MutS Homolog 2 (MSH2) and 3 (MSH3), two DNA mismatch repair genes required to maintain genome integrity, also play a major role in the formation of CTG repeat expansions in DM1 mice [13,39,40,41], In DM1 patients, single-nucleotide polymorphisms in MSH3 have been associated with a decreased level of somatic mosaicism and a decreased age of onset [42,43]. This evidence concerns the gene MSH2 and myotonic dystrophy type 1.