Note 2 - Summary of Significant Accounting Policies - Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Stock-based compensation expense	$ 627	$ 376	$ 1,672	$ 1,581
Research and Development Expense [Member]
Stock-based compensation expense	229	122	693	378
General and Administrative Expense [Member]
Stock-based compensation expense	$ 398	$ 254	$ 979	$ 1,203

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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