Propidium Iodide (PI) is a fluorescent vital dye that stains DNA and RNA. PI binds to both DNA and RNA, so the latter must be removed by digestion with ribonucleases. The content of DNA as determined by flow cytometry, can reveal useful information about the cell cycle and the proteins involved in the regulation of the cell cycle. Cells in G2 and M phases of the cell cycle have double the DNA content of those in G0 and G1 phases. Cells in S phase have a DNA content lying between these extremes. PI is detected in the orange range of the spectrum using a 562-588 nm band pass filter. This reagent may be used to analyze cell cycle by flow cytometry in addition to use with antibodies for examining the expression of proteins during the cell cycle.