Flow cytometric analysis of a representative transfection of purified CD34⁺ cells with the amphotropic MFG-EGFP retroviral vector after 2 days of prestimulation and 2 days of supernatant infection in the presence of IL-3, IL-6, and SCF. This particular transduction resulted in efficiencies of 30% within the CD34⁺ population (A). In (B) CD34⁺ cells were gated and the CD38 distribution of the EGFP-transduced cells was studied. Also, CD34⁺CD38⁻ cells expressed the EGFP gene with efficiencies similar to the total CD34⁺ population (30% EGFP⁺).