Over the past few decades, therapies have been developed for the treatment of the most predominant (estrogen receptor (ER)-positive) subset of breast cancers (BC) and anti-estrogens (e.g. tamoxifen, fulvestrant) and aromatase inhibitors (e.g. letrozole, anastrozole, exemestane) are in widespread use as adjuvant and metastatic therapies [1]. This evidence concerns the gene ESR1 and breast cancer.