Lastly, based on molecular aspects, breast cancer can be categorized into five clinical subtypes: luminal A (ER+, PR+, HER2−, low Ki-67 index), luminal B HER2− (ER+, PR+, HER2−, high Ki-67 index), luminal B HER2+ (ER+, PR+, HER2+, high Ki-67 index), HER2-enriched (ER−, PR−, HER2+), and triple-negative breast cancer (TNBC; ER−, PR−, HER2−, high Ki-67 index) [151]. Here, ESR1 is linked to breast cancer.