CDK4 and breast cancer: For example, the use of mTOR inhibitor, everolimus, or CDK4/6 inhibitor, palbociclib, in combination with endocrine therapy has proven to be among the most crucial advances in the management of HR+/HER2− advanced BC over the last 5 years.9 Since the development of these agents, further combinations of targeted drugs and endocrine therapies have been clinically approved.10, 11 However, the optimal choice and sequence of endocrine therapies is not clearly defined.