Checkpoint proteins such as programmed cell death protein 1 (PD-1), programmed death-ligand 1 (PD-L1), and cytotoxic T-lymphocyte associated protein 4 (CTLA-4) constitute a restraining mechanism of the immunity system that prevents it from autoimmune reactions but, at the same time, is related to immune escape by cancer cells. This evidence concerns the gene PDCD1 and cancer.