IDO1 and neoplasm: This may be due to the tumor-mediated suppression of the immune system by the release of cytokines, such as tumor growth factor ß (TGF-ß) and interleukin 10 (IL-10); enzymes, such as indoleamine 2,3-dioxygenase (IDO); and inhibitory immune checkpoint ligands, such as programmed death ligand 1 (PD-L1), that can lead to suppressed proliferation, exhaustion and death of the lymphocytes [40].