As previously mentioned, studies have shown that the ‘oxidative–antioxidant’ function of the body in patients with depression is dysfunctional, mainly manifested in the increased concentration of oxygen free radicals and the abnormal activity of some antioxidant enzymes; namely, copper–zinc superoxide dismutase (CuZn SOD), glutathione peroxidase (GPX), and catalase (CAT) [137]. The gene discussed is CAT; the disease is major depressive disorder.