Molecular basis of p(CCG)n repeat instability at the FRA16A fragile site locus.

Rare, folate-sensitive fragile sites are the result of the unstable expansion of trinucleotide p(CCG)n repeats, which are normally polymorphic in copy number. Differences in the number and frequency of alleles of the fragile site FRA16A p(CCG)n repeat were observed between different ethnic populations suggesting that certain alleles might be predisposed to instability. Sequence analysis demonstrated that the longer and more variable alleles were associated with loss of repeat interruption. Perfect repeat configuration therefore appears to be a necessary precondition for the instability associated with fragile site genesis.