This transgene consists of a 45 kb human genomic sequence extracted from lymphobalstoid cells from a patient with mild mytonic dystrophy. The 45 kb genomic sequence carries the DMWD, DMPK and SIX5 (DMAHP) genes. A part of the DMPK gene was replaced with a sequence extracted from a patient with severe mytonic dystrophy containing approximately 360 CTG repeats. In this line, the human DNA fragment shows a deletion of about 16 kb at the 5' end, eliminating the promoter and the first two exons of the DMPK gene. The founder had 362 CTG repeats. Intergenerational instability of CTG size is seen, with the majority of descendants showing expansion of +2 to +42 CTG in one generation. A greater amplification in CTG repeats is seen after paternal transmission than after maternal transmission.