Hemophilia B is less common than hemophilia A and arises when a person does not have enough clotting factor IX. Although hemophilia B is usually hereditary, a spontaneous mutation of the person’s own genes is responsible for about 30% of cases. The severity of hemophilia B is categorized as mild, moderate or severe, depending on the factor IX activity measured in the blood. About one-third of patients suffer from the severe form of this disease. Patients with hemophilia B have prolonged bleeding after injuries, operations, and dental treatment. In severe cases, there may also be spontaneous bleeding, that is to say, bleeding for no obvious reason. Severe complications may arise from bleeding into the joints, muscles, brain or other internal organs.