The activity of IDO1 is irrelevant under basal conditions, but strongly inducible by several inflammatory stimuli, such as interferon-γ (IFN-γ), lipopolysaccharide (LPS), tumor necrosis factor α (TNF- α), proinflammatory interleukins (ILs), infection, and transforming growth factor β (TGF-β) [37,38]. This evidence concerns the gene IDO1 and infection.