ESR1 and breast cancer: In the era of precision medicine, the optimal therapeutic strategies in patient management depend on the determination of BC classification dividing into four molecular subtypes, luminal A, luminal B, human epidermal growth factor receptor 2 (HER2)-positive, and triple-negative breast cancer (TNBC), which are based on estrogen receptor (ER), progesterone receptor (PR), HER2, and Ki-67 expression (2).