ESR1 and breast cancer: This is most likely explained by the fact that the expression of ER-related genes was more strongly associated with breast cancer-specific mortality in the tamoxifen-treated patients than in those not treated with tamoxifen (Figure 2), and is also consistent with the established relationship between ER status of the tumor (by ligand binding or immunohistochemistry assay) and response to tamoxifen [4,28].