In HA and HB, the level of deficient clotting factor VIII (FVIII) and IX (FIX),2respectively, correlates with the severity of symptoms, and distinction is, therefore, made between severe (less than 1% factor activity, which corresponds to <0.01 IU/mL), moderate (1–5% factor activity), and mild hemophilia (more than 5% factor activity). This evidence concerns the gene F8 and hemophilia.