E2F1 and esophageal squamous cell carcinoma: E2F1 appears to play different regulatory roles in human malignancies; E2F1 shows tumor-suppressing activity in esophageal, gastric, and colorectal adenocarcinoma, whereas it may function as a tumor promoter in pancreatic ductal adenocarcinoma and esophageal squamous cell carcinoma [5, 14, 15].