Indeed, in the kynurenine pathway, tryptophan is oxidized by indoleamine 2,3-dioxygenase 1 (IDO-1), an enzyme mainly found in macrophages, microglia, neurons and astrocytes that is up-regulated by certain cytokines and inflammatory molecules, such as lipopolysaccharides, amyloid peptides and interferon gamma (IFN-γ), which justifies the observed depressive states in individuals whose diseases are associated with chronic inflammation, like cancer and neurological disorders (Yabut et al., 2019). Here, IDO1 is linked to inflammatory response.