Regarding the immunophenotype classification, the presence/absence of molecular markers for estrogen receptors (ER), progesterone receptors (PR) (i.e., hormone receptors, HR), and human epidermal growth factor 2 (HER2), breast cancer can also be classified into: (i) HR positive/HER2 negative (about 70% of cases); (ii) HER2 positive (about 15–20% of cases) and; (iii) triple-negative (tumors lacking all three standard molecular markers, about 15% of the remaining cases) [28]. Here, ESR1 is linked to breast cancer.