We identified a PIK3CA mutation rate of 10.5% in familial MBCs but an absence of common activating mutations of AKT1, KRAS and BRAF. While limited by moderate numbers in our study, the absence of KRAS mutation contrasts with the only other study performed in sporadic MBCs by Dawson et al. who reported an overall incidence of 12% [20]. Here, BRAF is linked to maternal uniparental disomy of chromosome 20.