Of all types of breast cancer, triple-negative breast cancer (TNBC) is the most aggressive and lethal subtype, affecting 12–18% of breast cancer patients [8], and is characterized by the lack of expression of estrogen receptors, progesterone receptors, and human epidermal growth factor receptor 2 (HER2); for this reason, endocrine or anti-HER2 targeted therapies are not effective [9]. This evidence concerns the gene ESR1 and breast cancer.