MSH2 and mismatch repair cancer syndrome 1: Further studies in MEFs showed that MSH2 and OGG1 act independently, and have an additive effect on maintaining low levels of both spontaneous and exogenously induced 8-oxoG in genomic DNA, and that overexpression of MTH1 mitigates the mutator effect of MMR deficiency (Colussi et al., 2002; Russo et al., 2004).