HTR3A (5-HYDROXYTRYPTAMINE RECEPTOR 3A), also called SEROTONIN 5-HT-3 RECEPTOR, is a member of the ligand-gated ion channel receptor superfamily. HTR3A is a neurotransmitter in the central and peripheral nervous systems that plays a role in many physiologic processes such as sleep, appetite, thermoregulation, pain perception, hormone secretion, and sexual behavior. Using fluorescence in situ hybridization, the HTR3 gene is assigned to 11q23.1-q23.2. The single-channel conductance for homomers of HTR3A is 0.4 pS. Replacement of 3 arginine residues unique to the HA-stretch of the HTR3A subunit by their HTR3B subunit counterparts increases single-channel conductance 28-fold. Ultrastructural studies of the Torpedo nicotinic acetylcholine receptor indicated that the key residues might frame narrow openings that contribute to the permeation pathway. The genomic structure of the HTR3A gene contains 7 exons and spans approximately 14.5 kb.