CDK6 and esophageal squamous cell carcinoma: The perturbed cell cycle regulation pathway in ESCC mainly exhibited genetic alterations in the G1/S transition control, including mutations or deletions of TP53, RB1, CDKN2A, CHEK1, and CHEK2, and amplifications of CDK4, CCND1, CDK6, and MDM2 (Song et al., 2014).