We report the case of a child with myotonic dystrophy (DM) with symptoms begining at the age of seven, whose genetic study showed an additional DNA fragment, greater than of his father, an asymptomatic carrier. The clinical and molecular analysis of this parent-child pair are probably the first described in Brazil, since the recent discovery of genetic abnormality in DM by American and European researchers, that explained the long-debated phenomenon of "anticipation" in this disease. The main advances in molecular genetics in DM and its correlation with increasing severity and earlier onset of the symptoms in sucessive generations of a family are commented briefly.
myotonic dystrophy; Steinert disease; molecular genetics; expanding DNA sequence; anticipation phenomenon