In particular, for patients with invasive breast cancer, the statuses of hormone receptor (HR: estrogen receptor (ER) or progesterone receptor (PR))) and human epidermal growth factor receptor 2 (HER2) status using immunohistochemical (IHC) staining are the most helpful markers for the treatment decisions of patients with breast cancer in clinical practice. The gene discussed is ESR1; the disease is breast carcinoma.