TPO/SCF-displaying LVs promote transduction and survival of immature quiescent G₀ CD34⁺ cells. (A) FACS analysis of CD34⁺ CB cells stained with PY as described in “Materials and methods.” Quiescent cells residing in the G₀ phase of the cell cycle (G₀ CD34⁺) have minimal RNA content, as indicated by low PY staining, whereas cells residing into G_1b, S, G₂+M phase (G₁+) have a high RNA content. Sort windows to collect G₀ and G₁+ cells are shown in the dot blot (A) as R1 and R2, respectively. R1 and R2 were kept well separated to avoid contamination of the 2 cell categories. The G₀ population was checked for purity by 7-AAD (DNA)/PY (RNA) staining. The G₀/G_1a,G_1b, and SG₂M phases are indicated. In panels B and C, the G₀ and G₁+ CD34⁺ populations were transduced for 72 hours with TPO-, SCF-, or SCF/TPO-displaying vectors or with VSV-G–pseudotyped vectors. The percentage of CD34⁺ cell survival is indicated in panel B, and the numbers of GFP⁺ CD34⁺ marked cells (number of cells at start of infection [5 × 10⁴] × cell expansion × % cell transduction × % cell survival) for the G₀ and G₁+ populations are shown in panel C. Data are shown as means ± SD, n = 3.