Breast cancer is the most commonly diagnosed malignancy in women, and breast cancer is divided into luminal, human epidermal receptor 2 (HER2)-enriched, and triple-negative breast cancer (TNBC) subtypes, according to the presence of distinct molecular markers, including the estrogen receptor (ER), progesterone receptor (PR), and HER2. The gene discussed is ESR1; the disease is breast carcinoma.