Breast cancer (BC) is a heterogeneous malignancy that can be classified by estrogen receptor (ESR1) expression (ER+), human epidermal growth factor receptor 2 (ERBB2) expression (HER2+), the absence of ESR1, ERBB2 and the progesterone receptor in triple-negative breast cancer (TNBC) or the expression of driver oncogenes (e.g. MYC). The gene discussed is ESR1; the disease is breast carcinoma.