This guidance provides recommendations to sponsors developing human gene therapy (GT) products for the treatment of hemophilia including clinical trial design and related development of coagulation factor VIII (hemophilia A) and IX (hemophilia B) activity assays, including how to address discrepancies in factor VIII and factor IX activity assays. This guidance also includes recommendations regarding preclinical considerations to support development of GT products for the treatment of hemophilia. Additional clinical and preclinical recommendations are available in other guidances (Refs. 1 and 2). This guidance does not provide recommendations for products for the treatment of hemophilia C (factor XI deficiency) or for the treatment of any bleeding disorders other than hemophilia A and B, because of the unique nature of those bleeding disorders. This guidance finalizes the draft guidance of the same title dated July 2018.