How does DAPI work?

DAPI (4?,6-diamidino-2-phenylindole) is a blue DNA binding dye with a preference for A-T rich regions, whose quantum yield is significantly enhanced when bound to double-stranded DNA. Although DAPI can pass through an intact cell membrane, it is rarely used for live cell staining because the concentration needed is very high. Therefore, DAPI is generally considered excluded from viable cells, but can penetrate cell membranes of dead or dying cells and bind their intracellular nucleic acids. When bound to DNA, DAPI shows bright blue fluorescence, with excitation/emission maximum of 358/461 nm. DAPI is widely used in fluorescence microscopy and flow cytometry for cell viability measurement and DNA content analysis.