Propidium iodide

Propidium iodide (or PI) is a fluorescent molecule with a molecular mass of 668.4 Da that can be used to stain DNA. PI also binds to RNA, necessitating treatment with nucleases to distinguish between RNA and DNA staining. It can be used to differentiate necrotic, apoptotic and normal cells.

Propidium iodide is membrane impermeant and generally excluded from viable cells. PI is commonly used for identifying dead cells in a population and as a counterstain in multicolor fluorescent techniques. The absorption maximum of unbound PI is 488 nm (535 nm when bound to DNA) and the emission maximum is 590 nm (617 nm).