ESR1 and neoplasm: Importantly, we observed that tumor biopsies of patients who were receiving endocrine therapies such as tamoxifen and AIs as last treatment before their biopsy were significantly more frequently observed in “ER-low” cluster B. We speculate that, since the average ER target gene expression in the “ER-low” cluster is at the same level as ER-negative tumors, these tumors classified as ER-positive might have turned into phenotypically ER-negative tumors.