This study will examine two groups of subjects with factor IX (FIX) deficiency: 1) those with a current or history of inhibitors to FIX, and; 2) groups of two or more affected brothers, with or without inhibitors. The overall goal is to characterize the study groups in terms of their medical history, their patterns of bleeding, their care, quality of life, and complications including the development of joint disease, inhibitory antibodies to FIX, use of immune tolerance induction (ITI) and outcome.

Hemophilia B, FIX deficiency, is the second most common type of hemophilia, occurring in about one in 25,000 male births. This disease is in some ways more complex than hemophilia A, and is less well understood. Differences include a lower incidence and a greater risk of side effects to treatment, for example, allergic reactions and kidney disease. This study will examine two groups of subjects with FIX deficiency - those with a current or history of inhibitors to FIX, and groups of two or more affected brothers, with or without inhibitors. The overall goal is to characterize the study group in terms of their medical history, their patterns of bleeding, their care, quality of life, and complications including the development of inhibitory antibodies to FIX, allergies, kidney, and joint disease.

Inclusion Criteria:

1. A consent approved by the appropriate Institutional Review Board (IRB)/Independent Ethics Committee (IEC) has been obtained from the subject or his legally acceptable representative

2. Subject has FIX deficiency AND

   • Is part of an affected brother pair/group that will also enroll; AND/OR
   • Has a current or history of inhibitor, defined as >0.6 Bethesda units (BU)

Exclusion Criteria:

1. Subject has another congenital bleeding disorder
2. Subject is a carrier of hemophilia B with factor level >0.40 IU/mL

The population includes groups of two or more affected brothers, with or without a history of inhibitors, who share(d) one or both biological parents; and individuals with a history of an inhibitor. Most affected brother pairs will be concordant for no inhibitor and will serve as a control group for those with inhibitors. The study is open to subjects with mild (0.05-0.40 IU/mL), moderate (0.01-<0.05 IU/mL), or severe (<0.01 IU/mL) FIX deficiency. Females meeting the eligibility criteria may participate. There are no lower or upper age limits. Type of treatment, regimen, dosing and product(s) used are at the discretion of the investigator.