CDK6 and esophageal squamous cell carcinoma: All 22 of these genes had moderate to high expression in ESCC, including TP53 (75%), CCND1 (35%), CDKN2A (31%), CDKN2B (20%), CDK6 (8%), E2F1 (6%), Rb (5%) and CDK4 (2%), suggesting that variations in p53 and Cdk4/6-p16-Rb cell cycle checkpoint controls are critical events in ESCC [8].