To explain the restriction of early onset cases of myotonic dystrophy (DM) to maternal transmittance and the significant excess of male transmitters in the last asymptomatic generation, the Involvement of parental effects on the autosomal dominant mode of inheritance has been suggested. Using FISH we confirmed that the DM-kinase gene is proximal to the ApoE gene on mouse chromosome 7, close to an imprinted segment. To study whether there is any firm molecular basis for the speculation that imprinting may be involved in DM we have analysed the expression of paternal and maternal alleles of the DM-kinase gene in human and mouse tissues. Length polymorphisms In the 3' non coding exons of human and mouse DM kinase genes, i.e. the variable [CTG]n repeat motif in humans and a newly identified Cn stretch variation In mice, served as tools to distinguish between allelic RNA products in various tissues. In human tissues, presence of transcripts from both parental alleles could be demonstrated by RT-PCR. In mouse, similar observations were made using a RNAse protection assay on fetal and adult muscle RNAs. We conclude that imprinting does not play a role in the expression of the DM kinase gene.