In addition, BrCa is a heterogeneous disease comprising three subtypes; luminal BrCa is characterized by high expression of the estrogen (ER) and progesterone receptors (PR) but not human epidermal growth factor receptor 2 (HER2); or by overexpression of HER2 and low expression of ER and PR, and high expression of HER2. The gene discussed is ESR1; the disease is invasive breast carcinoma.