In addition to hormone markers such as estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2/neu), the expression of some critical overexpressed genes such as Ki-67 and mutated p53 is routinely determined by immunohistochemistry (IHC) in clinical laboratories for the diagnosis and staging of breast cancer. The gene discussed is ESR1; the disease is breast carcinoma.