Typically, breast cancers are classified into seven subtypes: luminal A, which is characterized by histologic low-grade malignancies and is ER-positive; luminal B is characterized by histologically high-grade malignancies and is ER-positive; the human epidermal growth factor receptor-2 (HER2) overexpression type of breast cancer—immunomodulatory (IM), basal-like (BL1 and BL2), mesenchymal (M), and mesenchymal stem-like (MSL) breast cancer [2], and normal breast-like tumors fall under this category [3]. The gene discussed is ESR1; the disease is breast cancer.