CCND1 and cancer: Cyclin-dependent kinases (CDK) are protein kinases that phosphorylate cellular proteins causing their activation or inactivation during the G1 cell cycle phase.1 2 In a dysregulated cell cycle, CDK4/6 proteins bind to cyclin D1 to form an activated complex, which then phosphorylates and inactivates tumour suppressor retinoblastoma protein and releases E2F transcription factors, thus resulting in cell cycle progression and cancer cell proliferation.1 Competitive inhibitors of this pathway have been introduced into clinical practice.