4',6-Diamidino-2-phenylindole dihydrochloride (DAPI) is a cell permeable fluorescent dye binding selectively to DNA. It binds to the minor groove of double-stranded DNA (preferentially to AT rich DNA), forming a stable fluorescent complex.DAPI has been used for demonstration of mycoplasmas and viruses in cells (1), for fluorescent chromosome staining (2) and as a dye for brain stem (3). 4',6-Diamidino-2-phenylindole dihydrochloride (DAPI) is a cell permeable fluorescent dye binding selectively to DNA. It binds to the minor groove of double-stranded DNA (preferentially to AT rich DNA), forming a stable fluorescent complex.DAPI has been used for demonstration of mycoplasmas and viruses in cells (1), for fluorescent chromosome staining (2) and as a dye for brain stem (3).