Myotonic dystrophy is a dominantly inherited disorder with multisystemic clinical features affecting skeletal muscle, the heart, the eye, the endocrine system. Two genetic loci have been identified. The mutation responsible for DM1 was identified as a CTG expansion located in 3' untranslated region of the myotonia dystrophica protein kinase gene (DMPK). The molecular pathogenesis of DM1 has been controversial. Myotonic dystrophy type 2 (DM2) which is caused by an untranslated CCTG expansion of zinc finger protein 9 (ZNF9), has been recently discovered. The clinical features common to both diseases are caused by a gain of function RNA mechanism in which the CUG and CCUG repeats alter cellular function. The long PCR based method is useful for the molecular diagnosis for these diseases.