Depression is one of the most common psychiatric illnesses, caused by Serotonin and Nor-epinephrine deficiency [10], but in cancer patients other pathogenic factors have been identified where major depressive disorder (MDD) has been mostly linked to an increase in cytokines including Interleukin (IL)-6, which further depletes the amino acid tryptophan and induces indolamine 2,3 dioxygenase (IDO) enzyme [21]. The gene discussed is IDO1; the disease is major depressive disorder.