Global Epidemiologic Study of Preexisting Immunity to AAV in Adults With Severe Hemophilia

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.