CDK4 and breast cancer: The signaling pathways recognized to be involved in BC are steroid hormone signaling, human epidermal growth factor receptor 2 (HER2/ERBB2) signaling pathway, cyclin D1/cyclin-dependent kinase 4/6/retinoblastoma protein (cyclin D1/CDK4/6/RB1), phosphatidylinositol 3-kinase/protein kinase B (PI3K-AKT) pathway, mitogen-activated protein kinase (MAPK) signaling pathway, and progranulin (PGRN) signaling pathway[2-4].