<p>DNA topoisomerases regulate the number of topological links between two DNA strands (i.e. change the number of superhelical turns) by catalysing transient single- or double-strand breaks, crossing the strands through one another, then resealing the breaks [<cite idref="PUB00005437"/>]. These enzymes have several functions: to remove DNA supercoils during transcription and DNA replication; for strand breakage during recombination; for chromosome condensation; and to disentangle intertwined DNA during mitosis [<cite idref="PUB00020794"/>, <cite idref="PUB00016842"/>]. DNA topoisomerases are divided into two classes: type I enzymes (<db_xref db="EC" dbkey="5.99.1.2"/>; topoisomerases I, III and V) break single-strand DNA, and type II enzymes (<db_xref db="EC" dbkey="5.99.1.3"/>; topoisomerases II, IV and VI) break double-strand DNA [<cite idref="PUB00020793"/>].</p><p>Type II topoisomerases are ATP-dependent enzymes, and can be subdivided according to their structure and reaction mechanisms: type IIA (topoisomerase II or gyrase, and topoisomerase IV) and type IIB (topoisomerase VI). These enzymes are responsible for relaxing supercoiled DNA as well as for introducing both negative and positive supercoils [<cite idref="PUB00020795"/>].</p><p>This entry represents DNA topoisomerase II enzymes from eukaryotes and viruses. Topoisomerase II primarily functions in introducing negative supercoils into DNA, and is of particular importance during the segregation of chromosomes during mitosis [<cite idref="PUB00020806"/>, <cite idref="PUB00004227"/>]. In eukaryotes and viruses, this enzyme occurs as a single polypeptide, with the N-terminal portion (homologous to subunit B of bacterial topoisomerase II, or gyraseB) responsible for ATPase activity and the C-terminal portion (homologous to subunit A of bacterial topoisomerase II, or gyraseA) responsible for the breaking-rejoining function through its capacity to form protein-DNA bridges. In mammals, there are at least two isozymes of this enzyme, topoisomerases II-alpha and II-beta, which are similar in structure and catalytic properties [<cite idref="PUB00020807"/>, <cite idref="PUB00020808"/>]. The alpha isoform is involved in chromosome condensation and segregation.</p><p>More information about this protein can be found at Protein of the Month: DNA Topoisomerase [<cite idref="PUB00035961"/>].</p>