ESR1 and cancer: The American Cancer Society reports that in 2020 there were nearly 2,261,419 new cases of breast cancer and 684,996 deaths.[1] Breast cancer is classified into four subtypes based on the expression of estrogen receptor, progesterone receptor, and human epidermal growth factor receptor 2 (HER2), including basal-like, luminal A, luminal B, and HER2 enriched molecular subtypes.[2] In the past few years, numerous advances have been made in the diagnosis and treatment of cancer.