Our previous findings of the importance of Aurora kinase A and ER for growth of tamoxifen and aromatase inhibitor resistant breast cancer cell lines [28,38] and for high Aurora kinase A expression as a marker for reduced response to tamoxifen therapy [28], indicate that both Aurora kinase A and B may be useful markers in endocrine resistant breast cancer and also targets for treatment. This evidence concerns the gene ESR1 and breast carcinoma.