According to the expression status of hormonal receptors (HR) and human epidermal growth factor receptor 2 (HER2), breast cancer is categorized into three major subtypes: HR positive [defined as estrogen receptor (ER)-positive and/or progesterone receptor (PR)-positive]/HER2-negative, HER2-positive (HR positive or negative), and triple-negative breast cancer (TNBC, defined as ER-negative, PR-negative, and HER2-negative) (Waks and Winer, 2019). The gene discussed is ESR1; the disease is breast cancer.