As described briefly in Section 1, host-guest systems are similar to protein-ligand systems in that they both involve the binding of a small molecule to a pocket in a receptor. Hosts often contain less than 100 non-hydrogen atoms, but are slightly larger than small molecules , so the broader field of such chemistry is often called supramolecular chemistry. Usually, hosts don't have large number of possible folds or conformational structures like a protein . Eventually some host-guest systems are well characterized and become part of the driving force behind methodology improvement, with the ultimate goal of transferability to protein-ligand systems .