Phenotypic expression of the myotonic dystrophy gene in monozygotic twins.

Phenotypic expression of the myotonic dystrophy (DM) gene is highly variable even within the same family. To investigate the importance of genetic and nongenetic factors on phenotypic variability, we studied phenotypic expression in two DM twin sets with monozygosity confirmed by DNA polymorphism analysis. Our observations suggest that genetic factors are primarily responsible for modulating pleiotropic expression of the DM gene.