ESR1 and breast cancer: Hence, BC is classified into four subtypes considering their receptor statuses: luminal A (ER-positive, PR-positive, and 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) (Figure 1).