PDCD1 and metastatic malignant neoplasm: The advent of immune checkpoint inhibitors (ICIs) has markedly improved patient survival in different subtypes of metastatic cancer, by enhancing cytotoxic T‐cells activity through blocking either cytotoxic T‐lymphocyte antigen 4 (CTLA‐4) or programmed cell death 1 (PD‐1) or its ligand (PD‐L1).1