Cleavage of the bond between a deoxyribose sugar in DNA's sugar-phosphate backbone and its associated nucleobase leaves an abasic site in double stranded DNA. These abasic sites are often generated as an intermediate and then restored in base excision repair. However, if these sites are allowed to persist, they can inhibit DNA replication and transcription. Abasic sites can react with amine groups on proteins to form DNA-protein crosslinks or with exocyclic amines of other nucleobases to form interstrand crosslinks. To prevent interstrand or DNA-protein crosslinks, enzymes from the BER pathway tightly bind the abasic site and sequester it from nearby reactive groups, as demonstrated in human alkyladenine DNA glycosylase (AAG) and "E. coli" 3-methyladenine DNA glycosylase II (AlkA). "in vitro" evidence demonstrated that the Interstand Cross-Links induced by abasic site (DOB-ICL) is a replication-blocking and highly miscoding lesion. 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.