The single nucleotide replacement pathway of base excision repair appears to facilitate the repair of most damaged bases. Following DNA glycosylase mediated cleavage of the damaged base, the endonuclease APEX1 is recruited to the site of damage where it cleaves the 5' side of the abasic (AP) deoxyribose residue. DNA polymerase beta (POLB) then cleaves the 3' side of the AP sugar phosphate, thus excising the AP residue. APEX1 is subsequently released, the XRCC1:LIG3 complex is recruited, and POLB mediates the synthesis of the replacement residue. Following LIG3 mediated ligation of the replaced residue, the XRCC1:LIG3 complex dissociates from DNA (Lindahl and Wood, 1999). An alternative BER pathway is employed when the structure of the terminal sugar phosphate is such that it cannot be cleaved by the AP lyase activity of POLB.