In vivo selection results in significant increases in F cells in mice transplanted with γ-globin/MGMT lentiviral vector-transduced β-thalassemic BM cells. (A) The percentages of F cells in the peripheral blood of mice transplanted with the indicated γ-globin/MGMT vector-transduced β-thalassemic BM cells are shown before (10-12 weeks after transplantation) and 1 month after completing (30-32 weeks after transplantation) administration of 3 doses of BG/BCNU. F-cell percentages are also shown for control mice that received no drug treatment. (B) Percentages of F cells in control untreated mice and in treated responding mice before and after drug administration. Data represent the mean ± SEM. Statistically significant differences are indicated by the P values, as determined by the Student t test.