AF555 NHS Ester is a bright orange-red fluorescent probe containing the AF555 fluorophore and an NHS active ester group. It efficiently forms covalent bonds with primary amines (R‑NH₂) in biomolecules, enabling stable labeling and tracking of target molecules. This reagent is suitable for fluorescent labeling of proteins, amine-modified oligonucleotides, and other amine-containing molecules. Its excitation/emission maxima are 555/572 nm, making it widely used in bioimaging, immunoassays, and other life science applications.