Increased probability of γ-globin vector expression at intergenic sites of vector integration. The percentage of HbF⁺ cells as determined by FACS analysis is shown for 22 CFU-S clones as a function of the site of insertion (intragenic or intergenic) within the cellular genome. The mean ± SEM of HbF⁺ cells for each group is shown, with the mean indicated by the solid bar. The P value indicates a statistically significant difference between the mean values of the 2 groups.