Propidium iodide is a fluorescent dye commonly used in biological research to stain cells. It is particularly useful for identifying dead cells, as it can penetrate only those with compromised cell membranes. When bound to DNA, it emits a bright red fluorescence, allowing researchers to visualize and analyze cell viability under a microscope. In addition to its role in cell viability assays, propidium iodide is often used in combination with other dyes in flow cytometry. This technique enables scientists to study various cell characteristics, such as size and granularity, providing valuable insights into cellular processes and health.