It is well known that cyclin D1 binds to cyclin-dependent kinases (CDKs) including CDK4 and CDK6 and executes its oncogenic effect by promoting unlimited cell cycle progression.27 We thus selected three sensitive ESCC cell lines with CCND1 copy number >2 and three resistant ESCC cell lines with CCND1 copy number <2 to interrogate cell cycle pathway focusing on G1 phase regulation. This evidence concerns the gene CDK6 and esophageal squamous cell carcinoma.