DMPK and myotonic dystrophy type 1: To exclude the possibility of confounding structural artifacts introduced by multiple rounds of PCR amplification, we have analyzed repeat sequences generated by direct amplification of 10–100 ng genomic DNA from DM1 transgenic mice carrying 20, 55 or 260 CAG•CTG repeats within a human DMPK transgene (Gourdon et al., 1997; Seznec et al., 2000).