For example, palbociclib-resistant breast cancer (MCF-7pR) cells that had undergone prolonged treatment with palbociclib acquired CCNE1 gene amplification and sustained high levels of CDK2 Thr160 phosphorylation, and the silencing of CCNE1 or CDK2 alone in these cells did not affect cell-cycle arrest. This evidence concerns the gene CDK2 and breast cancer.