IDO1 and neoplasm: The first pathway (effect of tryptophan depletion): Increased IDO expression facilitates the conversion of Trp to Kyn, depleting Trp in the tumor microenvironment, resulting in several effects: (1) Unbound tRNA accumulation activates the general control non‐derepressible 2 (GCN2) kinase pathway, phosphorylating translation initiation factor eIF2α and inhibiting cap‐dependent translation [72].