Furthermore, on the basis of the expression of four biomarkers, namely, estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2), and ki-67, on the surface of breast cancer cells, breast cancer can be classified into four subtypes: Luminal A, Luminal B, HER2-positive and triple-negative [4]. Here, ESR1 is linked to breast cancer.