ESR1 and breast cancer: Breast cancer classification, based on cellular markers that inform targeted therapies, comprises three major groups: (a) estrogen receptor (ER) or progesterone receptor (PR) positive, (b) human epidermal growth factor receptor 2 (HER2, encoded by the ERBB2 gene) positive, with or without ER and PR expression, and (c) triple-negative breast cancer (TNBC), which is defined by the lack of ER and PR expression and absence of HER2 amplification [4,5].