MSH5 and infection: The two remaining pairs showed high levels of genetic heterogeneity; in one pair we found this to be a result of infection by mixed genotypes, while the second was a result of nonsense mutations in the gene encoding the DNA mismatch repair proteins MSH2, MSH5, and RAD5. These nonsense mutations led to a hypermutator state, leading to dramatically elevated rates of synonymous and nonsynonymous substitutions.