Propidium iodide (PI) is a cell impermeable nucleic acid intercalating dye. Because PI is excluded from viable cells it can be used in apoptotic studies to distinguish necrotic cells or cells in the late stages of apoptosis from viable cells or cells in early apoptosis where cell membranes are still intact. PI is also utilized as a counterstain in multicolor fluorescent imaging. After entering cells and binding to DNA and RNA, the fluorescence of PI is enhanced 20- to 30-fold. PI can be used in flow cytometry and fluorescence microscopy.    &lamba;Ex/&lamba;Em (DNA-bound) = 535/617 nm  &lamba;Ex/&lamba;Em (no DNA) = 493/636 nm  Orange red solid soluble in H2O or DMSO  Store at 4°C and protect from light, especially in solution  C27H34I2N4  MW: 668.4  [25535-16-4]