Inositol-1,4,5-triphosphate receptors (IP₃R) are intracellular Ca²⁺ channels located on the membrane of the endoplamic reticulum (ER). They are regulated by IP₃ and Ca²⁺ and are modulated by many additional signals. Three isoforms of IP₃ receptors have been isolated to date, each with different affinities for IP₃. Functional IP₃ receptors are homo- or hetero-tetrameric structures, where each subunit has six transmembrane domains. In addition, each isoform has various splice variants, all leading to a large diversity of IP₃ Receptors.