Camptothecin (CPT) is a highly selective topoisomerase I inhibitor. This reagent converts topoisomerase 1 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).