Profile of BrdU and cyclin D1 expression through the cell cycle. MRC5, human diploid fibroblasts, were pulsed with BrdU for 20 min, fixed and stained for cyclin D1 and BrdU with fluorescent antibodies, while DNA was stained with DAPI. Images of each fluorochrome were subjected to image analysis to quantitate the level of each fluorochrome in each cell. (Top) BrdU levels are plotted vs. DNA content for each cell; and (Bottom) cyclin D1 levels are plotted vs. DNA content. BrdU negative cells in G1 (red), or G2 (blue) phases are indicated by open circles; while S phase cells (black) are represented by smaller closed circles.