 Compared to several other TLS pols examined, pol η is likely to contribute to the TLS-mediated repair of the DOB-ICL "in vivo".By using O6-2'-deoxyguanosine-butylene-O6-2'-deoxyguanosine (O6-dG-C4-O6-dG) DNA lesions which is a chemically stable structure, the bypassing activity of several DNA polymerases had been investigated and the results demonstrated that pol η exhibited the highest bypass activity; however, 70% of the bypass products were mutagenic containing substitutions or deletions.The increase in the size of unhooked repair intermediates elevates the frequency of deletion mutation.