ESR1 and breast cancer: In accordance with the more recent criteria [4], breast cancers are classified as: (1) luminal A (ER- and/or PR-positive, HER2-negative); (2) luminal B (ER- and/or PR-positive, HER2-positive or Ki67 > 20%); (3) HER2-enriched (HER2 amplified, ER- and PR-negative); and (4) triple-negative (ER-, PR-, and HER2-negative) subtypes.