Numerous studies have reported that increased circulating pro-inflammatory cytokines, such as interleukin-1 beta (IL-1β), IL-6, tumor necrosis factor-alpha (TNF-α), and interferon-gamma (IFN-γ), are associated with MDD (Dowlati et al., 2010; Fagundes et al., 2013; Farooq et al., 2017) by activating indoleamine 2,3 dioxygenase (IDO). The gene discussed is IFNG; the disease is major depressive disorder.