ESR1 and breast cancer: Breast cancer is divided into four different subtypes; luminal A (estrogen receptor (ER) and progesterone receptor (PR) positive, human epidermal receptor 2 (HER2) negative and low Ki-67 level), luminal B (ER and PR positive, HER2 positive or negative and high Ki-67 level), HER2 enrich (ER and PR negative), and triple negative (ER, PR, and HER2 negative) (3, 4).