CDK4 and breast carcinoma: Today, research has advanced our understanding of advanced breast cancer pathophysiology, leading to identification of new targets to complement ET such as mammalian target of rapamycin (mTOR) inhibitors, cyclin-dependent kinase 4/6 (CDK4/6) inhibitors, histone deacetylase (HDAC) inhibitors, and phosphoinositide 3 kinase inhibitors.