Atto 655 belongs to a new generation of fluorescent labels. The dye is designed for application in the area of life science, e.g. labeling of DNA, RNA or proteins. Characteristic features of the label are strong absorption, good fluorescence quantum yield, excellent thermal and photo-stability, outstanding ozone resistance, very good water solubility, and very little triplet formation. Atto 655is a zwitterionic dye with a net electrical charge of zero. The fluorescence is efficiently quenched by electron donors like guanine, tryptophan, etc. The aminederivative may be used for reactions with activated carboxy-groups like NHS-esters, TFP-esters etc.