ESR1 and breast cancer: Therefore, in this study, we aimed to evaluate the effects of adding EB1089 to the combined treatment with lapatinib and antiestrogens in HER2-positive breast cancer cells using two cell lines representing distinct HER2-positive breast cancer phenotypes: BT-474 that is characterized by its expression of ER (ER-positive/HER2-positive) and SK-BR-3 that lacks ER (ER-negative/HER2-positive).