ESR1 and breast cancer: Tumors with expression of either the estrogen receptor (ER) or progesterone receptor (PR) in at least 1% of tumor cells are categorized as hormone receptor-positive (HR+) [2], and intrinsic subtypes of breast cancer are classified into luminal A (ER+/PR+/−/HER2−), luminal B (ER+/PR+/−/HER2+/−), basal-like (ER−/PR−/HER2−), and HER2-positive (ER−/PR−/HER2+) [3,4].