Breast cancers are divided into three main groups on the basis of cellular markers: (1) estrogen receptor (ER)- or progesterone receptor (PR)-positive; (2) human epidermal growth factor receptor 2 (HER2)-positive with or without ER and PR positivity; and (3) triple-negative breast cancer (TNBC) defined by the absence of ER, PR, and HER2 expression [124]. This evidence concerns the gene ESR1 and breast cancer.