Figure 6.

Proposed model for MutSβ's role in promoting (CAG)•(CTG) repeat expansion. During DNA synthesis (i.e., DNA replication or DNA repair), (CAG)•(CTG) repeats form a hairpin via strand slippage. The hairpin can be removed by Polδ (I). However, hairpin binding by MutSβ recruits Polβ to the complex, where Polβ uses the hairpin as a primer to add several nucleotides to the 3′ end of hairpin. The resulting hairpin product can be utilized for the high-fidelity and highly processive DNA synthesis by Polδ, leading to hairpin retention (II). Alternatively, Polβ can carry out the hairpin retention DNA synthesis independent of Polδ (III).