A mouse model which presents typical symptoms of myotonic dystrophy when a normal DMPK 3′UTR mRNA is over-expressed [17] clearly raises issues as to the relative role of the expansion itself and of the non repeat DMPK 3′UTR in the pathogenicity of CUG expanded RNA in DM1 patients. This evidence concerns the gene DMPK and Steinert myotonic dystrophy.