We tested the hypothesis that the instability of the trinucleotide CTG at the myotonic dystrophy (DM) locus could be an intrinsic DNA damage recognisable by the p53 cell-cycle checkpoint system. p53 mRNA and protein levels were assayed in muscle biopsies and fibroblast cell lines of DM patients and unaffected controls. No differences in mRNA and protein levels were found between patients and controls, regardless of their expansion size. However, in the cells treated with adryamicin, p53 protein levels were comparable in DM and control cells. We conclude that the CTG trinucleotide expansion within the myotonin gene does not activate the p53 surveillance system, at least in adult tissues. The escape of trinucleotide expansion from the p53-mediated DNA repair system could explain some of the biological characteristics of genome instability.