Peripheral administration of endotoxin lipopolysaccharide (LPS), a component of Gram-negative bacterial cell wall, into mice has been shown to increase expression of pro-inflammatory cytokines, such as IL-1β, IL-6, and tumor necrosis factor (TNF)-α, in both the periphery and brain, especially in microglia and perivascular macrophages in the brain (Layé et al., 1994; Godbout et al., 2008), and cause abnormal behavior similar to major depression (O’Connor et al., 2009; Biesmans et al., 2013). The gene discussed is TNF; the disease is major depressive disorder.