DNA topoisomerase II (top 2) is the cellular target of several among the most potent anticancer agents (Doxorubicin, etoposides [VP-l6; VM-26], mitoxantrone, amsacrine, ellipticines). For this reason, it is one of the key targets in anticancer drug development. We have further characterized azatoxin derivatives that are both top 2 and tubulin inhibitors and have obtained pure top 2 and pure tubulin inhibitors. We have also shown that the anthrapyrazoles derivatives (DU937 and DU94l) that are in clinical trials are top 2 inhibitors. DNA topoisomerases 1 (top 1) has become an essential target for anticancer research since the discovery that camptothecin and several of its derivatives are specific top 1 poisons and that water-soluble camptothecin analogs exhibit promising anticancer activity. Saintopin is a dual top 1 and top 2 inhibitor. We have sequenced the saintopin cleavage sites and found for the first time that for both top 1 and top 2, a guanine is strongly preferred at the 5'-termini of the breaks. This result is consistent with our hypothesis that the drug bind at the interface of the DNA and the enzyme. Another approach to the drug-topoisomerase molecular interactions has been to develop and analyze camptothecin-resistant cells. We have characterized mutations in top 1 cDNA leading to amino acid point mutations in two camptothecin-resistant cell lines indicating that selective enzyme regions are important for both enzymatic activity and camptothecin sensitivity.