ESR1 and breast cancer: BC is divided into four subtypes by receptor status: luminal A (estrogen receptor (ER)-positive, progesterone receptor (PR)-positive, and human epidermal growth factor receptor 2 (HER2)-negative); luminal B (ER-positive and/or PR-positive, HER2-positive); HER2 overexpressing (ER-negative, PR-negative and HER2-positive); and triple negative (ER-negative, PR-negative, HER2-negative).