Breast cancer, despite its heterogeneity, can be categorized into five subtypes: luminal type A (estrogen receptor (ER) and (or) PR positive, HER2 negative), luminal type B (ER and (or) progesterone receptor (PR) positive, human epidermal growth factor receptor 2 (HER2) positive), HER2 overexpressing (ER and PR negative, HER2 positive), triple negative or basal-like (ER, PR, and HER2 negative)1, and claudine-low breast cancer2,3 (Table 1). The gene discussed is ESR1; the disease is breast cancer.