ESR1 and breast cancer: Breast cancer is graded based on the expression of progesterone and, estrogen receptors (PR&ER), and human epidermal growth factor receptor 2 (HER2), such as (1) ER+, a positive estrogen receptor; (2) HER2+, a HER2 overexpressor which may be ER+ or ER−; or (3) TNBCs, a triple-negative subtype that does not express any of these receptors.