In IBS patients mucosal mRNA expression (rho = 0.32, 0.43, 0.23 and 0.33, all p < 0.05), but not fecal protein levels of CgA, CgB, SgII and SgIII, was correlated with mucosal tryptophan hydroxylase 1 (TPH1) mRNA expression, suggesting that increased mucosal mRNA expression of granins are linked to increased production of mucosal serotonin. Here, TPH1 is linked to irritable bowel syndrome.