In terms of neurochemicals, altered levels of several neurotransmitters, including 5-hydroxytryptamine (5-HT), dopamine (DA), norepinephrine (NE), gamma-aminobutyric acid (GABA), and neurotrophin, brain-derived neurotrophic factor (BDNF), have been implicated in the occurrence of depressive disorders [111,127,128,129]. The gene discussed is BDNF; the disease is depressive disorder.