ESR1 and breast cancer: Regarding molecular classification, breast cancer comprises the following subtypes: Luminal A, expressing both the estrogen and progesterone receptor (ER + and PR +), Luminal B that in addition to ER and PR expresses a low level of the human epidermal receptor 2 (HER2) (ER + , PR + , HER2low), and the HER2-enriched subtype (ER-, PR-, HER2 +).