ESR1 and breast cancer: The breast cancer tissues were grouped into four subtypes according to the expression of estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER2), included ER/PR and HER2 negative (ER/PR-,HER2−; triple negative), ER/PR and HER2 positive (ER/PR+, HER2+), ER/PR positive and HER2 negative (ER/PR+, HER2−) and ER/PR negative and HER2 positive (ER/PR−, HER2+) breast cancer, respectively.