Cy5 fluoresces in the far-red region.

Cy5 is a fluorescent dye belonging to the Cyanine family of synthetic polymethine dyes. Its absorption wavelength peaks around 646-649 nm, and emission maximum is around 664-670 nm. Cy5 is widely used to label DNA oligos for use as hybridization probes in other applications, such as Fluorescent In-Situ Hybridization (FISH).