F9 and hemophilia B: Congenital hemophilia is a rare hemorrhagic disorder characterized by the deficiency of coagulation factor VIII (FVIII) in hemophilia A or factor IX (FIX) in hemophilia B. The severity of the disease depends on the plasma levels of FVIII or FIX and is therefore classified as mild (deficient factor 6–40%), moderate (1–5%), or severe (<1%).