Haemophilia B is less common than haemophilia A and arises when a person does not have enough clotting factor IX. Although haemophilia B is usually hereditary, a spontaneous mutation of the person’s own genes is responsible for about 30% of cases. In Switzerland, approximately 70 people are affected by haemophilia B. The severity of haemophilia B is categorised 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 haemophilia 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.