Myotonic dystrophy is caused by the expansion of a CTG repeat sequence. The mechanism by which this expanded repeat produces the pathophysiology of myotonic dystrophy is not clear. It has been shown previously that expansion of the repeat produces allele-specific effects on transcripts from two genes, DMPK and SIX5. We have examined the effect of repeat expansion on the level of RNA from a third gene, DMWD. We have identified a polymorphism in this gene and developed a quantitative allele-specific assay for DMWD RNA levels, which we have applied to nuclear and cytoplasmic fractions of RNA from DM cell lines. We have found that the level of the DM-associated allele in the cytoplasm of DM cell lines is reduced by 20-50% compared with the wild-type allele, similar to the level of reduction found for SIX5 in allele-specific analysis. However, no such reduction is observed in RNA from the nuclear fraction of DM cell lines. This may reflect the complex nature of processing transcriptional units at the DM locus.