N-(azide-PEG4)-3,3-Dimethyl-3H-indole-N'-(m-PEG3)-Benzothiazole Cy5 is a PEG derivative containing a cyanine dye with excitation/emmission maximum 649/667 nm as well as an azide group, which enables Click Chemistry. The hydophilic PEG spacer increases solubility in aqueous media.