PDCD1 and cancer: Undoubtedly, the unleashing of pre-existing anti-tumor immune responses through the blockage of the immune checkpoints—cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) and programmed cell death protein 1 (PD-1)/PD-ligand 1 (PD-L1)—represents the breakthrough of cancer therapy of the last decade [34].