Programmed cell death protein 1 (PD-1)–programmed death-ligand 1 (PD-L1) interactions are inhibitory signals regulating immune responses, and in cancer, expression of PD-1/PD-L1 or indoleamine 2, 3-dioxygenase 1 (IDO1) allows for an escape from immune surveillance [28]. Here, IDO1 is linked to cancer.