ESR1 and maternal uniparental disomy of chromosome 20: The majority of MBCs are triple negative for estrogen receptors (ER), progesterone receptors (PR) and human epidermal growth factor receptor 2 (HER2) expression (TNBC), and may express cytokeratin 5/6 (CK5/6) and/or epidermal growth factor receptor (EGFR).