CDK4 and breast cancer: Thus, the field of BET inhibitors will continue to evolve; strong mechanism-based translational studies that identify predictive biomarkers for improved response (such as Y537S mutation status) and rational combination treatments (such as CDK4/6 inhibitors) that synergize with BET inhibition and increase response rates will determine the clinical success of this class of drugs for treatment of endocrine therapy–resistant breast cancer.