DNMT1 and esophageal squamous cell carcinoma: In order to find new a diagnostic molecular marker in esophageal squamous cell carcinoma, this study focuses on the expression of the key enzyme in the process of DNA methylation - DNA methyltransferase 1 (DNMT1) - in esophageal squamous cell carcinoma tissues and the relationship with the promoter methylation status of CpG island in RASSF1A.