Since the DNA mismatch repair proteins MSH2, MSH3, and MSH6, are considered critical players in CTG repeat expansion, and their decreased expressions inhibit the expansion (Dragileva et al., 2009; Tomé et al., 2009), they provide a new insight into the mechanism of CTG repeat instability in DM1. The gene discussed is MSH3; the disease is myotonic dystrophy type 1.