It has been known for decades that MDD patients often have elevated levels of circulating granulocytes and monocytes (Maes et al., 1992; Maes, 1995; Seidel et al., 1996b), pro-inflammatory cytokines and their receptors, acute phase proteins, chemokines, and prostaglandins (Miller et al., 2009), indicative of activation of the peripheral immune system in MDD. Here, DDAH2 is linked to major depressive disorder.