The Topo I Drug Kit introduces transient nicks in DNA at specific sites. Detection of these transient DNA nicks requires trapping the enzyme on DNA in a nicked intermediate complex using protein denaturants. The resulting covalent DNA/topo I complexes contain nicked open circular DNA which can be detected by agarose gel electrophoresis (with ethidium bromide). Trapping nicked intermediates is relatively inefficient; however, inhibitors, such as the natural product camptothecin, stabilize the intermediate and lead to an increase in the nicked DNA product. This forms the basis for a mechanistic drug screening assay designed to allow detection of agents that affect topoisomerase I by stabilizing the cleaved intermediate complex. The TopoGEN Topo I Drug Screening Kit is designed to allow the investigator to quickly identify novel inhibitors of Human Top1 (included with kits denoted -A in catalog numbers). The kit will allow detection of novel compounds that either stabilize the nicked intermediate or otherwise inhibit catalytic activity of topoisomerase I.