Our whole-exome sequencing analyses of paired tissue samples of MBC and normal breast from the same patients identified somatic mutations common to all MBCs in five genes; TP53 in 70%, MUC17 in 60%, CRYBG2 in 40%, PLEC in 30%, and ZNF681 in 30%. The gene discussed is PLEC; the disease is maternal uniparental disomy of chromosome 20.