F9 and hemophilia: Hemophilia is a congenital X-linked recessive bleeding disorder causing low levels of Factor VIII (FVIII, hemophilia A [HA]) or Factor IX (FIX, hemophilia B [HB]), occurring in approximately 1 in 5000 for HA and 1 in 20,000–30,000 for HB) live male births [1], with a similar incidence across ethnic populations [2].