Congenital haemophilia A affects 1 in 5000–10,000 male births, and coagulation factor VIII encoded by the F8 gene is deficient in haemophilia A. In X-linked inheritance, a female heterozygote with a mutation on only one of the two X-chromosomes, the so-called carrier, often has FVIII:C of more than 50 IU/dL of normal activity. This evidence concerns the gene F8 and hemophilia A.