The absence of one Msh3 allele (Msh3/null mice) is sufficient to decrease CAG expansion frequencies in HD and DM1 mice, suggesting that MSH3 may be a limiting factor in the process leading to the formation of expansions, and that CAG instability could tightly depend on MSH3 protein levels [16], [27], [35]. This evidence concerns the gene MSH3 and Huntington disease.