Hemophilia, a rare bleeding disorder with X-chromosomal recessive inheritance pattern, is characterized by the deficiency of coagulation factor VIII (FVIII) in the case of hemophilia A or IX (FIX) in the case of hemophilia B. Severe hemophilia (defined as factor activity < 1%) is associated with a high risk of spontaneous bleeding, mostly affecting joints. The gene discussed is F8; the disease is hemophilia B.