One of the most mutation-prone sequences is the variable number of repeating DNAs at the myotonic dystrophy (DM1) gene, causing muscular dystrophy.  In DM1 individuals the mutant gene has an expansion of a repeated DNA sequence CTG CTG CTG to hundreds or thousands of repeats.  We report a new kind of mutation at the DM1 region.  We found a DM1 individual who, in addition to an expanded repeat, also had the repeat tract completely replaced with a non-repeat DNA sequence from another chromosome.  Replacement mutations occurred only skeletal muscle and cerebellar cortex.  This non-CTG configuration expands our understanding of the sequence variations that can arise at this hypermutable site. (By Christopher E. Pearson, Ph.D., http://jmg.bmj.com/content/early/2011/05/27/jmg.2010.085944.abstract?papetoc )