Furthermore, we reported that the TPH1 rs211105 T/T genotype in Japanese patients with IBS was associated to the efficacy of ramosetron, a 5-HT3 antagonist.(23) This result and our findings implied that upregulated 5-HT signaling is evoked in TPH1 rs211105 T/T subjects. Here, TPH1 is linked to irritable bowel syndrome.