Flow cytometric analysis of progression of cells synchronized at the G1/S boundary. A, B: Upper panel demonstrates two-parameter histogram of distribution of BPHLacZ cells (A) and BPHL cells (B) in G1 (2N) and G2 (4N) phases at different times after release from thymidine block. Lower panel shows dot plots of BrDU incorporation in these cells as they progress through the S phase. Appearance of BrDU stained cluster of cells in G1 (upper left quadrant) at 12 h indicates entry of cells to the next cycle. C, D: Graphical representation of the percentage distribution of BPHLacZ cells (C) and BPHL cells (D) in initial G1 (IG1) (lower left quadrant), initial G2 (IG2) (lower right quadrant), final G1 (FG1) (upper left quadrant) and final G2 (FG2) (upper right quadrant) at different time points after release. Data shows temporary arrests of BPHL cells in G1/S phase. Similar profile was obtained in two separate experiments.