IDO1 and cancer: 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).