Anti–human factor IX IgG levels in B10.A congenic mice following intravenous injection of AVC3FIX5 vector indicate role of MHC H-2 genes. B10.A-H2^aH2-T18^a/SgSnJ and B10.A-H2ⁱ⁵H2-T18^a(5R)/SgSnJ mice were injected with 6.8 × 10¹¹ vector particles/kg AVC3FIX5 adenovirus vector and IgG levels versus human factor IX were measured 1 month later. Ten males and 10 females from each strain were assessed. Mean IgG anti–human factor IX levels are shown with 95% CIs indicated by vertical lines (▴, males; •, females). Mouse strains are A/J (A/J), B10.A-H2^aH2-T18^a/SgSnJ (B10.A), B10.A-H2ⁱ⁵H2-T18^a(5R)/SgSnJ (B10.A5R), and C57BL/6J (C57).