ESR1 and breast neoplasm: Breast tumors are routinely classified by estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) status, with ER/PR defining hormone receptor–positive disease and HER2 defining HER2−positive disease; within the HER2−negative category, HER2−low has recently emerged as a clinically relevant subset.