In a phase 3 clinical trial of palbociclib (CDK4/6 inhibitor) for advanced breast cancer, it was revealed that CCNE1 amplification was observed in CDK4/6 inhibitor-resistant cells, and it has been reported that when CDK2 inhibitors were administered to these cells, sensitivity to CDK4/6 inhibitors was maintained [18,19,20,21]. This evidence concerns the gene CCNE1 and breast cancer.