Primary resistance: About 15% of patients treated with CDK4/6 inhibitor (CDK4/6i) + aromatase inhibitor, and up to 30% of those treated with CDK4/6i + fulvestrant, will develop recurrent disease within 6 months Acquired resistance: Almost all patients will eventually develop progressive disease Multiple pathways are implicated in resistance CCNE1 amplification and cyclin E1 overexpression are