CDK4 and breast cancer: Currently, the major targets of clinical utility in breast cancer include the human epidermal growth factor receptor 2 (HER2) and epidermal growth factor receptor (EGFR), vascular endothelial growth factor (VEGF) receptor, mechanistic target of rapamycin (mTOR) pathway and the cyclin-dependent kinase 4/6 (CDK4/6) pathway [1].