Immunohistochemical analyses have shown that >90% of MBCs are negative for estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER2/neu), and that they have a consistent expression of basal markers such as cytokeratin 5/6 (CK 5/6) and/or epidermal growth factor receptor (EGFR) [4]. Here, ESR1 is linked to maternal uniparental disomy of chromosome 20.