Clinically, BC can be subdivided into three major subtypes: estrogen (ER) and/or progesterone receptor (PgR) expressing tumors (commonly referred to as hormone receptor positive [HR+]), ErbB2 amplification [HER2+]), and triple-negative BC (TNBC), in which ER/PgR expression is normal and HER2 expression is normal or lacking [4]. Here, ESR1 is linked to breast cancer.