F9 and hemophilia: Hemophilia is an X-chromosome linked recessive hereditary bleeding disorder, which can be classified into 2 types: hemophilia A and hemophilia B.[1] The former is coagulation factor VIII deficiency, and the latter is coagulation factor IX (FIX) deficiency, caused by mutations in the corresponding coagulation factor genes, respectively.[1] The severity of hemophilia depends on the level of clotting factors.