PDCD1 and head and neck squamous cell carcinoma: To date, antibodies to therapeutically target the PD-1:PD-L1axis have been approved for the treatment of a variety of cancers in the metastatic and adjuvant setting, including melanoma, non-small cell lung cancer (NSCLC), renal cell carcinoma (RCC), Hodgkin’s lymphoma, bladder cancer, head and neck squamous cell carcinoma (HNSCC), Merkel-cell carcinoma, and microsatellite instable-high (MSI-H) or mismatch repair-deficient (dMMR) solid tumours.