2.1 Search strategy
Searches strategy were carried out in PubMed, Web of Science, the Cochrane Library and Scopus for all studies, and search terms were developed as follows: (( oral OR mouth OR mouth [mesh] OR mouth neoplasms [mesh] OR tongue OR tongue neoplasms [mesh] OR alveolar OR gingiva* OR lip OR gum OR buccal OR palate* OR retromolar OR head and neck OR head and neck neoplasms[Mesh]) AND (squamous cell carcinoma OR squamous cell carcinoma [mesh] OR cancer)) AND (PD-L1 OR PDL1 OR Programmed Death Ligand 1 OR B7-H1 OR B7H1 OR CD274 OR PDCD1L1 ).