The Cell Clock assay can be used to monitor the four major phases of the mammalian cell cycle during in vitro culture. The assay uses a redox dye that is imported by cycling cells. After dye uptake and incubation, a distinct color change occurs within cells, with particular color changes being associated with cells in the G0–G1, S, G2, and M phases. The ratio of cells within the different phases can be calculated using this assay supplemented by visible evidence from photomicrographs. The calculation of phase ratios can be obtained by computer software analysis (ImageJ) of the digitized images of photomicrographs.