Myotonic dystrophy (DM) is an autosomal dominant neuromuscular disorder associated with defects in many tissues, including skeletal muscle myotonia, progressive myopathy, and abnormalities in the heart, the brain, and the endocrine system (Harper 1995). The clinical phenotype, which is notably variable, is subject to genetic anticipation, the progressive worsening of symptoms, and/or presentation of symptoms at an earlier age in successive generations (Harper 1995). Both genetic anticipation and variability of phenotype in patients with DM were explained when an unstable CTG triplet–repeat expansion was identified in the myotonin protein kinase (DMPK) gene and shown to be responsible for the disease (Aslanidis et al. 1992; Brook et al. 1992; Buxton et al. 1992; Fu et al. 1992; Harley et al. 1992; Mahadevan et al. 1992). Unaffected individuals usually have 5–40 CTG repeats in the DMPK gene, but in an affected population the number of repeats is dramatically higher, sometimes even up to several hundred or thousands of repeats. Six years after the discovery of unstable CTG triplet–repeat expansion on chromosome 19q, significant amounts of experimental data have been collected, but the molecular mechanisms involved in the induction of the disease are still uncertain. DM is unusual among triplet-repeat disorders in that it is associated with a trinucleotide repeat occurring in the 3 UTR of a disease gene. On the basis of the dominant transmission of DM, one would ordinarily expect the disease allele mutation to affect protein structure, but it is difficult to see how a CTG triplet repeat outside the gene’s open reading frame (ORF) could affect the DMPK protein. Thus, it has been hypothesized that CTG expansion might affect DMPK expression at the transcriptional or posttranscriptional level (Fu et al. 1993; Krahe et al. 1995; Wang et al. 1995), and, indeed,