Ionotropic glutamate receptors are integral membrane proteins composed of four large subunits that form a central ion channel pore. Sequence similarity among all known glutamate receptor subunits, including the δ receptors, suggests they share a similar architecture. Glutamate receptor subunits are modular structures that contain four discrete semi-autonomous domains: the extracellular amino-terminal domain (ATD), the extracellular ligand-binding domain (LBD), the transmembrane domain (TMD), and an intracellular carboxyl-terminal domain (CTD).