However,because of the nonself-origin of both plasma-derived and recombinantFVIII and FIX concentrates, patients with hemophilia who receive theseproducts may develop neutralizing antibodies against FVIII or FIX,a condition known as “hemophilia with inhibitors.”4,5 The incidence of neutralizing antibody production is approximately20%–30% in patients with severe hemophilia A (FVIII levelsof <1% of the normal value),4,6,7 while lower rates are observed in patients with moderate hemophiliaA and severe hemophilia B (5%–10%).4,6,7. The gene discussed is F8; the disease is hemophilia B.