S-phase suicide assay of HSPCs assayed in vitro. Human CD34⁺ cells were stimulated (+cyt) or not (–cyt) with cytokines for 18 hours, then exposed or not to Ara-C for another 12 hours, as indicated, and analyzed immediately (A) or after 1 week (B) of culture. For B, the cells were also transduced with LV-GFP during the first 18 hours of incubation. (A) Cell-cycle analysis: representative FACS histograms of propidium iodide (PI) staining; distribution of the analyzed cell populations in the indicated cell-cycle phases is shown in Table 2. M1 indicates apoptotic/necrotic cells; M2, G₀/G₁ phases; M3, S phase; and M4, G₂/M phases. (B) Representative FACS analysis of GFP expression in suspension culture. Percentages of GFP-positive cells are given.