PDCD1 and head and neck cancer: In particular, immune checkpoint blockades have been developed using antibodies against cytotoxic T-lymphocyte antigen 4 (CTLA-4), programmed cell death-1 (PD-1), and programmed cell death ligand-1 (PD-L1), and PD-1 inhibitors have been approved for melanoma, non–small cell lung cancer, renal cancer, Hodgkin’s lymphoma, bladder cancer, and head and neck cancer (Table 1).