It can interfere by several means with the anti-cancer immune response, e.g., by inducing the upregulation of immune checkpoints such as programmed cell death protein 1 (PD-1), PD-1 ligand 1 (PD-L1), and cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4) [19,75]. Here, PDCD1 is linked to cancer.