A total of 406 patients with stages I–III BC with ER expression <10% by IHC (and HER2-negative) treated with neoadjuvant and/or adjuvant chemotherapy were included in the analysis (Supplementary Fig. 1): 364 (90%) with ER-negative (ER < 1%) BC and 42 (10%) with low ER expression (ER1–9%) BC. This evidence concerns the gene ESR1 and breast cancer.