CRISPR/Cas9 technology inserted a 170 CTG repeat expansion into the 3'UTR. Protein levels are decreased by about 50% in heterozygous and more than 90% in homozygous tibialis anterior muscles. This is due to the inhibition of mRNA nuclear export as mice show an accumulation of numerous nuclear CUG repeat expansion RNA foci in the nucleus of myoblasts and differentiated myotubes.The CTG expansions are stable with no germline contractions or expansions observed up to 12 months of age. (J:284198)