Several studies have shown that breast cancers can be classified into at least four “intrinsic” subtypes (basal-like, HER2 enriched, luminal A, and luminal B) which can only be partially recapitulated by IHC definitions of the three fundamental breast cancer biomarkers: estrogen receptor (ER), progesterone receptor (PgR), and human epidermal growth factor receptor 2 (HER2) [2–5]. Here, ESR1 is linked to breast cancer.