This transgenic mice model ("DMSXL") contains large human genomic DNA sequence carrying the DMPK gene with very large repeat (>1200 CTG). They show intergenerational and somatic instability biased towards expansions. Homozygous mice, expressing enough toxic RNA carrying CUG expansion display a clear phenotype and some DM1 features: Growth retardation, muscle weakness, myotonia, splicing defects in muscle heart and brain, especially at young ages. Brain, muscles and heart abnormalities are under characterisation.