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.