CDK4 and breast cancer: The most important pathways acknowledged in breast cancer including estrogen receptor (ER) pathway,32,33 HER2 signaling pathway,34,35 Phosphoinositide 3-kinase/protein kinase B/mammalian target of rapamycin (PI3K/AKT/mTOR) pathway,36 mitogen-activated protein kinases (MAPK) pathway,36,37 and cyclin D1/cyclin-dependent kinase 4/6/retinoblastoma protein (cyclin D1/CDK4/6/RB1) pathway (Fig. 2).