Collectively, breast cancer can be classified into several sub-types based on the observed presence of certain breast cancer-associated biomarkers, such as estrogen receptor (ER), progesterone receptor (PR), Ki-67 (a protein marker with prognostic and predictive potential for adjuvant chemotherapy), and human epidermal receptor 2 (HER2), in the tumors (Table 1) [2]. Here, ESR1 is linked to breast cancer.