What does propidium iodide measure?

Propidium iodide measures the DNA content of cells in different stages of the cell cycle. The measurements are done to separate the cells based on the 3 cell-cycle phases, G0/G1 phase, S phase, and the G2/M phase. The amount of DNA content depends on which cell cycle is being measured and is also based on the ploidy of cells. The ploidy of the cells are correlated to the specific cell cycle phase. For example, in the G0/G1 phase, cells have 2n ploidy. In the G2/M phase, the cells have 4n ploidy, which is double the DNA content. Additionally, the cells in the G2/M phase will have double the fluorescence intensity than in the G0/G1 phase. When viewed on the mass spectrum, the fluorescence intensities show up as two separate peaks, reflecting the different phases in the cell cycle. This means that the cell population between the G0/G1 and G2/M peaks reflects the cells in the S phase. To conclude, PI measures not only the DNA content of cells in different stages of the cell cycle, but also measures their fluorescence on the mass spectrum.