Overexpression or activation of RTKs such as EGFR, HER2 and IGF1R can cause resistance to tamoxifen through activation of PI3K/AKT and MAPK pathways in breast cancer (BC) cell lines14–16 and are associated with poor outcome in tamoxifen-treated patients.17,18 As MCF7 cells express very low levels of EGFR/HER2 but high levels of HER3 (data not shown), we used heregulin (HRG) to activate HER3 in MCF7 and B6-9 cells. This evidence concerns the gene ERBB3 and breast cancer.