PDCD1 and head and neck cancer: Currently, treatments with inhibitors targeting immune checkpoints related to T cells, such as programmed cell death protein 1/programmed death-ligand 1 (PD-1/PD-L1) and cytotoxic T-lymphocyte-associated protein 4, have been used for various cancers, including melanoma, non-small-cell lung cancer, and head and neck cancer (12–14).