Chang Gung medical journal
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Myotonic dystrophies or dystrophia myotonica (DM) is a clinical syndrome that includes myotonic dystrophy type 1 (DM1), myotonic dystrophy type 2 (DM2), myotonic dystrophy type 3 (DM3), and so forth. The terminology was recommended by the new nomenclature for myotonic dystrophies of an International Panel for Consensus. Previous studies have shown that DM1 is caused by the expansion of a cytosine-thymine-guanine (CTG) repeat in the DM protein kinase gene on chromosome 19, and DM2 is caused by an expansion of a cytosine-cytosine-thymine-guanine (CCTG) repeat in the zinc finger protein 9 (ZNF9) gene on chromosome 3. ⋯ The inverse correlation between age at onset and CTG repeat length is significant only in patients with small expansions of about 100-250 triplet repeats. Transmission contraction of the repeat size is likely to occur in alleles with large repeats and is associated with paternal transmission. In congenital DM1, individual variability of muscle differentiation does occur, in spite of the same number of CTG repeats in the leukocytes.