The enzyme 8-oxo-7,8-dihydrodeoxyguanosine triphosphatase (8-oxo-dGTPase) hydrolyzes 8-oxo-dGTP to 8-oxo-dGMP, thereby preventing misincorporation of 8-oxo-dGTP into DNA. We investigated expression of MTH1 gene encoding 8-oxo-dGTPase. Large amounts of MTH1 mRNA were present in thymus and testis, embryonic tissues, and certain cell lines. In peripheral blood lymphocytes, the level of MTH1 mRNA was significantly increased after concomitant treatment with phytohemagglutinin and interleukin-2. Analyses of the 5' regions of the MTH1 transcripts revealed that 7 types of MTH1 mRNAs, which may be produced by transcription initiation at different sites and/or alternative splicing. The MTH1 gene consists of 5 major exons, some of which are composed of differentially processed segments. All types of MTH1 mRNAs carry the entire coding region, and may be functional. Three ATG initiation codons in-frame were found in the 5' regions of some of the MTH1 mRNAs. There is a polymorphic alteration at the 5' splicing site (GT to GC) located in exon 2, an event which affects splicing patterns of the MTH1 transcript. Allele frequency of this polymorphism is about 20% among healthy volunteers.