Indeed, recurrent spontaneous hemarthrosis episodes frequently observed in severe and moderate forms of the diseases (<1%, and 1% to <5% of residual factor IX activity, respectively), are rare when coagulation factor IX activity is higher than 5%, as observed in mild cases (5 to <30% of the normal FIX value) (Mannucci and Tuddenham, 2001). This evidence concerns the gene F9 and hemarthrosis.