Breast cancer is an heterogeneous disease with different subtypes defined by the following molecular classification: Luminal A and B (estrogen receptor-positive ER+), HER-2 positive (HER-2+ and ER−), basal-like (triple-negative breast cancer TNBC; ER−, PR−, and HER-2−) breast cancers [4–6]. Here, ESR1 is linked to triple-negative breast carcinoma.