PCNA and esophageal squamous cell carcinoma: p21 interacts with cyclin/cyclin‐dependent kinase (CDK) complex and functions to negatively control cell cycle.3 p21 also directly binds with proliferating cell nuclear antigen (PCNA) and thereby inhibits DNA replication.4 Although p21 has been reported as a useful prognostic factor in a variety of human tumors, the clinicopathologic significance of the p21 expression for ESCC patients remains controversial.5, 6, 7, 8, 9, 10, 11, 12, 13