In patients with estrogen receptor (ER)-positive breast cancer, following prolonged exposure to adjuvant endocrine therapy, resistance mutations can emerge on metastatic progression.9 This is particularly true for aromatase inhibitors, given as first-line treatment in postmenopausal women with ER-positive breast cancer10 where around 30% of these cancers acquire activating ESR1 gene mutations that confer resistance to treatment.11 With endocrine therapy considered as effective as chemotherapy,12 it is a preferred treatment by many women, given the lower incidence of adverse side effects. This evidence concerns the gene ESR1 and breast cancer.