Definition of Factor VIII

Factor VIII: Factor eight, a key factor in the process of blood coagulation (clotting). Lack of normal factor VIII causes hemophilia (hemophilia A).

The gene for classic hemophilia was long known to be on the X chromosome. Females carry the gene and transmit it to their hemophiliac sons. Female carriers are normal since they have another X chromosome that contains a normal gene to make factor VIII. Each of their sons has a one-half (50:50) risk of being a hemophiliac.

Because the gene for hemophilia was clearly on the X chromosome, it was correctly inferred that the normal gene for Factor VIII was on the X. Factor VIII is also known as antihemophiliac factor or antihemophiliac globulin.