Camptothecin (CPT) is a highly selective topoisomerase I inhibitor (Liu et al., 2000 and Pommier and Cherfils, 2005). This reagent converts topoisomerase I, an essential enzyme in higher eukaryotes, to a cellular poison when replication forks collide with CPT-trapped topoisomerase I cleavage complexes (Pommier, 2006). The resulting lesions are replication-mediated DNA double-strand breaks (DSBs) (Strumberg et al., 2000).