In vivo gene therapy with LVs expressing FVIII and FVIII-modified forms under the control of pF8. (A) aPTT assay on plasma of C57BL/6-HA mice up to 52 weeks after LV delivery. Compared with pF8.FVIII, the injection of pF8.RH (n = 3) and pF8.N6 (n = 5) significantly improves FVIII activity 1.4-fold (∼11%) and 1.3-fold (∼10%), respectively (*P ≤ .0001). (B) ELISA assay showing no anti-FVIII antibodies over time among treated mice. Bleeding assay (C) and bleeding time (D) highlighting hemophilic phenotype correction in all FVIII-injected mice compared with HA control mice (*P ≤ .0001). Saline-injected HA mice are used as control (n = 6).