ESR1 and breast cancer: Although various chemotherapeutic agents such as docetaxel, tamoxifen, cisplatin, carboplatin, doxorubicin, gemcitabine, mitoxantrone etc. have improved the overall survival and quality of life for patients, the 5 year survival of stage IV breast cancer patients is still very low (∼20%) for most estrogen receptor (ER) positive/progesterone receptor (PR) negative subtypes [3].