to carcinogenesis. Mutagenesis Yasui et al. examined the fate of 8-oxo-dG when this oxidized derivative of deoxyguanosine was inserted into the thymidine kinase gene in a chromosome within human lymphoblastoid cells in culture. They inserted 8-oxo-dG into about 800 cells, and could detect the products that occurred after the insertion of this altered base, as determined from the clones produced after growth of the cells. 8-oxo-dG was restored to G in 86% of the clones, probably reflecting accurate base excision repair or translesion synthesis without mutation. G:C to T:A transversions occurred in 5.9% of the clones, single base deletions in 2.1% and G:C to C:G transversions in 1.2%. Together, these more common mutations totaled 9.2% of the 14% of mutations generated at the site of the 8-oxo-dG insertion. Among the other mutations in the 800 clones analyzed, there were also 3 larger deletions, of sizes 6, 33 and 135 base pairs. Thus 8-oxo-dG, if not repaired, can directly cause frequent mutations, some of which may contribute to carcinogenesis. Role of DNA oxidation in gene regulation As reviewed by Wang et al., oxidized guanine appears to have multiple regulatory roles in gene expression. As noted by Wang et al., genes prone to be actively transcribed are densely distributed in high GC-content regions of the genome. They then described three modes of gene regulation by DNA oxidation at guanine. In one mode, it appears that oxidative stress may produce 8-oxo-dG in a promoter of a gene. The oxidative stress may also inactivate OGG1. The inactive OGG1, which no longer excises 8-oxo-dG, nevertheless targets and complexes with 8-oxo-dG, and causes a sharp (~70o) bend in the DNA. This allows the assembly of a transcriptional initiation complex, up-regulating transcription of the associated gene. The experimental basis establishing this mode was also reviewed by Seifermann and Epe A second mode of gene regulation by DNA oxidation at a guanine, occurs when an 8-oxo-dG is formed in a guanine rich, potential G-quadruplex-forming sequence (PQS) in the coding strand of a promoter, after which active OGG1 excises the 8-oxo-dG and generates an apurinic/apyrimidinic site (AP site). The AP site enables melting of the duplex to unmask the PQS, adopting a G-quadruplex fold (G4 structure/motif) that has a regulatory role in transcription activation. A third mode of gene regulation by DNA oxidation at a guanine, occurs when 8-oxo-dG is complexed with OGG1 and then recruits chromatin remodelers to