Accumulating evidence has indicated that several pro-inflammatory cytokines including interleukin-1β (IL-1β), tumor necrosis factor-α (TNF-α), and interleukin-6 (IL-6) were all increased in patients with major depressive disorder (MDD) (Schiepers et al., 2005; Song et al., 2009; Hannestad et al., 2011). Here, IL6 is linked to major depressive disorder.