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Note 2 - Summary of Significant Accounting Policies (Details Textual) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2023 USD ($) | Dec. 31, 2022 USD ($) | Dec. 31, 2021 USD ($) | Dec. 31, 2020 USD ($)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest | $ (21,948) | $ (36,280) | $ (68,208) |
Retained Earnings (Accumulated Deficit) | (396,302) | (375,276) |  |
Net Cash Provided by (Used in) Operating Activities | (16,474) | (28,151) | $ (47,242) |
Net Current Assets | 10,461 |  |  |
Cash, Cash Equivalents, and Short-Term Investments | 17,943 |  |  |
Debt Securities, Available-for-sale, Allowance for Credit Loss, Ending Balance | 0 |  |  |
Accounts Receivable, Allowance for Credit Loss | 0 |  |  |
Impairment of Long-Lived Assets to be Disposed of | $ 0 | $ 0 |  | $ 0
Number of Reportable Segments | 2 |  |  |
Depreciation of RMB Against U.S. Dollar, Percent | 2.90% | 8.20% | 2.30% |
Cash Subsidies for Research and Development Expenses [Member] |  |  |  |
Government Assistance, Income, Increase (Decrease) | $ 831 |  |  |

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