Based on the expression of estrogen and progesterone receptors (ER/PR) and human epidermal growth factor receptor (HER2) status, breast cancer is classified into further subgroups that include: luminal A (ER+/PR+/HER2 negative), luminal B (ER+/PR+/HER2+), HER2 positive (ER-/PR-/HER2+), basal-like or triple negative (ER-/PR-/HER2−), claudin-low and normal-like [2]. The gene discussed is ESR1; the disease is breast cancer.