Most breast cancers express specific hormone receptors (HRs) for estrogen (estrogen receptor α, ERα) and progesterone (progesterone receptor, PR), and some cancers exhibit human epidermal growth factor receptor 2 (HER2)/erb-b2 receptor tyrosine kinase 2 (ERBB2) gene amplification or overexpression. The gene discussed is ESR1; the disease is cancer.