Assess the seroprevalence of neutralizing antibodies (NAb) to AAV in adults with severe hemophilia A (coagulation factor VIII [FVIII] <1%) or moderately severe to severe hemophilia B (coagulation factor IX [FIX] ≤2%).

Inclusion Criteria:

1. Participant is male between 18 and 75 years old at the time of screening.
2. Established severe hemophilia A (plasma Factor VIII (FVIII) activity <1%) or B (plasma Factor IX (FIX) activity ≤2%).
3. Provision of signed informed consent form (ICF).
4. Participant is willing and able to comply with the requirements of the protocol.

Exclusion Criteria:

1. Bleeding disorder(s) other than hemophilia A or B.
2. Personal laboratory evidence of having developed inhibitors to FVIII or FIX protein at any time (≥0.6 Bethesda Units [BU] on any single test).
3. Currently receiving systemic immunosuppressive therapy, cytotoxic chemotherapy, immunoglobulin therapy, or monoclonal antibody therapy.
4. Received systemic antiviral and/or interferon therapy within 30 days of blood draw, with the exception of treatment for human immunodeficiency virus (HIV).
5. Currently receiving ribavirin and/or interferon based therapy for active hepatitis C virus (HCV).
6. Received immunoglobulin therapy or plasma infusion within 120 days of the blood draw.
7. Has a known immune deficiency other than HIV.
8. Has lymphocyte or plasma cell malignancies.
9. Participant is a family member or employee of the investigator.

Hemophilia A and hemophilia B patients who receive treatment at Hemophilia Treatment Centers.

Takeda provides access to the de-identified individual participant data (IPD) for eligible studies to aid qualified researchers in addressing legitimate scientific objectives (Takeda's data sharing commitment is available on https://clinicaltrials.takeda.com/takedas-commitment?commitment=5). These IPDs will be provided in a secure research environment following approval of a data sharing request, and under the terms of a data sharing agreement.

IPD from eligible studies will be shared with qualified researchers according to the criteria and process described on https://vivli.org/ourmember/takeda/. For approved requests, the researchers will be provided access to anonymized data (to respect patient privacy in line with applicable laws and regulations) and with information necessary to address the research objectives under the terms of a data sharing agreement.