A major role of DNA topoisomerase II in vivo is to catalyze the double-stranded cleavage of DNA, allowing passage of a second DNA duplex through the break. This activity requires adenosine triphosphate (ATP) and is necessary for separating catenated DNA duplexes found at the end of replication. The decatenation of DNA molecules is a topoisomerase II-specific reaction, and is a convenient assay for measuring topoisomerase II activity in vitro (1).