Studies of EC cell numbers, tryptophan hydroxylase 1 (Tph1) mRNA levels, serotonin transporter (SERT) expression and/or 5-HT synthesis indicate alterations in 5-HT signaling and bioavailability in IBS and IBD (Coates et al., 2004; Manocha et al., 2012). Here, SLC6A4 is linked to irritable bowel syndrome.