Escherichia coli endonuclease III (EC 4.2.99.18) [1] is a DNArepair enzyme that acts both as a DNA N-glycosylase, removingoxidised pyrimidines from DNA, and as an apurinic/apyrimidinic(AP) endonuclease, introducing a single-strand nick at the sitefrom which the damaged base was removed. Endonuclease III is aniron-sulfur protein that binds a single 4Fe-4S cluster. The4Fe-4S cluster does not seem to be important for catalyticactivity, but is probably involved in the proper positioning ofthe enzyme along the DNA strand [2]. The 4Fe-4S cluster is boundby four cysteines which are all located in a 17 amino acidregion at the C-terminal end of endonuclease III. A similarregion is also present in the central section of mutY and in theC-terminus of ORF-10 and of the Micro-coccus UV endonuclease[4].