The proportions of PD-1+ NK subset and PD-1 expression on NK cells are increased in the TME of various cancers, including colon cancer, liver cancer, gastric cancer, esophageal cancer, ovarian cancer, head and neck cancer, and Hodgkin lymphoma, where its ligands, PD-L1 or PD-L2, are expressed at high level (67). This evidence concerns the gene PDCD1 and head and neck cancer.