Topoisomerase enzymes (topo) are critical to normal function of any living cell. They adjust DNA winding (topology) by making temporary cuts in the DNA for transcription and replication. They are key targets for a group of anti-tumor chemotherapy compounds (such as camptothecin and topotecan)and some environmental toxins. Topo inhibitors freeze the enzyme-DNA complex at the stage after the cut is made and before the DNA is reconnected (called the stabilization complex) so the DNA and enzyme can't go on with normal function.