CDK4 and esophageal squamous cell carcinoma: Since the CDK4-Cyclin D1 (CCND1) complexes and cyclin-dependent kinase inhibitors (CKIs) CDKN1A and CDKN1B are key regulators in controlling cell-cycle progression, we then detected their protein levels in ESCC cells following ESCCAL-1 overexpression or knockdown by western blot.