By3 Azide NHS is a bifunctional fluorescent labeling reagent designed for efficient and selective conjugation. The reagent combines three components: a Cy3 fluorophore, which provides bright fluorescence; an N‑hydroxysuccinimide (NHS) ester, which reacts readily with amines or lysine/N‑termini under mild conditions; and an azide group, which enables subsequent copper‑catalyzed azide–alkyne cycloaddition (CuAAC) “click” chemistry. This dual‑function design allows researchers to introduce both a fluorescent tag and a bioorthogonal handle in a single step, streamlining workflows in imaging, proteomics, and chemical biology.