ESR1 and breast cancer: Receptor status is used as a guide to classify breast cancers into four biological subtypes: (a) low grade luminal A subtype, (b) high grade luminal B subtype (High grade, ER/PR positive or HER2+/ER+/PR+), (c) HER2 subtype (HER2+/ER−/PR−), and (d) triple negative/basal like subtype (HER2−/ER−/PR−) (22).