A compelling evidence has suggested that 5-HT is involved in CD pathogenesis, which is characterized by the prevalence of enterochromaffin cells, higher 5-HT mucosal content, increased activity of tryptophan hydroxylase I (TPH-1), and more importantly, lower expression of SERT, a critical regulator of the intestinal 5-HT availability10. This evidence concerns the gene TPH1 and Cowden disease.