CELF1 and myotonic dystrophy type 1: Using DM1 cell and mouse models, our group showed that mutant CUG repeats not only alter CUGBP1 levels [19], but also change CUGBP1 activity due to an increase in GSK3β kinase; this prevents CUGBP1 phosphorylation at S302 by cyclin D3-CDK4 [20], converting active (phosphorylated at S302) CUGBP1ACT into inactive CUGBP1 repressor CUGBP1REP, which is unphosphorylated at S302.