The expression of the indoleamine 2,3-dioxygenase 1 (IDO-1) enzyme takes place in peripheral tissues (macrophages, dendritic cells (DCs)) and the central nervous system (CNS) (microglia) immune cells, and is stimulated by an underlying infection and associated pro-inflammatory cytokines, such as interferon-γ (IFN-γ), tumor necrosis factor α (TNF-α), and interleukins (IL) 6 (IL-6) and 1β (IL-1β) [6]. The gene discussed is IDO1; the disease is infection.