Efforts to improve the effectiveness of ET by adding medicines that target potential mechanisms of resistance are ongoing.5–12 One of the most successful approaches is the combination of cyclin-dependent kinase 4 & 6 (CDK4 & 6) inhibitors with ET.3,4,7–10,12 These combinations have improved progression-free survival (PFS) and objective response rates (ORR) in patients with HR+ advanced breast cancer (ABC), both as initial therapy and after progression on ET. Here, CDK4 is linked to breast cancer.