Among the four MMR genes mentioned above, mono-allelic inherited PVs in MLH1 and MSH2 are the most common and account for approximately 70–90% of all LS cases, while 7–10% are due to PVs in the MSH6 gene and less than 5% are due PVs in the PMS2 gene [10,11]. This evidence concerns the gene MSH6 and Leigh syndrome.