TPH1 and irritable bowel syndrome: The study of Dussik et al. (2018[14]) on gene expression profiling and assessment of vitamin D and serotonin pathway variations in patients with irritable bowel syndrome showed that IBS patients’ derived RNA exhibited lower levels of tryptophan hydroxylase-1 (TPH1) expression, the enzyme that catalyzes the rate-limiting step in serotonin biosynthesis in enterochromaffin cells.