Both share a similar molecular mechanism in which deleterious expansion of microsatellite repeats in non-coding regions, (CTG)n in 3′UTR of dystrophia myotonica protein kinase (DMPK) gene in DM1 (2–4) and (CCTG)n in intron 1 of CNBP gene in DM2 (5) are transcribed into expanded C/CUG-RNA that are retained in the nucleus as discrete foci. This evidence concerns the gene DMPK and myotonic dystrophy type 1.