Aiming to produce a model of severe, congenital DM1 phenotypes, the high level of intergenerational instability of CTG repeats in DM300 mice was exploited through successive breeding, leading to increased CTG repeats over generations and culminating in the creation of DMSXL mice, which carry over 1000 CTG repeats in the human DMPK gene [128,129]. Here, DMPK is linked to myotonic dystrophy type 1.