CY5 (Sulfo-Cyanine5) is a far-red fluorescent dye widely used in molecular biology for labeling nucleic acids, proteins, and other biomolecules. With excitation and emission peaks at approximately 649 nm and 670 nm, respectively, it is particularly suited for fluorescence-based applications such as Förster Resonance Energy Transfer (FRET). In single-molecule RNA studies, Cy5 is frequently attached to a biotinylated DNA tether, functioning as an acceptor dye to track RNA folding dynamics, molecular interactions, and enzymatic activity with high precision.