ESR1 and breast cancer: The majority of breast cancers are estrogen (ER+) and progesterone (PgR+) hormone receptor-positive, of which the luminal A (LUMA) subtype is the least aggressive (low grade) while the more proliferative (Ki-67-high) luminal B1 (LUMB1) and human epidermal growth factor receptor 2-positive (HER2+) luminal B2 (LUMB2) subtypes are usually of higher grades.