Apoptosis and cell-cycle analyses. Cells were incubated with or without flavopiridol for 24 hours and then harvested for flow cytometry. Percentage of apoptotic cells with sub-G₁ DNA content (M₁ phase) was indicated in the histograms, and percentages of cells in different phases of cell cycle were analyzed using Modfit software. Similar results were obtained from 3 independent experiments. After exposure to flavopiridol, the population of the cells in S phase decreased, whereas the populations in G₁ or G₂ phases (or both) and percentage of the apoptotic cells increased.