However, there is evidence suggesting the existence of homomeric glycine receptors in adult animals. Each GlyR subunit is composed of three domains: an extracellular domain (ECD), a transmembrane domain (TMD), and an intracellular domain (ICD). The ECD is mostly made of β-sheets and hosts five orthosteric ligand-binding sites at the interface between subunits. The TMD is made of four α-helixes (M1-M4) embedded in the cell membrane.