CD8A and esophageal squamous cell carcinoma: The analysis identified 56 genes significantly upregulated at both mRNA and protein levels in ESCC (Fig. 1A and Additional file 1: Table S1), and interestingly, Pearson correlation analysis using TCGA_ESCC dataset revealed that PES1 expression assumed the strongest negative correlation with the levels of CD8A and CD8B, two markers of CD8+ CTL (Fig. 1B, C) (Additional file 1: Tables S2 and S3), which promoted us to focus on PES1 for further investigation.