PDCD1 and metastatic malignant neoplasm: Immune checkpoint inhibitors (ICIs), primarily inhibitors of programmed cell death protein 1 (PD-1) or its ligand, programmed cell death ligand 1 (PD-L1), and cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), are now a standard treatment for many types of advanced/metastatic cancer (8).