Here, the ATP-binding, ATP hydrolysis, mispair-binding, sliding clamp formation, and Mlh1–Pms1 complex interaction properties of dominant mutant Msh2–Msh6 complexes have been characterized. The results demonstrate two mechanisms for dominance. In one, seen with the Msh6-S1036P and Msh6-G1067D mutant complexes, the mutant complex binds mispaired bases, is defective for ATP-induced sliding clamp formation and assembly of ternary complexes with Mlh1-Pms1, and occludes mispaired bases from other mismatch repair pathways. In the second, seen with the Msh6–G1142D complex, the mutant complex binds mispaired bases and is defective for ATP-induced sliding clamp formation but assembles ternary complexes with Mlh1–Pms1 that either occlude the mispaired base or prevent Mlh1–Pms1 from acting in alternate mismatch repair pathways.