The two major types of myotonic muscular dystrophy (MMD) — MMD1 and MMD2 — are both caused by genetic defects.
MMD1, the most common type, results from an abnormal DNA expansion in the DMPK gene on chromosome 19.
MMD2 arises from an abnormal expansion of DNA in the ZNF9 gene on chromosome 3.
Within MMD1 there are additional subtypes, depending on a person’s age at onset of symptoms. The age of onset is roughly correlated with the size of the DNA expansion, with larger expansions associated with earlier disease onset.