Myotonic dystrophy (DM) is a trinucleotide repeat syndrome which can contain 50 to over 2,000 CTG repeats in affected individuals, but does not express a fragile site. Although one prior study [Jalal et al., Am J Med Genet 46:441-443, 1993] did not find evidence of fragility at 19q13.3 in six individuals affected with DM using induction protocols for folate sensitive fragile sites, other chemicals may induce fragile site expression at this site. In an attempt to induce fragile sites at 19q13.3, blood cultures from four congenital DM cases and four control individuals treated with fluorodeoxyuridine (folate-sensitive rare fragile sites), bromodeoxyurdine (rare and common fragile sites), aphidicolin (common fragile sites), and 5-azacytidine (common fragile sites) were harvested using routine cytogenetic technique. Slides were solid stained and 100 cells were examined for fragile site expression, particularly on F group chromosomes. The latter were photographed prior to destaining and G-banded to verify chromosome and band location of breakage. No culture conditions induced a fragile site at band 19q13.3 at > 1% expression in patients with congenital DM. Our results suggest that CTG repeats, even when present in > 1,000 copies, may behave differently from other large expansions which are associated with fragile sites. The CTG repeats in DM are not associated with a methylated CpG island, as are folate-sensitive fragile sites, which most likely plays a role in the expression of fragile sites at the trinucleotide repetitive site.