One study found that blood levels of cytokines, including IL-6 and TNF-α, were significantly higher in depressed patients than in healthy controls (202), and that these pro-inflammatory cytokines, especially IL-6, IL-1β, and TNF-α, promote the production of Th17, which has been implicated in depression and other CNS disorders closely associated with them (203). This evidence concerns the gene TNF and major depressive disorder.