778 | $ 306 | 272,779,507 | (245,565,035)
Ending balance (in shares) at Jun. 30, 2019 |  | 30,603,534 |  |
Beginning balance at Dec. 31, 2019 | 19,012,874 | $ 333 | 282,153,201 | (263,140,660)
Beginning balance (in shares) at Dec. 31, 2019 |  | 33,203,806 |  |
Stock-based compensation - restricted stock units ("RSUs") | 2,321,820 |  | 2,321,820 |
Stock-based compensation - performance share units ("PSUs") | (88,348) |  | (88,348) |
Stock-based compensation - employee stock purchase plan ("ESPP") | 42,827 |  | 42,827 |
Issuance of shares for RSUs |  | $ 4 | (4) |
Issuance of shares for RSUs (in shares) |  | 396,559 |  |
Proceeds from contributions to the ESPP | 113,059 |  | 113,059 |
Issuance of shares in an at-the-market ("ATM") offering, net of issuance costs | 5,506,880 | $ 44 | 5,506,836 |
Issuance of shares in an at-the-market ("ATM") offering, net of issuance costs (in shares) |  | 4,351,652 |  |
Net loss | (8,597,736) |  |  | (8,597,736)
Ending balance at Mar. 31, 2020 | 18,311,376 | $ 381 | 290,049,391 | (271,738,396)
Ending balance (in shares) at Mar. 31, 2020 |  | 37,952,017 |  |
Beginning balance at Dec. 31, 2019 | 19,012,874 | $ 333 | 282,153,201 | (263,140,660)
Beginning balance (in shares) at Dec. 31, 2019 |  | 33,203,806 |  |
Net loss | (16,802,402) |  |  |
Ending balance at Jun. 30, 2020 | 21,497,373 | $ 417 | 301,440,018 | (279,943,062)
Ending balance (in shares) at Jun. 30, 2020 |  | 41,685,310 |  |
Beginning balance at Mar. 31, 2020 | 18,311,376 | $ 381 | 290,049,391 | (271,738,396)
Beginning balance (in shares) at Mar. 31, 2020 |  | 37,952,017 |  |
Stock-based compensation - restricted stock units ("RSUs") | 2,028,599 |  | 2,028,599 |
Stock-based compensation - employee stock purchase plan ("ESPP") | 41,308 |  | 41,308 |
Issuance of shares for RSUs |  | $ 2 | (2) |
Issuance of shares for RSUs (in shares) |  | 232,995 |  |
Proceeds from contributions to the ESPP | 104,145 | $ 1 | 104,144 |
Proceeds from contributions to the ESPP (in shares) |  | 144,370 |  |
Issuance of shares in an at-the-market ("ATM") offering, net of issuance costs | 9,216,611 | $ 33 | 9,216,578 |
Issuance of shares in an at-the-market ("ATM") offering, net of issuance costs (in shares) |  | 3,355,928 |  |
Net loss | (8,204,666) |  |  | (8,204,666)
Ending balance at Jun. 30, 2020 | $ 21,497,373 | $ 417 | $ 301,440,018 | $ (279,943,062)
Ending balance (in shares) at Jun. 30, 2020 |  | 41,685,310 |  |

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