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In this study we demonstrate that the different substrate recognition properties of bacterial and human AP endonucleases might be used to quantify and localize apurinic (AP) sites formed in DNA in vivo. By using a model oligonucleotide containing a single AP site modified with methoxyamine (MX), we show that endonuclease III and IV of E. coli are able to cleave the alkoxyamine-adducted site whereas a partially purified HeLa AP endonuclease and crude cell-free extracts from HeLa cells are inhibited by this modification. In addition MX-modified AP sites in a DNA template retain their ability to block DNA synthesis in vitro. Since MX can efficiently react with AP sites formed in mammalian cells in vivo we propose that the MX modified abasic sites thus formed can be quantitated and localized at the level of the individual gene by subsequent site specific cleavage by either E. coli endonuclease III or IV in vitro.