5-HT4 receptor (HTR4) is a member of the Serotonin Receptor subfamily and has been shown to affect the sensory and motor functions of the gut. Some of the most promising emerging therapies in Irritable Bowel Syndrome revolve around targeted pharmacotherapeutic modulation of serotonin receptors (ie, 5-HT3 and 5-HT4 subtypes). At least 9 isoforms (5-HT4A-5-HT4G, and 5-HT4n) have been described, which differ in the length and composition of their intracellular C termini after a common splicing site (L358). A ninth isoform (5-HT4H) results in the insertion of 14 amino acids into the second intracellular loop. References: The UniProt Consortium. Nucleic Acids Res. 47: D506-515 (2019); Nucleic Acids Res. 2016 Jan 4;44(D1):D733-45, PMID:26553804