Enzymes (EC.184.108.40.206) that control and alter the topologic states of DNA in both prokaryotes and eukaryotes. Type I topoisomerases break a single DNA strand, whereas the type II enzymes break both strands. Topoisomerase I in eukaryotic cells normally functions during transcription. Topoisomerase II catalyses the relaxation of supercoiled DNA molecules, catenation, decatenation, knotting, and unknotting of circular DNA. The topoisomerase III gene is commonly deleted in patients with the Smith–Magenis syndrome; the product of this gene apparently specifically reduces the number of supercoils in highly negatively supercoiled DNA and is found in many somatic tissues. There is a marked reduction of topoisomerase II in some cell lines from patients with ataxia-telangiectasia. Topoisomerase II of E. coli is also known as DNA gyrase.