Propidium iodide (CAS 25535-16-4), commonly abbreviated as PI, is a fluorescent nucleic acid intercalating dye widely employed in biomedical research. Structurally analogous to ethidium bromide, PI intercalates into double-stranded DNA molecules without sequence specificity, binding approximately one dye per 4–5 base pairs. Due to membrane impermeability, PI selectively enters cells only when plasma membrane integrity is compromised, as occurs during cell death or late apoptosis. Upon DNA binding, its fluorescence notably increases, facilitating detection via fluorescence microscopy, spectrometry, or flow cytometry. PI is routinely utilized for distinguishing necrotic or late apoptotic cells in viability assays, apoptosis studies (often paired with Annexin V), and cell cycle analyses.