ESR1 and neoplasm: In their research, they found that all three major histological subtypes of invasive breast cancer (i.e. ER+/PR+ subtype; HER2+ subtype; and the ER–/PR–/HER2– triple negative subtype) contained rare tumor cells with epithelial morphology that stained with both E and M markers, while that benign breast tissue and tumor cells in pre-invasive ductal carcinoma in situ (DCIS) lesions and reactive stromal cells were exclusively epithelial or mesenchymal.