ESR1 and breast cancer: Of 99 confirmed cases of breast cancer, 53 were estrogen receptor positive (ER+), and 43 were estrogen receptor negative (ER-); 49 were progesterone positive (PR+), and 41 were progesterone negative (PR-); and 38 were human epidermal growth factor receptor positive (HER2+), and 52 were human epidermal growth factor receptor negative (HER2-) (Table 1).