High-level gene transfer and β-globin production in individual BFUe's from thalassemia major bone marrow transduced with the BGI vector. Individual BFUe's were picked at 2 weeks and HbA analysis performed after intracellular labeling of half the colony with human zeta-globin (A) as a negative control and the other half with HbA (B) and analyzed by FACS. Panels C to F show the MFI from 20 individual BFUe colonies from NBM, TBM, and TBM-BGI from experiments 1, 2, 3, and 4 respectively. The average MFIs ± SEMs of HbA were as follows: experiment 1, NBM 549 ± 34, TBM 17 ± 4, TBM-BGI 304 ± 41; experiment 2, NBM 140 ± 6, TBM 4 ± 2, TBM-BGI 118 ± 4; experiment 3, NBM 132 ± 8, TBM 44 ± 5, TBM-BGI 167 ± 11; experiment 4, NBM 650 ± 49, TBM 33 ± 9, TBM-BGI 369 ± 39. The horizontal lines in panels C-F represent the average MFI of the 20 BFUe's in that group.