ESR1 and breast cancer: At the molecular level, breast cancer can be categorized into five molecular subtypes depending on the hormone receptors the cells express: Luminal A (ER/PR+, HER2−), luminal B (ER/PR+, HER2+), HER2-enriched (ER/PR−, HER2+), triple-negative (ER/PR−, HER2−), and normal-like (ER/PR+, HER2−, KI67−)24.