Based on genetic and epigenetic characteristics, breast cancer cell lines can be classified as luminal A, luminal B, HER2 positive, triple negative A and triple negative B (4) Clinically, BC can be classified into estrogen receptor (ER)-positive and ER-negative as well as HER2 (human epidermal growth factor receptor 2)-positive and HER2-negative subtypes. The gene discussed is ESR1; the disease is breast cancer.