Topoisomerases solve the topological problems encountered by DNA throughout the lifetime of a cell. Topoisomerase II␣, which is highly conserved among eukaryotes, untangles replicated chromosomes during mitosis and is absolutely required for cell viability. A homozygous lethal mutant, can4, was identified in a screen to identify genes important for cell proliferation in zebrafish by utilizing an antibody against a mitosis-specific marker, phospho-histone H3. Mutant embryos have a decrease in the number of proliferating cells and display increases in DNA content and apoptosis, as well as mitotic spindle defects. Positional cloning revealed that the genetic defect underlying these phenotypes was the result of a mutation in the zebrafish topoisomerase II␣ (top2a) gene. top2a was found to be required for decatenation but not for condensation in embryonic mitoses. In addition to being required for development, top2a was found to be a haploinsufficient regulator of adult liver regrowth in zebrafish. Regeneration analysis of other adult tissues, including fins, revealed no heterozygous phenotype. Our results confirm a conserved role for TOP2A in vertebrates as well as a dose-sensitive requirement for top2a in adults.The accurate and complete replication and separation of chromosomes during mitosis is vital for the viability of cells. One potential complication encountered during cell division is the topological and tensional pressures put on DNA during these processes. Topoisomerases are responsible for enzymatically winding, unwinding, knotting, and unknotting reactions that are necessary for solving the topological problems of DNA. Genetic and biochemical studies in bacteria and yeast have revealed two classes of topoisomerases, type I and type II (38). Type I topoisomerases act on DNA by creating a temporary single-strand nick in DNA, passing the intact strand through the broken strand and then religating the nick. Type II topoisomerases, alternatively, function by binding to two double-stranded DNA molecules, generating a double-stranded break in one of the strands, passing the intact strand through the broken strand, and religating the broken strand.During DNA replication, sister chromatids become tangled with each other as a by-product of their duplication. The primary cellular function of type II topoisomerases during mitosis is the decatenation (untangling) of sister chromatids. Consistent with this essential role, genetic studies Saccharomyces cerevisiae have revealed that temperature-sensitive topoisomerase II mutants (TOP2) are unviable at nonpermissive temperatures (10,16,36). These mutants arrest during M phase due to their incompletely decatenated chromatids.Humans have two TOP2 homologues, topoisomerase II alpha (TOP2A) and topoisomerase II beta (TOP2B). Although they are both functionally redundant with the yeast homologue, TOP2A and TOP2B are genetically unique. As the products of different loci, TOP2A and TOP2B have different expression patterns. Specifically, TOP2A expression peaks during M phase, wh...