Fig. 1. Incorporation of BrdU into synchronous populations of CHO cells at entry into S phase. Mitotic cells were pulse-labeled with BrdU for the last 30 minutes before the end of each interval. Fixed cells were subjected to fluorescence immunocytochemistry designed to visualize nuclear BrdU incorporation, indicative of S phase cells. FITC images across the top are from optical sections through individual cells, showing one example of a BrdU-negative cell (little to no fluorescence, 0 hours) and three examples of BrdU-positive cells with clear nuclear staining (3, 5 and 7.5 hours). The nucleus occupies most of the cell volume in these cells, and the cell periphery is not indicated. The percentage of intact cells (n=500 per time point) showing nuclear staining of BrdU is indicated at the bottom of the figure. The time after mitosis (in hours) is indicated at the top.