Expression of estrogen receptors (ERs) and/or progesterone receptors in tumour cells is a major determinant of systemic therapy – hormone-receptor (HR)-negative BCs are typically treated using cytotoxic chemotherapy, whereas endocrine therapy is the adjuvant therapy of choice for HR+ tumours (Anderson et al, 2002; Early Breast Cancer Trialists' Collaborative Group (EBCTCG), 2005). This evidence concerns the gene ESR1 and breast cancer.