Breast cancer is a heterogeneous disease, with treatment decisions and prognosis traditionally guided by immunohistochemistry (IHC) markers such as estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2), and Ki67 (a proliferation index marker), along with tumor size, tumor grade, and nodal status [1]. This evidence concerns the gene ESR1 and breast cancer.