The discovery that the coagulation defect in the blood of one hemophiliac could be normalized by infusion of blood from another hemophiliac in 1940 [1] led to finding that there were two types of hemophilia: hemophilia A, due to deficiency of coagulation factor VIII (FVIII) and hemophilia B, caused by the reduction of coagulation factor IX (FIX) [2]. The gene discussed is F9; the disease is hemophilia.