ESR1 and breast cancer: Further, in the ERα positive subgroup, multivariable analysis revealed that the double high group (HR=1.980, 95% CI: 1.308–2.996, p=0.001) as well as grade (HR=1.612, 95% CI: 1.086–2.394, p=0.018) and HER2 positivity (HR=1.716, 95% CI: 1.027–2.867, p=0.039) were significantly associated with reduced BCSS in ERα positive breast cancer and that tamoxifen treatment significantly improved ERα positive BCSS (HR=0.628, 95% CI: 0.431–0.915, p=0.015) (C-index: 0.637) (see Additional file 5: Table S7).