Sustained fVIII expression levels in secondary hemophilia A transplant recipients demonstrating successful genetic modification at the level of primary HSCs. Human efVIII and sfVIIIΔB antigen (fVIII:Ag) levels in the plasma of secondary transplant recipients that received BM cells from FB.efVIII- or MSGV-sfVIIIΔB–corrected primary BAL-fVIIIKO mice measured by ELISA. Mice were conditioned by lethal (800 cGy) TBI before transplantation with BM cells from primary corrected hemophilia A mice as indicated: I indicates BM cells from primary transplant recipients that had received a 550 cGy TBI conditioning regimen; B, BM cells from primary transplant recipients that had received BU-ATS conditioning.