Cy3 and Cy5 are reactive watersoluble fluorescent dyes of the cyanine dye family used for labelling proteins, peptides, DNA, RNA, and other biomolecules. They are usually synthesized with reactive groups on either one or both of the nitrogen side chains so that they can be chemically cross-linked to nucleic acids or protein molecules. Thay are small, pH insensitive (between pH 3 and pH 10) , soluble in aqueous solution and have tolerance to DMSO. They are more photostable than fluorescein, have high molar extinction coefficients and favorable quantum yields. They are used in many different biological assays, including DNA microarrays, protein microarrays, two-dimensional protein analysis, fluorescence resonance energy transfer, and immunocytochemistry. Cy3 is excited maximally at 550 nm and emits maximally at 570 nm, in the green part of the spectrum. Cy5 is excited maximally at 649 nm and emits maximally at 670 nm, in the red part of the spectrum.