Based on the expression of estrogen receptor (ER), progesterone receptor (PgR) and epidermal growth factor receptor 2 (HER2, also known as ERBB2), Perou and Sørlie’s team first classified breast cancer into five molecular subtypes: luminal subtype A; luminal subtype B; basal-like subtype; HER2 positive subtype; and normal breast-like subtype [2,3]. The gene discussed is ESR1; the disease is breast cancer.