The hexahistidine-tag (His6, see His-tag) provides binding sites for three Ni-NTA moieties. Tris-NTA is thus perfectly suited for non-covalent, stable, highly selective Protein Labeling of hexahistidine tags in a stoichiometric ratio of 1:1 at sub-nanomolar affinity. In RED-tris- NTA labeling kits the tris-NTA scaffold is conjugated with TRIC optimized NanoTemper Technologies Fluorophores.