PDCD4 and esophageal squamous cell carcinoma: To investigate the potential mechanism of CASC9 regulating PDCD4 expression in ESCC, we first performed bioinformatic analysis and found that the promoter region of PDCD4 was enriched in the repressive mark H3K27me3 and EZH2 binding sites in various cells (Additional file 2: Fig. S7), implying PDCD4 expression under regulation of epigenetic modification.