Toxicity, fVIII activity, and donor cell engraftment following BU conditioning with costimulation blockade. Mice were conditioned with 35 mg/kg BU on days − 3 and − 2 and then received a transplant of 3 × 10⁵ BDDpfVIII-transduced sca-1⁺ cells on day 0. CTLA4-Ig and anti-CD40L were administered on days 0 and + 2 relative to transplantation. (A) The percentages of B and T cells over time were determined by flow cytometry after administration of BU + costimulation blockade. (B) eGFP⁺ donor-cell chimerism in the peripheral blood versus fVIII activity at 2 weeks, 8 weeks, and 24 weeks after transplantation. (C) Percent eGFP⁺ donor-cell chimerism versus mean proviral copy number in the peripheral blood versus fVIII activity at the time of death, 6 months after transplantation.