Analysis of DM1 mouse models, HSALR mice that express 250 CTG repeats in the 3′-UTR of human skeletal muscle actin [34] and DMSXL mice, which carry the human DMPK gene with more than 1,000 CTG repeats in the 3′-UTR [48] showed that GSK3β is increased and cyclin D3 is respectively reduced in skeletal muscle of these mice [31,49,50]. This evidence concerns the gene DMPK and myotonic dystrophy type 1.