PDCD1 and metastatic malignant neoplasm: In particular, immune checkpoint inhibitors—therapeutic antibodies that enhance antitumoral T-cell responses by blocking inhibitory molecules such as cytotoxic T lymphocyte antigen 4 (CTLA-4) and programmed cell death 1 (PD-1)—have proven very effective and have even resulted in long-term remission in patients with metastatic cancers (9,10).