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Income Taxes | 12 Months Ended
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Dec. 31, 2023
Income Taxes [Abstract] |
Income taxes |
8. | Income taxes
As of December 31, 2023 and 2022, a valuation allowance was recorded against all deferred tax assets due to our cumulative net loss position. The components of our provision for income taxes are as follows for the periods indicated:
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ |  | Year Ended
​ | ​ | ​ | December 31,
(in thousands) | ​ | ​ | 2023 | ​ |  | 2022
Current | ​ | ​ | ​ | ​ | ​ | ​
Federal and state | ​ | $ | — | ​ | $ | —
Foreign | ​ |  | 147 | ​ |  | 109
Total provision for income taxes | ​ | $ | 147 | ​ | $ | 109
​ The reconciliation of taxes at the federal statutory rate to our provision for income taxes are as follows for the periods indicated: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ |  | Year Ended
​ | ​ | ​ | December 31,
​ | ​ | ​ | 2023 | ​ |  | 2022 |
Tax at federal statutory rate | ​ | ​ | 21.0 | % | ​ | 21.0 | %
Permanent differences | ​ | ​ | (4.3) | ​ | ​ | (2.1) | ​
Research and development ("R&D") tax credit | ​ | ​ | 1.4 | ​ | ​ | 0.8 | ​
Uncertain tax position | ​ | ​ | (0.3) | ​ | ​ | (0.3) | ​
State, net of federal benefit | ​ | ​ | 2.6 | ​ | ​ | (0.5) | ​
Deferred rate change | ​ | ​ | 0.4 | ​ | ​ | 0.3 | ​
Change in valuation allowance | ​ |  | (21.2) | ​ |  | (19.5) | ​
Total | ​ | ​ | (0.4) | % | ​ | (0.3) | %
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ Significant components of net deferred tax assets were as follows for the periods indicated: ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ |  | Year Ended
​ | ​ | ​ | December 31,
(in thousands) | ​ | ​ | 2023 | ​ |  | 2022
Deferred tax assets | ​ | ​ | ​ | ​ | ​ | ​
Net operating loss carryforwards | ​ | $ | 89,173 | ​ | $ | 82,050
R&D tax credits | ​ | ​ | 9,664 | ​ | ​ | 9,072
Capitalized R&D expenses | ​ | ​ | 8,963 | ​ | ​ | 8,412
Non-qualified stock options | ​ | ​ | 1,473 | ​ | ​ | 752
Start-up costs | ​ | ​ | 680 | ​ | ​ | 894
Accrued vacation | ​ | ​ | 179 | ​ | ​ | 212
Property and equipment | ​ | ​ | 59 | ​ | ​ | 144
Other | ​ | ​ | 361 | ​ | ​ | 300
Total deferred tax assets | ​ | ​ | 110,552 | ​ | ​ | 101,836
Valuation allowance | ​ | ​ | (110,552) | ​ | ​ | (101,836)
Net deferred tax assets | ​ | $ | — | ​ | $ | —
​ As of December 31, 2023, we had federal and state net operating loss carryforwards (“NOLs”) of approximately $389.9 million and $7.3 million, respectively. The federal NOLs began expiring in 2021 and the state NOLs began expiring in 2020. As of December 31, 2023, we had federal and state tax credit carryforwards of approximately $9.8 million and $2.0 million, respectively. The federal tax credit carryforwards began expiring in 2021 and the state tax credits will begin expiring in 2028. ​ Utilization of NOLs may be subject to an annual limitation due to the ownership change limitations provided by Section 382 of the Internal Revenue Code of 1986, as amended, and similar state provisions. We have not performed a detailed analysis to determine whether an ownership change has occurred. Such a change of ownership would limit our utilization of the NOLs and could be triggered by subsequent sales of securities by us or our stockholders. The changes to our gross unrecognized tax benefits were as follows for the periods indicated:
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ |  | Year Ended
​ | ​ | ​ | December 31,
(in thousands) | ​ | ​ | 2023 | ​ |  | 2022
Gross unrecognized tax benefits at beginning of year | ​ | $ | 2,046 | ​ | $ | 1,939
Gross increases: | ​ | ​ | ​ | ​ | ​ | ​
Prior year tax positions | ​ | ​ | 12 | ​ | ​ | 4
Current year tax positions | ​ | ​ | 185 | ​ | ​ | 113
Gross decreases: | ​ | ​ | ​ | ​ | ​ | ​
Prior year tax positions | ​ | ​ | (22) | ​ | ​ | (10)
Gross unrecognized tax benefits at end of year | ​ | $ | 2,221 | ​ | $ | 2,046
​ All of these unrecognized tax benefits, if recognized, would impact the effective tax rate before taking consideration of the valuation allowance. The amount of unrecognized tax benefits subjected to the valuation allowance was $1.7 million and $1.6 million for the years ended December 31, 2023 and 2022, respectively. We recognized approximately $0.1 million of interest or penalties for each of the years ended December 31, 2023 and 2022. As of December 31, 2023 and 2022, total accrued interest and penalties were $0.5 million and $0.4 million, respectively. We recognize accrued interest and penalties related to unrecognized tax positions as a component of income tax expense. We do not expect a significant change in the amount of unrecognized tax benefits in the next year. ​ We are subject to U.S. federal income tax as well as income tax of multiple state and foreign jurisdictions. Tax years from 2004 through present remain open for audit under the applicable statute of limitations due to the carryover of the unused NOLs and tax credit carryforwards. We do not have any tax audits or other proceedings pending.

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- Definition The entire disclosure for income taxes. Disclosures may include net deferred tax liability or asset recognized in an enterprise's statement of financial position, net change during the year in the total valuation allowance, approximate tax effect of each type of temporary difference and carryforward that gives rise to a significant portion of deferred tax liabilities and deferred tax assets, utilization of a tax carryback, and tax uncertainties information. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 13 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480990/946-20-50-13 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 235 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08(h)(2)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480678/235-10-S99-1 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 740 -Name Accounting Standards Codification -Publisher FASB -URI https://asc.fasb.org//740/tableOfContent Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 740 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 14 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482685/740-10-50-14 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 740 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 21 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482685/740-10-50-21 Reference 6: http://www.xbrl.org/2003/role/disclosureRef -Topic 740 -SubTopic 270 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482526/740-270-50-1 Reference 7: http://www.xbrl.org/2003/role/disclosureRef -Topic 740 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 17 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482685/740-10-50-17 Reference 8: http://www.xbrl.org/2003/role/disclosureRef -Topic 740 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 6.I.5.Q1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479360/740-10-S99-1 Reference 9: http://www.xbrl.org/2003/role/disclosureRef -Topic 740 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 2 -Subparagraph (SAB Topic 11.C) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479360/740-10-S99-2 Reference 10: http://www.xbrl.org/2003/role/disclosureRef -Topic 740 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482603/740-30-50-2 + Details
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