The main theories of the underlying pathophysiology of depression include deficiency of monoamine neurotransmitters such as serotonin, noradrenaline and dopamine in the central nervous system, dysregulation of the hypothalamic–pituitary–adrenal (HPA) axis, and reduction in brain-derived neurotrophic factor (BDNF). The gene discussed is BDNF; the disease is depressive disorder.