Flow cytometric analysis of a cell culture immunolabeled after incorporation of BrdU, showing strong positivity in the cells in S phase. BrdU staining intensity is plotted against DNA content as determined by PI staining intensity. The cell population in the lower left corner represents cells with 2n DNA (G0 and G1 phases of the cell cycle). Cells with 4n DNA (G2 and M phases of the cell cycle) are observed in the lower right corner. Cells with 2n and 4n DNA are negative for BrdU staining, whereas cells in the S phase of the cell cycle (in between cell populations with 2n and 4n DNA) are positive for BrdU staining. FITC, fluorescein isothiocyanate. Provided by Bert Schutte, Department of Molecular Cell Biology, Maastricht University Medical Centre.