TPH2 and major depressive disorder: Studies of the human brain have revealed two isoforms of the gene coding for tryptophan hydroxylase 1 and 2, termed TPH1 and TPH2. One of them, the TPH2 gene, is of interest because this tryptophan hydroxylase 2 isoform regulates the biosynthetic pathway of 5-HT in the serotoninergic neurons of raphe nuclei and has been implicated in the pathogenesis of major depressive disorder and the mechanism of antidepressant action [14].