Eight MBCs harbored germline mutations associated either with loss‐of‐heterozygosity or a second somatic mutation (BRCA1, n = 6; BRCA2, n = 1; and RBBP8, n = 1), and one MBC displayed a BRCA2 homozygous deletion (Table S4). Here, BRCA1 is linked to maternal uniparental disomy of chromosome 20.