N-Hydroxysulfosuccinimidyl-4-azidobenzoate (sulfo-SADB) is a water-soluble, heterobifunctional reagent used in bioconjugation, protein labeling, and photoaffinity studies. The sulfo-NHS ester reacts selectively with primary amines on biomolecules to form stable amide bonds, attaching the 4-azidobenzoyl group. The aryl azide moiety enables two key applications: (1) copper-catalyzed azide-alkyne cycloaddition (CuAAC) click chemistry for precise conjugation of fluorophores, biotin, or other detection tags in immunoassays and cell surface labeling; (2) UV irradiation to generate a reactive nitrene for photocrosslinking to proximal proteins, ligands, or cellular components, facilitating studies of protein-protein interactions, structural biology, and localization under physiological conditions without organic solvents.