The DNA of the DMPK-gene in myotonic dystrophy is expanded. In the DMPK-gene a repeat of the code CTG is present. The normal number of CTG repeats (CTG-CTG-CTG- etc) in the DMPK gene is three to 37. In patients with myotonic dystrophy the CTG-repeats are increased, over 40 up to thousands of CTG repeats.

In general, the number of repeats correlates with the age of onset and the severity of the disorder. When a parent transmits the disease to a child, the repeat number often increases; the child will develop the disease at a younger age, with more severe symptoms. This is called anticipation.