The HaloTag® Succinimidyl Ester (O2) Ligand contains a reactive succinimidyl ester (SE) group connected to an alkylchloride separated by an ethylene glycol repeat (O2). The HaloTag® Succinimidyl Ester (O2) Ligand can be successfully conjugated to any reporter group, protein, or nucleic acid derivative containing an amine, forming stable amide bond linkages (see figure). The ligand with functional group can then be used with the HaloTag® protein for any application of interest.