The most influential hypotheses of depression are as follows: monoamine (assumes a decrease in monoamine levels as a cause of depression), neurotrophic (a decrease in neurotrophic factors levels, mainly BDNF), impaired neurogenesis in the hippocampus, neuroendocrine (hyperactivation of the HPAA), glutamate (altered glutamatergic excitation), and immune/inflammatory (increased levels of inflammatory cytokines) hypotheses [18,19,20,21]. This evidence concerns the gene BDNF and major depressive disorder.