The HaloTag® Amine (O2) Ligand contains a reactive amine group connected to an alkylchloride, separated by an ethylene glycol repeat (O2). The HaloTag® Amine (O2) Ligand can be successfully conjugated to any reporter group, protein, or nucleic acid derivative containing an activated carboxylic acid, sulfonyl halide or isocyanate. Examples of activated carboxylic acids are succinimidyl esters (see figure), STP esters, acid halides and TFP esters. The ligand with functional group can then be used with the HaloTag® protein for any application of interest.