Haemophilia A (also spelled hemophilia A or hæmophilia A) is the most common form of haemophilia which is the most common genetic disorder associated with serious bleeding. It is caused by a reduction in the amount or activity of factor VIII. This protein serves as a cofactor for factor IX in the activation of factor X in the coagulation cascade. The lack of this section of the coagulation cascade results in the formation of fibrin deficient clots which makes coagulation much more prolonged, and the the clot more unstable.
Haemophilia A is inherited as an X-linked recessive trait, and thus occurs in males and in homozygous females. However, mild haemophilia A has been described in heterozygous females, presumably due to extremely unfavourable lyonization (inactivation of the normal X chromosome in most of the cells). Approximately 30% of patients have no family history; their disease is presumably caused by new mutations . Approximately, 1 in 5,000 males are affected.
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