IDO1 and neoplasm: IDO is a hemoglobinase that degrades tryptophan (Trp), a necessary amino acid, through the kynurenine (Kyn) pathway (KP).328 IDO is crucial for sufficient levels of kynurenine, which is necessary for the proper proper functioning of human tissues, such as endothelial cells, epithelial cells, and mature DCs.329 Acquired immunological tolerance is associated with IDO activity, which can induce Treg activation and inhibit T-cell activation, thus enabling tumour cells to evade immune surveillance.330