ERBB2 and neoplasm: Interestingly, CDK4 gene expression was higher in luminal B and basal-like CCND1-amplified tumours only (Fig. 3, “CDK4” P = 0.004 and P = 0.029 respectively) whilst CDK6 expression was lower in luminal A, luminal B and HER2-enriched CCND1-amplified tumours (Fig. 3, “CDK6” P <  0.001 for all comparisons).