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Note 6 - Stock-based Compensation - Stock Option Activity (Details)
 3 Months Ended
Mar. 31, 2021
USD ($)
$ / shares
shares
Options outstanding, number (in shares) | shares 2,240,204
Options outstanding, weighted-average exercise price (in dollars per share) | $ / shares $ 8.28
Granted (in shares) | shares 876,615
Granted, weighted-average exercise price (in dollars per share) | $ / shares $ 1.11
Options outstanding, number (in shares) | shares 3,116,819
Options outstanding, weighted-average exercise price (in dollars per share) | $ / shares $ 6.26
Options outstanding, weighted-average remaining contractual life (Year) 9 years 3 days
Options outstanding, aggregate intrinsic value | $ $ 475,258
Exercisable (in shares) | shares 1,363,373
Exercisable, weighted-average exercise price (in dollars per share) | $ / shares $ 13.10
Exercisable, weighted-average remaining contractual life (Year) 8 years 91 days
Exercisable, aggregate intrinsic value | $ $ 331,618
Vested and expected to vest (in shares) | shares 3,116,819
Vested and expected to vest, weighted-average exercise price (in dollars per share) | $ / shares $ 6.26
Vested and expected to vest, weighted-average remaining contractual life (Year) 9 years 3 days
Vested and expected to vest, aggregate intrinsic value | $ $ 475,258