Liver Biopsy Following Gene Therapy For Hemophilia

This observational study will obtain liver biopsy samples and evaluate the long-term effect of adeno-associated virus (AAV)-mediated gene therapy on the liver tissue in adult patients with hemophilia A or hemophilia B who have previously been treated with a factor VIII or factor IX gene-containing AAV-vector for liver-targeted gene transfer. Participants are from a cohort of patients treated with AAV-mediated gene transfer and at least 6 months after vector infusion.

Inclusion Criteria: * Age ≥18 to 80 years * Patients, who were enrolled and treated in one of the following clinical trials: * AGT4HB (EudraCT number: 2005-005711-17; NCT00979238) - FIX AAV gene therapy trial (sponsor: St. Jude Children's Research Hospital) * GO8 (EudraCT number:2014-003880-38; NCT02576795) - FVIII AAV gene therapy trial (sponsor: University College, London) * Able to give informed consent * Able to comply with study requirements Exclusion Criteria (Do not apply to participants who will not undergo liver biopsy, and have leftover liver tissue from a previous biopsy procedure, because all exclusion criteria only cover the safety considerations for the biopsy procedure.): * Any condition that, in the opinion of the investigator or sponsor of the ongoing clinical trial in which the patient is participating in, would prevent the patient from fully complying with the requirements of the clinical trial and/or would influence or interfere with evaluation and interpretation of subject safety or efficacy result of that ongoing clinical trial * Platelet count \1.5 * Abnormal kidney function with estimated GFR \