It is caused by extended (CTG)n expansions at 3′-UTR of DMPK gene (DM type 1, DM1) or (CCTG)n repeats in the intron 1 of CNBP gene (DM type 2, DM2) and the sequestration of Muscleblind-like (MBNL) family proteins by transcribed (CUG)n RNA hairpin is the main pathogenic mechanism for DM. This evidence concerns the gene DMPK and myotonic dystrophy type 1.