ESR1 and breast cancer: A retrospective analysis on 774 biopsy samples of post-menopausal women with ER+ HER2- breast cancer treated for 5 years with tamoxifen or anastrazole and enrolled in the ATAC study, compared the prognostic value of 6 tests: Oncotype DX, Prosigna, EndoPredict, Breast Cancer Index (BCI), Clinical Treatment Score and immunohistochemistry with 4-markers.