BC is a genetically heterogeneous group of tumors with a variety of morphologic features and is classified into four distinct molecular subtypes based on the immunohistochemical expression of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (Her2): ER/PR+, Her2– (Luminal A); ER/PR+, Her2+ (Luminal B); ER/PR–, Her2+ (HER2+); and ER/PR–, Her2– (basal-like/triple-negative breast cancer [TNBC]) [2]. The gene discussed is ESR1; the disease is breast cancer.