It is clear that the details of binding of different classes of agonist ligands to the same receptor may be distinct. This could result in subtle differences in the profile of G protein activation by individual ligands at the same receptor due to agonist-induced selection of conformational states of the receptor which favor interaction and ternary-complex formation with different G proteins. This can result in differences in the details of receptor pharmacology when measured at the level of effector output. Such differences are also likely to be dependent upon both the levels of expression of the receptor and the relevant G proteins and the G protein and effector enzyme isoform expression profile of a particular tissue. It would be foolish, however, to use such differences, in isolation, as a means to expand the classification of G protein-coupled receptors particularly when more molecular approaches to receptor classification are readily available.