Expression of the mLARβΔγV5 γ-globin vector is less susceptible to stable position effects. (A) FACS analysis for TER119 and HbF expression in representative unique, single-copy secondary CFU-S clones derived from primary mice receiving cells transduced with the indicated vector. For the TER119 analysis, M1 represents the gate for cells staining above the isotype control background; the percentage of TER119-staining cells is indicated in each histogram. R2 designates the brightly staining TER119-positive cells that were analyzed for HbF expression. The percentage of HbF⁺ cells for each clone is indicated in the histogram to the right of the TER119 histogram. (B) The percentage of HbF⁺ cells in TER119 clonal erythroblast populations containing the indicated γ-globin vector was determined by FACS analysis. Each column of diamonds represents individual secondary CFU-S clones, derived from primary mice that underwent transplantation, harboring a unique, single integration of the indicated vector. The mean ± SEM of HbF⁺ cells for each group is shown, with the mean indicated by the solid horizontal bar. The P value indicates a statistically significant difference between the mean values of the 2 groups.