According to the American Cancer Society, stage IV patients have a 5-year relative survival rate of 26%, compared to nearly 100% for stage I patients.[2] Based on the presence or absence of estrogen receptors (ER), progesterone receptors (PR), and human epidermal growth factor receptor 2 (ERBB2; formerly HER2) molecular markers, breast cancer can be classified into 3 main subtypes: hormone receptor-positive/ERBB2-negative (70% of patients), ERBB2-positive (15–20%), and triple-negative (lacking all 3 standard molecular markers; 15%). Here, ESR1 is linked to cancer.