Triplet repeats form secondary structures that escape DNA repair in yeast

Several human neurodegenerative diseases result from expansion of CTG/CAG o r CGG/CCG triplet repeats. The finding that single-stranded CNG repeats for m hairpin-like structures in vitro has led to the hypothesis that DNA secon dary structure formation is an important component of the expansion mechani sm. We show that single-stranded DNA loops containing 10 CTG/CAG or CGG/CCG repeats are inefficiently repaired during meiotic recombination in Sacchar omyces cerevisiae. Comparisons of the repair of DNA loops with palindromic and nonpalindromic sequences suggest that this inefficient repair reflects the ability of these sequences to form hairpin structures in vivo.