FIG. 4.

Model for TNR expansions and breakage due to hairpin formation at flap structures. Displaced 5′ flaps, formed either on the lagging strand during replication or during nick or gap repair, that contain CTG repeats are predicted to fold into a hairpin structure at some frequency. The likelihood of hairpin formation is increased in strains defective for flap processing, such as strains lacking Fen1/Rad27. Four possible pathways for resolution of the hairpin/nick structure and their consequences for repeat instability are shown. NHEJ, nonhomologous end joining.