GenSci agreement	9 Months Ended
Sep. 30, 2026
Organization, Consolidation and Presentation of Financial Statements [Abstract]
GenSci agreement	GenSci agreement On December 15, 2025, GenSci and Yarrow entered into an exclusive license agreement (the “GenSci License Agreement”), pursuant to which Yarrow obtained from GenSci an exclusive, royalty-bearing license to develop, manufacture, and commercialize YB-101 (also known as GenSci098), an antibody targeting the TSHR, outside Greater China for all fields of use, including the treatment of GD and TED. GenSci retained rights to exploit these assets in Greater China. Under the GenSci License Agreement, and subject to limited exceptions in which GenSci will perform certain development activities outside Greater China, Yarrow is responsible for all development and commercialization activities for YB-101 outside Greater China. GenSci is obligated to provide Yarrow with clinical data relating to YB-101 that exists as of the effective date in connection with the initial know-how transfer. Additionally, each party is obligated to provide the other party with clinical data generated by that party during the development of YB-101 as part of the ongoing knowledge sharing. In addition, Yarrow currently relies on GenSci for formulation and manufacturing of Yarrow’s Phase 2a/2b clinical trial materials. More specifically, clinical data generated by GenSci will be shared with Yarrow for inclusion in global safety reports and regulatory submissions by Yarrow to global health authorities including the FDA. Yarrow is the manager of the YB-101 global safety database; as a result, data sharing between Yarrow and GenSci will continue during the term of the GenSci License Agreement. Yarrow does not currently anticipate outsourcing preclinical or clinical research to GenSci, but could consider doing so in the future. Manufacturing data generated by GenSci related to the manufacturing and testing of YB-101 will be shared with Yarrow on an ongoing basis to support global regulatory filings related to manufacturing. Subject to customary exceptions, during the term of the GenSci License Agreement, neither Yarrow (with respect to activities outside Greater China) nor GenSci (with respect to activities in Greater China), nor their respective affiliates, may directly or indirectly clinically develop or commercialize specified categories of antibodies directed to TSHR. Upon execution of the GenSci License Agreement, the Company was required to pay GenSci a non-refundable upfront cash payment of $70.0 million. The upfront payment was recorded as acquired in-process research and development in the Company’s statements of operations in the period from October 3, 2025 (inception) to December 31, 2025 since further development and regulatory approval of the licensed product candidates are necessary and there is no alternative use that the Company could benefit from. GenSci is also eligible to receive up to approximately $1.295 billion in additional contingent payments based on GenSci’s completion of the manufacturing technology transfer, GenSci’s achievement of a development milestone, as well as Yarrow’s achievement of development, regulatory approval, and commercial sales-based milestones. Specifically, GenSci is eligible to receive up to approximately $100 million in contingent payments based on the achievement of specified clinical development milestones by Yarrow or GenSci, including a $50 million near-term development milestone, as applicable, and up to $150 million in contingent payments based on Yarrow’s achievement of specified regulatory approval milestones. In addition, GenSci is eligible to receive tiered royalties ranging from the low teens to the low-mid teens on annual net product sales outside Greater China during the applicable royalty term. The royalty term for a licensed product in a given country commences upon the first commercial sale of the licensed product in that country and continues until the latest of: (a) the expiration of the last royalty-bearing valid claim of the licensed patents covering the licensed product in that country; (b) the tenth anniversary of the first commercial sale of the licensed product in that country; and (c) the expiration of all regulatory exclusivity for the licensed product in that country. The expected expiry of the last-to-expire royalty payment obligation is January 20, 2046. The GenSci License Agreement will remain in effect until the expiration of all royalty terms. Either party may terminate the GenSci License Agreement for an uncured material breach or insolvency of the other party. GenSci may terminate the GenSci License Agreement in the event of a specified patent challenge by Yarrow or its affiliates or if Yarrow ceases all development activities outside Greater China for a substantial period of time prior to achieving a specified regulatory approval milestone. Following a specified near-term triggering event, Yarrow may terminate the GenSci License Agreement for convenience upon providing the required notice.

On December 15, 2025, GenSci and Yarrow entered into an exclusive license agreement (the “GenSci License Agreement”), pursuant to which Yarrow obtained from GenSci an exclusive, royalty-bearing license to develop, manufacture, and commercialize YB-101 (also known as GenSci098), an antibody targeting the TSHR, outside Greater China for all fields of use, including the treatment of GD and TED. GenSci retained rights to exploit these assets in Greater China. Under the GenSci License Agreement, and subject to limited exceptions in which GenSci will perform certain development activities outside Greater China, Yarrow is responsible for all development and commercialization activities for YB-101 outside Greater China. GenSci is obligated to provide Yarrow with clinical data relating to YB-101 that exists as of the effective date in connection with the initial know-how transfer. Additionally, each party is obligated to provide the other party with clinical data generated by that party during the development of YB-101 as part of the ongoing knowledge sharing. In addition, Yarrow currently relies on GenSci for formulation and manufacturing of Yarrow’s Phase 2a/2b clinical trial materials.

More specifically, clinical data generated by GenSci will be shared with Yarrow for inclusion in global safety reports and regulatory submissions by Yarrow to global health authorities including the FDA. Yarrow is the manager of the YB-101 global safety database; as a result, data sharing between Yarrow and GenSci will continue during the term of the GenSci License Agreement. Yarrow does not currently anticipate outsourcing preclinical or clinical research to GenSci, but could consider doing so in the future. Manufacturing data generated by GenSci related to the manufacturing and testing of YB-101 will be shared with Yarrow on an ongoing basis to support global regulatory filings related to manufacturing.

Subject to customary exceptions, during the term of the GenSci License Agreement, neither Yarrow (with respect to activities outside Greater China) nor GenSci (with respect to activities in Greater China), nor their respective affiliates, may directly or indirectly clinically develop or commercialize specified categories of antibodies directed to TSHR.

Upon execution of the GenSci License Agreement, the Company was required to pay GenSci a non-refundable upfront cash payment of $70.0 million. The upfront payment was recorded as acquired in-process research and development in the Company’s statements of operations in the period from October 3, 2025 (inception) to December 31, 2025 since further development and regulatory approval of the licensed product candidates are necessary and there is no alternative use that the Company could benefit from.

GenSci is also eligible to receive up to approximately $1.295 billion in additional contingent payments based on GenSci’s completion of the manufacturing technology transfer, GenSci’s achievement of a development milestone, as well as Yarrow’s achievement of development, regulatory approval, and commercial sales-based milestones. Specifically, GenSci is eligible to receive up to approximately $100 million in contingent payments based on the achievement of specified clinical development milestones by Yarrow or GenSci, including a $50 million near-term development milestone, as applicable, and up to $150 million in contingent payments based on Yarrow’s achievement of specified regulatory approval milestones. In addition, GenSci is eligible to receive tiered royalties ranging from the low teens to the low-mid teens on annual net product sales outside Greater China during the applicable royalty term. The royalty term for a licensed product in a given country commences upon the first commercial sale of the licensed product in that country and continues until the latest of: (a) the expiration of the last royalty-bearing valid claim of the licensed patents covering the licensed product in that country; (b) the tenth anniversary of the first commercial sale of the licensed product in that country; and (c) the expiration of all regulatory exclusivity for the licensed product in that country. The expected expiry of the last-to-expire royalty payment obligation is January 20, 2046.

The GenSci License Agreement will remain in effect until the expiration of all royalty terms. Either party may terminate the GenSci License Agreement for an uncured material breach or insolvency of the other party. GenSci may terminate the GenSci License Agreement in the event of a specified patent challenge by Yarrow or its affiliates or if Yarrow ceases all development activities outside Greater China for a substantial period of time prior to achieving a specified regulatory approval milestone. Following a specified near-term triggering event, Yarrow may terminate the GenSci License Agreement for convenience upon providing the required notice.