In light of its high heterogeneity, breast cancer is clinically divided into three main subtypes based on the expression of hormone receptors, namely estrogen receptor (ER) and progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2): luminal, HER2-positive, and triple-negative breast cancer (TNBC). The gene discussed is ESR1; the disease is triple-negative breast carcinoma.