This reagent is a dibenzocyclooctyne (DBCO) derivative featuring a strained alkyne for copper-free strain-promoted azide-alkyne cycloaddition (SPAAC) and a reactive N-hydroxysuccinimide (NHS) ester group connected via a short butyrate linker. The NHS ester enables selective covalent attachment to primary amines (e.g., lysine residues) on proteins, peptides, antibodies, and other biomolecules, introducing a bioorthogonal DBCO handle for subsequent click chemistry with azide-modified payloads. It is particularly valuable in the synthesis of antibody-drug conjugates (ADCs) for targeted cancer therapies, fluorescence labeling for real-time imaging and diagnostics, and crosslinking studies to probe protein-protein interactions and molecular architectures in biochemistry.