Trp metabolism is up-regulated in depression which results in impaired serotonin synthesis, upregulation of indolamine 2,3 dioxygenase (IDO) activity, and 3-hydroxykynurenine and quinolinic acid which are neurotoxic in nature. Furthermore, there is a decrease in neuroprotective molecules such as kynurenine and kynurenic acid. This evidence concerns the gene IDO1 and major depressive disorder.