ESR1 and breast cancer: This classification system proposes the following three main molecular subtypes: luminal (LUM) BC, which expresses hormonal estrogen and progesterone receptors (ER+ and PR+) but no human epidermal growth receptor 2 (HER2-); overexpressed HER2 BC (ER+/− PR+/− HER2+); and triple negative (TN) BC, which lacks these receptors HER2, ER and PR1, 2.