Serotonin is a monoamine neurotransmitter that is made in serotonergic neurons in the CNS (central nervous system) and is important in the regulation of mood, sleep, vomiting, sexuality and appetite. 5-HT3 (5-hydroxytryptamine-3) receptor is the only ligand-gated ion channel within the family of serotonin receptors. It is composed of five subunits consisting of SR-3A, SR-3B, HTR3C, HTR3D and HTR3E. HTR3D (5-hydroxytryptamine receptor 3D), also known as Serotonin receptor 3D, is a 454 amino acid multi-pass membrane protein that is one components of the pentaheteromeric complex that forms the 5-HT3 receptor. HTR3D must be co-expressed with SR-3A to form a functional 5-HT3 receptor complex on the plasma membrane. Until it is complexed with SR-3A, HTR3D is localized within the endoplasmic reticulum. Expression of HTR3D is restricted to kidney, colon and liver. There are three different isoforms of HTR3D that are expressed as a result of alternative splicing events.