Clinical data have shown in both the metastatic and adjuvant treatment settings that tamoxifen is less efficacious in ER + PR− tumors than in ER + PR+ tumors [1-3], and single HR+ breast cancers, especially ER + PR- breast cancers, have aggressive features and poorer prognosis in comparison to double HR+ (ER + PR+) breast cancer [4,5]. This evidence concerns the gene ESR1 and breast cancer.