ESR1 and breast cancer: BC is immunohistochemically classified into four major subtypes: (1) luminal A (ER+ and/or PR+ and HER2-; low Ki-67 level/low proliferation rate), (2) luminal B (ER+ and/or PR+ and HER2+; high Ki-67/high proliferation rate), (3) HER2-positive (ER-, PR-, and HER2+), and (4) basal/triple-negative (HER2-, ER-, and PR-) [4,5].