Note 6 - Stock-based Compensation - Stock Option Activity (Details)	9 Months Ended
	Sep. 30, 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	1,732,515
Granted, weighted-average exercise price (in dollars per share) | $ / shares	$ 0.91
Forfeited (in shares) | shares	(172,904)
Forfeited, weighted-average exercise price (in dollars per share) | $ / shares	$ 1.76
Options outstanding, number (in shares) | shares	3,799,815
Options outstanding, weighted-average exercise price (in dollars per share) | $ / shares	$ 5.22
Options outstanding, weighted-average remaining contractual life (Year)	8 years 9 months 14 days
Options outstanding, aggregate intrinsic value | $	$ 33,791
Exercisable (in shares) | shares	1,839,079
Exercisable, weighted-average exercise price (in dollars per share) | $ / shares	$ 9.85
Exercisable, weighted-average remaining contractual life (Year)	8 years 2 months 1 day
Exercisable, aggregate intrinsic value | $	$ 30,113
Vested and expected to vest (in shares) | shares	3,542,971
Vested and expected to vest, weighted-average exercise price (in dollars per share) | $ / shares	$ 5.51
Vested and expected to vest, weighted-average remaining contractual life (Year)	8 years 8 months 26 days
Vested and expected to vest, aggregate intrinsic value | $	$ 33,791