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Topoisomerase inhibitors block the enzyme DNA topoisomerases (I and II) which reduce torsional stress in super coiled DNA, allowing selected regions of DNA to become untangled and relaxed to permit its replication, recombination, repair, and transcription. Camptothecin analogs (eg, Camptosar, Hycamtin) inhibit the function of topoisomerase I and are S-phase specific. Epipodophyllotoxins (eg. Etopophos, Vumon) induce DNA strand breaks by inhibiting the function of topoisomerase II in cells in the S and G2 phases.