Total assayable (A) and G418-resistant (B) progenitor cells produced in long-term hematopoietic cultures initiated with 5 groups of transduced CD34⁺ cells as outlined in Fig 1. Cells isolated as d5 CR (⧫), d5 CNR (▪), d6 CR (○), or d6 CNR (□) or maintained as 2° TC (▴) were used to establish LTCs supplemented with cytokines. Cultures were demidepopulated every week, followed by replenishment of half of the medium and cytokines. Harvested cells were used for clonogenic assays in the presence or absence of G418. For clarity and ease of presentation, data from each of the 5 groups of cells are presented as the mean ± SE of assayable and G418-resistant clonogenic cells detected at the indicated time points in 6 separate LTCs initiated with BM cells from 6 different normal donors. Values were normalized to represent data obtained from cultures initiated with 10⁴ cells. In 4 experiments, LNL6 was used on d1 and G1Na on d5 and d6, while the reverse order was used in the remaining 2 experiments. *P < .05, the indicated CNR group v the respective CR group. At weeks 3 and 4, d5 CNR and d6 CNR values (in A and B) were also statistically different (P < .05) from those observed in 2° TC. No significant differences were detected between d5 and d6 CNR cells.