the 3' untranslated region of this gene contains 5-37 copies of a ctg trinucleotide repeat. expansion of this unstable motif to 50-5,000 copies causes myotonic dystrophy type i, which increases in severity with increasing repeat element copy number. repeat expansion is associated with condensation of local chromatin structure that disrupts the expression of genes in this region. as the dmpk repeat is replicated, the hairpin loop that is formed leads to repeat expansion (a) or contractions (b).