20(S)camptothecin has shown promising activity against a range of tumour types.  This CPT derivative causes cell toxicity by stabilizing a ternary complex between the nuclear enzyme topoisomerase I (topo I) and double-stranded DNA, leading to single and double strand breaks.  TopoGEN provides this reagent as a control poison/inhibitor of topoisomerase I.  For in vitro cleavage detection, we recommend that the drug be tested over a wide range of uM concentrations; however, cleavages are strongly influenced by the amount of topoisomerase in the reaction as well as the type of assay being employed (SDSK+ vs. Radioac- tive End Labeled DNA vs. testing for plasmid cleavages).  Note that large amounts of enzyme will be required to detect cleavage complex, compared to assays that measure catalytic activity.