ESR1 and neoplasm: Of note, ERα expression would allow for the prediction of the response to antiestrogen hormone therapy, due to its relevance as a therapeutic target in breast cancer, and malignancy with ER expression may have a better prognosis while low ERα expression has been associated with increased tumor growth and/or a poorer prognosis in both human and canine patients [16,21,23,24,25,26].