70% of breast cancer cases are hormone receptor–positive type which has either estrogen receptor (ER) or progesterone receptor (PR) protein in the cancer cells; 15–20% of cases are ERBB2-positive (formerly known as human epidermal growth factor receptor 2 (HER2)-positive) which has high levels of ERBB2 protein on the cancer cells; and the remaining 15% of cases are triple-negative (TNBC) type that does not have ER, PR, or ERBB2 protein in the cancer cells [3, 4]. Here, ESR1 is linked to breast cancer.