TPH1 and irritable bowel syndrome: In IBS, several studies reported associations of symptoms of IBS and numbers of EC cells, expression of mRNA levels of Tph1, and SERT expression in mucosal biopsy; 5-HT content is also altered in IBS (Miwa et al., 2001; Coates et al., 2004; Spiller, 2008; Camilleri, 2009; Malinen et al., 2010; Cremon et al., 2011).