Propidium iodide (or PI) is an intercalating agent and a fluorescent molecule with a molecular mass of 668.4 Da that can be used to stain cells. When PI is bound to nucleic acids, the fluorescence excitation maximum is 535 nm and the emission maximum is 617 nm. Excitation energy can be supplied with a xenon or mercury-arc lamp or with the 488 line of an argon-ion laser. Propidium iodide is used as a DNA stain for both flow cytometry, to evaluate cell viability or DNA content in cell cycle analysis, and microscopy to visualise the nucleus and other DNA-containing organelles. It can be used to differentiate necrotic, apoptotic and normal cells.