Figure 2.

Bidirectionality of eukaryotic MMR. The 5′-nicked (a) and 3′-nicked (b) heteroduplexes are used as model substrate. The shorter path is chosen to remove the mismatched base. The 5′–3′ exonuclease activity of EXO1 is required for excision reaction in both 5′- and 3′-nicked heteroduplexes.