IDO is activated by proinflammatory cytokines, such as interferon (IFN)-α, IFN-γ, and tumor necrosis factor (TNF)-α6, 9, 10, 11, 12, which in addition to other inflammatory molecules such as interleukins, chemokines, and prostaglandins have been associated with major depressive disorder (MDD)2, 6, 11, 13, 14, 15. The gene discussed is IFNG; the disease is major depressive disorder.