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.