Cyanine5 NHS ester is widely used in bioconjugation for labeling biomolecules such as proteins, peptides, and antibodies. Its NHS ester group reacts efficiently with primary amines (e.g., lysine residues) under mild conditions, forming stable amide bonds. This makes it ideal for fluorescent labeling in applications like fluorescence microscopy, flow cytometry, and immunohistochemistry. Cyanine5 emits in the far-red region (~670 nm), allowing for low background fluorescence and compatibility with common optical filters. It is also used in multiplex assays where multiple fluorophores are needed, due to its distinct spectral properties. Additionally, it is employed in in vivo imaging and nucleic acid labeling when conjugated to oligonucleotides. The dye’s brightness and photostability enhance detection sensitivity in various biological assays.