Hemophilia A is an inherited blood coagulation disorder caused by deficiencies of clotting factor VIII. Symptoms of hemophilia A include spontaneous internal bleeding and prolonged bleeding or oozing after injury, surgery, and tooth extraction. The severity of hemophilia A is defined by the amount of normal factor VIII present in the blood circulation. In severe hemophilia A (50% of cases), there is less than 1% normal factor VIII activity; in moderate hemophilia A (10% of cases), factor VIII levels range from 1% to 5%; in mild hemophilia A (40% of cases), factor VIII levels range from 6% up to 35%. Factor VIII is encoded by the F8 gene, and variants in F8 lead to the development of hemophilia A. Hemophilia A occurs worldwide with an incidence of approximately 1 in 5000 newborn males. Hemophilia A is inherited in an X-linked recessive manner and, consequently, hemophilia A is normally only seen in males. All males who have an F8 variant are affected, while females who have an F8 variant on one of their X chromosomes are carriers of the disorder. Approximately 10% of carriers exhibit excessive bleeding symptoms. F8 variants are numerous and consist of several different types of genetic abnormalities. The most common F8 variants are the intron 22 and intron 1 inversions. The intron 22 and intron 1 inversions together are responsible for more than half of severe hemophilia A cases, but only rarely result in a milder phenotype. Treatment of hemophilia A with factor VIII infusion has raised life expectancy for patients from 11 years to 63 years.