Genetic analysis revealed an inherited condition involving MBC in 10 out of 37 (27%) cases, including 7 carriers of BRCA1/2 germline pathogenic variants (3 BRCA1 and 4 BRCA2) and 3 carriers of pathogenic variants of other genes including APC, MUTYH, and CDH1. Invasive ductal carcinomas were diagnosed in 35 out of 37 MBCs including sporadic and inherited cases. The gene discussed is MUTYH; the disease is maternal uniparental disomy of chromosome 20.