Why is DAPI used as a counterstain?

DAPI is used as a counterstain to detect nuclei in multicolor fluorescence applications. Its blue fluorescence is distinctive to other fluorescent dyes (e.g. red, yellow or green colors) utilized to stain other structures. This is ideal, as it is necessary to differentiate the signals from the secondary antibody and counterstain. DAPI is also used as a counterstain to ensure a cell has absorbed the stain and not any external toxins. This is done after a specific cell has been localized. DAPI is also useful for multiplexing, as there is very little overlap between DAPI and other fluorescent molecules such as GFP and FITC.