According to these characteristics, BC are classified in four main subtypes, namely luminal A (ER+, PR+, HER2−, Ki-67−), luminal B (ER+, PR±, HER2±, Ki-67 ±), HER2+ (ER−, PR−, HER2+), and triple negative (ER−, PR−, HER2−) [4]. The gene discussed is ESR1; the disease is breast cancer.