8-Oxo-7,8-dihydroguanine (GO) can originate as 8-oxo-7,8-dihydro-2'-deoxyguanosine 5'-triphosphate (8-oxo-dGTP), an oxidized form of dGTP in the nucleotide pool, or by direct oxidation of guanine base in DNA. Accumulation of GO in cellular genomes can result in mutagenesis or programmed cell death, and is thus minimized by the actions of MutT homolog-1 (MTH1) with 8-oxo-dGTPase, OGG1 with GO DNA glycosylase and MutY homolog (MUTYH) with adenine DNA glycosylase. Studies on Mth1/Ogg1/Mutyh-triple knockout mice demonstrated that the defense systems efficiently minimize GO accumulation in cellular genomes, and thus maintain low incidences of spontaneous mutagenesis and tumorigenesis. Mth1/Ogg1-double knockout mice increased GO accumulation in the genome, but exhibited little susceptibility to spontaneous tumorigenesis, thus revealing that accumulation of GO in cellular genomes induces MUTYH-dependent cell death. Cancer cells are exposed to high oxidative stress levels and accumulate a high level of 8-oxo-dGTP in their nucleotide pools; cancer cells consequently express increased levels of MTH1 to eliminate 8-oxo-dGTP, indicating that increased expression of MTH1 in cancer cells may be detrimental for cancer patients. Mth1/Ogg1-double knockout mice are highly vulnerable to neurodegeneration under oxidative conditions, while transgenic expression of human MTH1 efficiently prevents neurodegeneration by avoiding GO accumulation in mitochondrial genomes of neurons and/or nuclear genomes of microglia, indicating that increased expression of MTH1 may be beneficial for neuronal tissues.