ESR1 and breast cancer: Breast cancer is classified to determine treatment options based on the following molecular markers: The expression of estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor-2 (HER-2), and Ki-67 can be divided into the following four subtypes 4: Luminal A (ER+ and/or PR+ and HER2-, low expression of Ki67) and Luminal B (ER+ and/or PR+ and HER2-/HER2+, high expression of Ki67) 5 accounted for 40% and 30% of cases, respectively, and were the most common types of breast cancer.