Currently, the most extensively studied checkpoint pathways in the field of cancer research are programmed death protein 1 (PD-1/CD279) and its ligands PD-L1/CD274/B7-H1 and PD-L2/CD273/B7-DC, cytotoxic T lymphocyte-associated antigen 4 (CTLA-4), T-cell immunoglobulin and mucin domain 3 (TIM3), and lymphocyte activation gene 3 (LAG3), among others (92). Here, PDCD1 is linked to cancer.