Breast cancer is classically divided into three types: (a) estrogen receptor (ER) or progesterone receptor (PR)-positive; (b) human epidermal growth factor receptor 2 (HER2)-positive (erbB2 amplification) with or without ER/PR positivity; (c) triple-negative breast cancer (TNBC), which is defined by the lack of ER/PR expression and HER2 amplification. This evidence concerns the gene ESR1 and breast cancer.