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.