Due to its molecular heterogeneity, breast cancer has been classified according to the expressions of the estrogen receptor (ER), the progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2), resulting in three subtypes: luminal A and B, HER2-enriched, and triple-negative or basal-like breast cancer [3,4,5]. The gene discussed is ESR1; the disease is breast cancer.