PCNA and esophageal squamous cell carcinoma: Generally, positive regulation of CDK activity is mediated by the accumulation of cyclins, whereas negative regulation is achieved by phosphorylation of the catalytic subunit or by binding with CDK inhibitors, including p21 and p27.46 Cyclin B1 contributes to the transition of cells from the G2 to M phase and is inactivated by phosphorylation of CDK1.47 In ESCC, cyclin B1 was reported to be an independent prognostic factor in patients,48 indicating that cyclin B1 plays a key role in ESCC cell proliferation.