CDK4 and breast cancer: Currently, the clinical trials evaluating the clinical efficacy of chemotherapy as a monotherapy or in combination with other compounds in patients with ER+/HER2 BC that presented resistance to CDK4/6i and ET are the phase III TROPiCS-02 (NCT03901339), the phase III KEYNOTE-B49 (NCT04895358), the phase II TATEN (NCT04251169) and a phase I trial (NCT04134884).