PI solution is used for dead cell staining. Propidium iodide (PI) is an ethidium bromide analog that emits red fluorescence upon intercalation with double-stranded DNA. PI does not permeate viable cell membranes, but passes through disturbed cell membranes and stains the nuclei. PI is often used in combination with a fluorescein compound, such as Calcein AM or FDA, for simultaneous staining of viable and dead cells. The excitation and emission wavelengths of PI-DNA complex are 535 nm and 615 nm, respectively.