BDNF and major depressive disorder: The author included candidate genes, such as brain-derived neurotrophic factor (BDNF), serotonin transporter polymorphisms (5-HTTLPR) [15], stress sensitivity, and neuroendocrinological process regulation response to stress [16] as mediators of depression risk and cognitive styles [17,18], interpersonal factors [19], early adversities [20], and neurobiological factors (such as changes in hippocampus and amygdala [16]) as moderating mechanisms of risk.