Objective：To study the change of CTG repeat numbers in patients and their family members of DM， SEP and MEP.Methods: CTG repeat numbers of DM, SEP and MEP gene from 5 patients diagnosed as DM clinically, and their 16 family members and a healthy subject were analysed by polymerase chain reaction and Southern blot. Results: The numbers of CTG repeat were 19-30 in 10 normal individuals,SEP and MEP were normal. It ranged from 80 to more than 1 605 in 5 patients with DM which were much higher than the normal person. The repeat numbers of CTG in 12 of 16 family members were higher than the normal. The severity degree of clinical symptoms of these 5 patients was related to the degree of amplification of CTG, SEP and MEP.Conclusion: The gene diagnosis of DM is consistent with its clinical diagnosis, SEP and MEP.