ESR1 and breast carcinoma: Estrogen receptor (ER)/progesterone receptor (PR) positive (predominantly luminal) tumors or human epidermal growth factor receptor 2 (HER2) positive (HER2-enriched) tumors, which collectively make up the majority of breast cancer cases, are treated with ER- or HER2-targeted drugs, respectively, largely contributing to the current overall breast cancer 5-year survival rate of nearly 90%1,2.