PDCD1 and head and neck squamous cell carcinoma: In a meta-analysis of 18 datasets of head and neck cancers gene expression profiling, the increased CD274 (gene encoding PD-L1) and CD279 (gene encoding PD-1) DNA copy number, as well as increased mRNA expression of this genes, was significantly increased in HNSCC as compared with the controls (P < 0.05, Fig. S1A–S1C).