Induction of cell cycle in stem-cell–containing populations. Mice were treated with a single dose of 5-FU and bone marrow was harvested 1, 2, 3, 5, and 10 days after treatment. Bone marrow from both femurs and tibias of 5 mice was pooled for each time point, depleted of erythroid cells, and counted (A). To analyze the frequency of progenitor cells (HSC) in the S/G₂/M phases of the cell cycle, progenitor populations from each time point were purified by depleting lineage⁺ cells (excluding Mac-1), by MACS, and staining the remaining cells with anti–Sca-1, c-kit, and Thy-1.1. The lineage⁻, Sca-1⁺, Thy-1.1^low, c-kit⁺ population (including both c-kit^high and c-kit^low cells) was then sorted and analyzed for DNA content by propidium iodide uptake (B). At least 7,000 cells at each time point were collected for DNA analysis. For an analysis of progenitor frequency, WBM from each time point was stained with the lineage cocktail (including anti–Gr-1, CD3, CD4, CD8, B220, and Ter119) and with anti–Sca-1, Thy-1.1, and c-kit. The percent of lineage⁻, Sca-1⁺, Thy-1.1^low, c-kit⁺ (including c-kit^high and c-kit^low) cells in WBM was determined by integration analysis using FACSDESK22 software (C). The total number of lineage⁻, Sca-1⁺, Thy-1.1^low, c-kit⁺ cells in the femurs and tibias of mice at each time point was calculated by multiplying the frequency of the population (C) by the cellularity of the bone marrow (A) and is shown in D.