TPH1 and irritable bowel syndrome: Alterations in mucosal 5-HT levels linked to variations in the expression of Tph-1 and SERT genes, or in the expression of 5-HT intestinal receptors genes 5-HT3 and the 5-HT4, has been reported in intestinal biopsies of patients with Irritable Bowel Syndrome (IBS) [80].