Inhibitors develop much more rarely in patients with a moderate (FVIII 1%–5%) or a mild form (FVIII > 5%–40%) of haemophilia A and, unlike that in those with severe haemophilia A, the risk of inhibitor formation has been found to increase in parallel with exposure to FVIII concentrates in these patients, so that inhibitors often develop during adulthood, frequently after a period of intensive treatment [6,7,8,9]. This evidence concerns the gene F8 and hemophilia A.