CD274 and head and neck squamous cell carcinoma: PD-L1 expression has been identified in melanoma, ovarian tumors, lung tumors, renal cell tumors, urothelial tumors, squamous cell carcinomas of the head and neck, esophageal tumors, cervical tumors, breast tumors, pancreatic tumors, stomach tumors, Wilms tumors, and glioblastoma, as well as infiltrating lymphocytes (15,19–30).