PDCD1 and cancer: Current cancer immunotherapy focuses on the use of immune checkpoint inhibitors (ICIs)—antibodies that block the function of immune checkpoints namely cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), programmed cell death protein 1 (PD-1), programmed cell death 1 ligand 1 (PD-L1), or lymphocyte activation gene 3 (LAG3) [1–3].