CTG repeat number in the nonaffected allele of myotonic dystrophy patients is not critical for disease expression.

To investigate whether unusual allele segregation might explain the dominant negative effect of the expanded allele for myotonic dystrophy on myotonin protein kinase mRNA metabolism, which is suggested to cause the disease, we determined the number of CTG repeats at the DM locus in the nonaffected alleles of 64 DM (dystrophia myotonia) patients. The relative distribution was then compared with the distributions obtained from alleles of the normal parents and normal siblings of DM patients. Comparison was also made with the allele distribution of normal subjects from the same geographic area. It appears that the CTG repeat number of the nonaffected allele in DM patients is not critical for the expression of the disease.