PDCD1 and neoplasm: The most successful immunotherapeutic agents to date consist of immune checkpoint inhibitors (ICIs), which block immune co-inhibitory receptors, such as cytotoxic T-lymphocyte antigen 4 (CTLA-4) and programmed cell death protein 1 (PD-1), or associated ligands such as programmed cell death ligand 1 (PD-L1), to dis-inhibit TILs and permit tumor-specific cytotoxicity.