PDCD1 and cancer: Numerous immune checkpoint proteins, such as PD-1 (programmed cell death protein 1)/ PD-L1 (programmed cell death ligand 1) and CTLA-4 (cytotoxic T lymphocyte-associated antigen-4), are involved in tumor proliferation, angiogenesis, metastasis and chemoresistance via suppressing cytotoxic T cell activation, resulting in cancer immune escape and immune tolerance 39, 40.