A hereditary sex-linked disease (see sex linkage) in which there is a deficiency or defect of certain clotting factors, causing the blood to clot very slowly. The most common form is haemophilia A, which involves a deficiency of Factor VIII, whereas the much rarer haemophilia B is caused by deficiency of Factor IX. There may be prolonged bleeding following injury and, in severe cases, spontaneous bleeding into the joints and muscles. The disorder is due to a defective recessive allele of the respective gene, both of which in humans are located on the X chromosome. Female carriers of a defective allele are unaffected, whereas all males who inherit a defective allele exhibit the disease. Treatment is by infusion of the deficient clotting factor, obtained either from human donated blood plasma or recombinant tissue culture.