Transduction of CD34⁺ cells with a cell surface marker gene and expression of the gene product in the enucleated RBC progeny. (A) is an FACS analysis of CD34⁺cells at day 4 posttransduction with the retroviral vector L-tNGFR-SN (solid histogram) when labeled with the antibody against tNGFR, MC192 (shown on the X-axis). Sham-transduced cells (open histogram) were used to set gates for the flow cytometry of tNGFR-expressing [tNGFR(+)] cells and the nonexpressing [tNGFR(−)] cells. The tNGFR(−) and tNGFR(+) cells were subjected to erythroid differentiation and reanalyzed by three-color FACS analyses at day 18 of culture by labeling cells with biotinylated MC192 and streptavidin PE, FITC-conjugated anti-glycophorin A, and Hoechst 33342. Cultures derived from the tNGFR(−) (B and D) and the tNGFR(+) (C and E) populations are shown.