Since then, there have been numerous advances in targeted therapy for breast cancer, including tyrosine kinase inhibitors, poly (ADP-ribose) polymerase (PARP) inhibitors, vascular endothelial growth factor(VEGF) inhibitors, phosphoinositide 3-kinase(PI3K) inhibitors, immune checkpoint inhibitors, and cyclin-dependent kinase 4/6 (CDK4/6) inhibitors, and etc. (7, 8). Here, CDK4 is linked to breast cancer.