Several studies have focused on a functional polymorphism in the promoter region of the serotonin transporter gene (HTTLPR), which is supposed to moderate the influence of stressful life events on depression and the brain-derived neurotrophic factor (BDNF), which is supposed to exert a prophylactic effect against neuronal toxicity induced by stress [91-93]. This evidence concerns the gene BDNF and major depressive disorder.