While the effects of MMR proteins on instability can vary according to the repeat sequence and its context [21]–[28], it is notable that Msh2 and Msh3 enhance CAG/CTG expansion both in HD and DM1 mouse models [18]–[23], [25]–[27], and Pms2, encoding a subunit of the MutLα (MLH1-PMS2) complex that acts downstream of mismatch recognition by MutSα (MSH2–MSH6 heterodimer) or MutSβ, was identified as a genetic enhancer of CTG expansion in a DM1 mouse model [24]. The gene discussed is MSH3; the disease is myotonic dystrophy type 1.