ESR1 and breast cancer: Breast cancer is a heterogeneous disease, and its distinctive biological subtypes include triple-negative phenotype (ER (estrogen receptor)−/PR (progesterone receptor)−/HER2 (human epidermal growth factor receptor 2)−) and hormone receptor-positive/human epidermal growth factor 2 (ERBB2; formerly HER2) ERBB2-negative and ERBB2-positive.