According to the presence of immunohistochemistry markers, such as estrogen receptor (ER), progesterone receptor (PR), or human epidermal growth factor 2 (HER2) molecular markers, breast cancer can be classified into three main subtypes: hormone receptor-positive (ER+, PR+, HER2−), HER2 positive (ER−, PR−, HER2+), and triple-negative breast cancer TNBC (ER−, PR−, HER2−). This evidence concerns the gene ESR1 and breast cancer.