ThioLinker-DBCO is a water-soluble, bis-alkylating labelling reagent that enables the site-specific incorporation of DBCO moiety onto antibodies to form well-defined conjugates. This approach involves a disulfide reduction to release the two cysteine thiols followed by the re-forming the disulfide by bis-alkylation, via a three-carbon bridge to which DBCO moiety is covalently attached. During this process, irreversible denaturation of the protein, which would disrupt either their tertiary structure or their biological activity, does not occur. The covalent, site-specific conjugation of PEG to a polyhistidine tag (His-tag) on a protein with bis-alkylating reagents was also reported in literature.