CDK4 and breast cancer: Currently, first-line salvage therapy for tamoxifen-resistant breast cancer patients includes fulvestrant (a selective ER down-regulator) [4, 5], cyclin-dependent kinase 4/6 (CDK4/6) inhibitors [6], aromatase inhibitors, everolimus (a mammalian target of rapamycin inhibitor) [7], and histone deacetylase (HDAC) inhibitors [8].