ESR1 and breast cancer: Due to its heterogenous nature, BC is categorized into three main groups based on cellular receptor markers reflecting available targeted therapies: (a) estrogen receptor (ER) or progesterone receptor (PR) positive; (b) human epidermal growth factor receptor 2 (HER2) positive (amplification of erbB2) with or without ER and PR positivity; and (c) triple‐negative breast cancer (TNBC) defined by the absence of all kinds of receptor markers expression like ER, PR, and HER2 [2].