Transduction efficiency detected 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 (○), and 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 every time point, transduction efficiency was calculated as (no. of G418-resistant colonies/no. of cells plated in G418) × (no. of cells plated without G418 × 100/no. of unselected colonies). For clarity and ease of presentation, data from each of the 5 groups of cells are presented as the mean ± SE of transduction efficiency calculated at each indicated time point in 6 separate LTCs initiated with BM cells from 6 different normal donors. *P < .05, the indicated CNR group vthe respective CR group. At weeks 3 and 4, d5 CNR and d6 CNR values were also statistically different (P < .05) from those observed in 2° TC. No significant differences were detected between d5 and d6 CNR cells at any time point.