CDK4 and neoplasm: This was true for Ccnd1−/− fibroblasts, R3327-5′A tumour cells wherein Ccnd1 was knocked down with RNA interference (see also Supplementary Fig. 3C), or Ccnd1+/+ fibroblasts treated with 2 μM Palbociclib, a specific inhibitor of Ccnd1·Cdk4 (Fig. 4a).