ESR1 and breast cancer: Gene expression studies with cDNA microarray technology have demonstrated that triple negative (TN, ER−/PR− as well as human epidermal HER2-negative, HER2−) breast cancers are often characterized by a basal-like molecular profile, which exhibits overexpression of a number of genes involved in cell proliferation and differentiation, p-21 mediated pathway, and G1-S checkpoints of cell cycle signaling pathways; whereas ER/PR+/HER2− and ER/PR+/HER2+ are often characterized by luminal molecular profiles, which are associated with the ER signaling pathway 33,34.