F9 and hemophilia B: Early research hinted at mutational heterogeneity, and this has been confirmed by the wide variety of mutations discovered in the coagulation factor VIII gene in hemophilia A and the coagulation factor IX gene in hemophilia B. Both genes are located on the X chromosome: the factor VIII gene is near the end of the long arm at Xq28, and the factor IX gene is also on the long arm, closer to the centromere at Xq27.2.