Breast cancer can be categorized into three primary subtypes, hormone-receptor-positive (HR+ BC), HER2-positive (HER2+ BC), and triple-negative (TNBC), based on the presence or absence of the estrogen receptor (ER), progesterone receptor (PR), and ERBB2 (commonly known as HER2) [1,2,4]. The gene discussed is ESR1; the disease is breast cancer.