A recent example is that stress-susceptible mice treated systemically with an IL-6 antibody show significant reductions in depressive-like behaviors in the chronic social defeat stress model of depression, which generally upregulates systemic levels of IL-6 (Hodes et al., 2014; Stewart et al., 2015; Zhang J.C. et al., 2017). This evidence concerns the gene IL6 and major depressive disorder.