In clinical practice, immunohistochemical (IHC) staining for estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor-2 (HER2), and the proliferation marker Ki-67 is used to broadly categorize breast tumors into four subtypes: luminal type A, luminal type B, the Her-2 type, and triple negative [1,2]. Here, ESR1 is linked to breast neoplasm.