CDK4 and breast cancer: Increased levels of Cyclin E1 lead to drug resistance in many cancer cells, including ovarian cancer that is resistant to the standard platinum-taxane chemotherapy, the human epidermal growth factor receptor -2 (HER2) positive breast cancers resistant to trastuzumab and breast cancer resistant to cyclin-dependent kinase 4/6 (CDK 4/6)-inhibitors (45).