Breast cancer is a heterogeneous disease: ∼20% of breast cancers show amplification of the human epidermal growth factor receptor 2 (HER2+), but 10% to 20% of breast cancers lack expression of the estrogen receptor (ER), the progesterone receptor (PR), and HER2 gene amplification, a condition known as triple-negative breast cancer (TNBC). The gene discussed is ESR1; the disease is breast cancer.