Breast tumors can be classified by (1) the expression of three receptors, namely, the estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) and (2) differential transcriptomic signatures such as the PAM50, which, among additional clinical biomarkers, are used to parse breast cancers into one of five intrinsic molecular subtypes (luminal A, luminal B, HER2-enriched, basal-like, and normal-like) [2,3]. Here, ESR1 is linked to breast cancer.