Based on their expression, BC is classified in four main subtypes that differ in terms of prognosis and outcome: luminal A (ER+, PR+, HER2−, Ki-67 low), luminal B (ER+, PR+/−, HER2+/−, Ki-67 high), HER2 overexpression (ER−, PR−, HER2+) and triple negative (TNBC/ER−, PR−, HER2−) [3,4]. Here, ESR1 is linked to breast cancer.