CDK4 and breast cancer: Finally, CDK4/6 inhibitors such as Ribociclib, Palbociclib, and Abemaciclib are administered in combination with endocrine therapy as first-line therapy for treatment of advanced ER+ breast cancer (Hortobagyi et al, 2016; Finn et al, 2015; Turner et al, 2015; Dickler et al, 2017; Sledge et al, 2017).