It is possible that depression involves dysregulation of both central and peripheral serotonergic systems, while successful antidepressant treatment normalizes the function of both.[29,30] A large quantity of studies reported that allelic variants in tryptophan hydroxylase (TPH)1 expressed in peripheral tissue may be genetic predictors of depression[29,31,32] to confirm the above hypothesis that both central and peripheral serotonergic systems were involved in the pathophysiology of depression. Here, TPH1 is linked to major depressive disorder.