The 5-HT4 receptor, a member of the Serotonin Receptor subfamily, 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. 5-HT4 receptor expression has been documented primarily in the brain, and in colon, heart, and vessel. An EST has been isolated from a human cancerous nerve library.