We here report on an analysis towards the role of cell type and cell stage in CTG•CAG instability in different tissues of a mouse knock-in DM1 model, where a human (CTG•CAG)n repeat segment (n ~ 110) replaces the cognate 3' segment of the endogenous mouse Dmpk gene [13]. This evidence concerns the gene DMPK and myotonic dystrophy type 1.