DAPI (
4',6-diamidino-2-phenylindole) is a
fluorescent stain that binds strongly to A-T rich regions in
DNA. It is used extensively in
fluorescence microscopy. As DAPI can pass through an intact
cell membrane, it can be used to stain both live and
fixed cells, though it passes through the membrane less efficiently in live cells and therefore the effectiveness of the stain is lower.