The classification of BC adopted in clinical evaluation is based on immunohistochemistry expression and stratifies BC in luminal A (ER+ (estrogen receptor positive), PR+ (progesterone receptor positive), HER2− (human epidermal growth factor receptor 2 negative), Ki-67 ≤ 14), luminal B (ER+, PR+/−, HER2+/−, Ki-67 > 14), HER2 overexpression (ER−, PR−, HER2+), and triple-negative (ER−, PR−, HER2−) [10]. The gene discussed is ESR1; the disease is breast cancer.