ESR1 and breast cancer: Previous studies have demonstrated that among highly proliferative ER+/HER2− breast cancers, tumours with low ER-related signalling (i.e. MKShi/ERSlo) exhibit poor response rates to neoadjuvant AI and high rates of early relapse during adjuvant tamoxifen, indicating an enrichment of primary endocrine-resistant tumours in this group [9].