As we have seen above, the theory that depression derives from a deficiency of cerebral serotonin of genetic origin has no evidence; however, the supporters of the chemical imbalance extend the chemical factors of imbalance to serotonin, norepinephrine, dopamine, glutamate, hypothalamic–pituitary–adrenal (HPA) axis, GABA, glucocorticoids, inflammatory cytokines and brain-derived neurotrophic factor (BDNF) [21]. Here, BDNF is linked to major depressive disorder.