For rare inherited diseases an important question is what type of clinical diagnostic test to select, for instance Sanger-based single genesequencing; a high read depth gene panel; whole exomesequencingor whole genome sequencing. There is emerging recognitionthat a transmissible parental variant present at less than expected heterozygous frequency (due to mosaicism) may escape detection by certain methods. This risk has been proposed as a factor infavourof higher depth sequencingstrategies. Here we report a case where barely 30-fold depth whole genome sequencing through the 100,000 Genomes Project identified low grade mosaicismthat had been missed by conventional Sanger sequencing.