DAPI

DAPI is a fluorescent dye that strongly binds to A-T-rich regions of DNA. It becomes highly fluorescent when bound to the minor groove of the DNA helix. DAPI has an absorption maximum at 358 nm (ultraviolet) and its emission maximum is at 461 nm (blue). Therefore, DNA can be visualized with a fluorescent microscope by shining ultraviolet light on the DAPI stained sample and filtering for blue light.