However, it is known that biogenic amine deficiency (particularly 5-HT, dopamine, and noradrenaline), neurotrophic factors such as brain-derived neurotrophic factor (BDNF), gut microbiota deregulation, genetic, immunologic, endocrine, environmental factors, and neurogenic problems are underlying the origin of depression [11]. The gene discussed is BDNF; the disease is depressive disorder.