Cell-cycle analysis is a method in cell biology that employs flow cytometry to distinguish cells in different phases of the cell cycle. Before analysis, the cells are and treated with a fluorescent dye that stains DNA quantitatively, usually propidium iodide (PI). The fluorescence intensity of the stained cells at certain wavelengths will therefore correlate with the amount of DNA they contain. As the DNA content of cells duplicates during the S phase of the cell cycle, the relative amount of cells in the G₀ phase and G₁ phase (before S phase), in the S phase, and in the G₂ phase and M phase (after S phase) can be determined, as the fluorescence of cells in the G₂/M phase will be twice as high as that of cells in the G₀/G₁ phase.