We evaluated the natural history of NAb titers to AAV serotype 5 (AAV5) in adult males ≥18 years old with hemophilia B (factor IX ≤ 2%) during the lead-in period of a phase 3 trial prior to the infusion of etranacogene dezaparvovec to characterize NAb in addition to immunoglobulin G (IgG) and immunoglobulin M (IgM) anti-AAV5 binding antibody changes over time. This evidence concerns the gene CD40LG and hemophilia B.