In order improve the clinical management of breast cancer, tumors are classified by histologic type [14,15] and by the expression of human epidermal growth factor receptor 2 (HER2) [16,17], PR, and ER [18] into four intrinsic subtypes: luminal A, luminal B, HER2-enriched, and basal-like breast cancer [19,20,21]. The gene discussed is ESR1; the disease is breast cancer.