DBCO-Sulfo-NHS Ester is a water-soluble version of the popular DBCO NHS Ester (#4529). It is an excellent building block for labeling antibodies, proteins, and other primary amine-bearing macromolecules (e.g., peptides and amino-modified DNA/RNA oligos) in aqueous buffers. Dibenzocyclooctyne (DBCO) compounds might be the most common class of Cu-free click reaction reagents that possesses reasonably fast kinetics and good stability in aqueous buffers. The Cu-free click reaction, i.e., the strain-promoted alkyne-azide cycloaddition (SPAAC) is a bioorthogonal reaction utilizing a pair of reagents, cyclooctynes and azides that exclusively and efficiently react with each other while remain inert to naturally occurring functional groups such as hydroxy, amino and thiol groups. SPAAC enables labeling a wide variety of biomolecules without any auxiliary reagents in an aqueous and otherwise complex chemical environment through the formation of a stable triazole.