Effect of cell cycle inhibitors. Exponentially growing BJ1 cells were switched to medium with 0.03% FBS and inhibitors. Controls (no switch or addition) and treated cells were extracted with TX-100, fixed with MeOH. Controls were processed at time zero; treated samples were processed at 6 or 8 hours. Samples were then stained for Mcm6, PCNA, phospho-S10-histone H3, and DNA and subjected to cytometry as described in the Methods section. The percentages of G1p+eS, eS, G1a were normalized to the same fraction in the control sample. Normalized eS (early S phase), G1p+eS (high-Mcm6 cells+ early S phase), and G1a (low-Mcm6 G1 cells) are plotted for cells treated with mimosine (A), aphidicolin (B), and nocodazole (C). Serum withdrawal stopped forward movement of cells from the G1a compartment but not G1p or S (middle bars). Serum withdrawal plus mimosine prevented cells from entering or exiting the G1p compartment (A). Serum withdrawal plus aphidicolin arrested cells in S (B). The frequency of eS increased over 6 hours as cells moved from G1p to eS. Serum withdrawal plus nocodazole prevented cells from entering the G1a compartment (C).