CD274 and esophageal squamous cell carcinoma: Noteworthy, PD-L1 copy number analysis revealed PD-L1 amplifications to be a far less frequent event in ESCC (2%) than in squamous cell carcinoma of the oral cavity (19%) [37], indicating an intertumoral spread width regarding the biologic mechanisms underlying PD-L1 expression between primary squamous cell carcinomas of different localizations [38].