Endogenous factor VIII synthesis from the intron 22–inverted F8 locus may modulate the immunogenicity of replacement therapy for hemophilia A

Patients with hemophilia lacking functional coagulation factor VIII (FVIII) are treated with replacement FVIII proteins. The development of neutralizing antibodies to the replacement FVIII, a major clinical problem, depends on the nature of the mutation in the gene encoding FVIII. The authors find that a prevalent inversion allele can unexpectedly produce FVIII protein, explaining why individuals with this allele do not frequently develop neutralizing antibodies.