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N-(m-PEG4)-3,3-Dimethyl-3H-indole-N'-(acid-PEG3)-Benzothiazole Cy5 is a PEG linker containing a cyanine dye with excitation/emmission maximum 649/667 nm and a free terminal carboxyl acid. Terminal carboxylic acid can react with primary amine groups in the presence of of activators (e.g. EDC, or HATU) to form a stable amide bond. The hydophilic PEG spacer increases solubility in aqueous media.