Preclinical mechanistic experiments are needed to dissect the functional relevance of ESR1 amplifications more clearly in HR+ HER2- BC cells exposed to different types of pharmacological treatments, including in vitro estrogen deprivation (which mimics the in vivo effects of AIs) +/- CDK 4/6 inhibitors or everolimus, fulvestrant +/- CDK 4/6 inhibitors, or fulvestrant +/- alpelisib (in PIK3CA-mutated cancers). This evidence concerns the gene ESR1 and breast cancer.