BRCA1 and maternal uniparental disomy of chromosome 20: In particular, most BRCA1-related MBCs are grade 3, HER2-negative (HER2-) tumours with high proliferative activity, whereas BRCA2-associated MBCs are high-grade, HER2-positive (HER2+) tumours and show absence of progesterone receptor (PR) expression (7, 9).