CD274 and head and neck squamous cell carcinoma: The expression levels of PD-L1 in the tumor tissues of cervical squamous cell carcinoma (CESC), cholangiocarcinoma (CHOL), esophageal carcinoma (ESCA), head and neck squamous cell carcinoma (HNSC), and stomach adenocarcinoma (STAD) were higher than those in the corresponding control tissues.