More than two-thirds of women with breast cancer have hormone receptorsexpression (positive estrogen receptor / positive progesterone receptor: ER+/PR+;positive estrogen receptor / negative progesterone receptor: ER+/PR-; negative estrogenreceptor / positive progesterone receptor: ER-/PR+), and hormone therapy is often usedin such cases, especially tamoxifen.2 The gene discussed is ESR1; the disease is breast cancer.