DNA topoisomerases are nuclear enzymes that regulate the topological structure of DNA by transiently breaking and rejoining DNA strands. Although DNA topoisomerase I and DNA topoisomerase II relax both positive and negative supercoils, DNA topoisomerase III relaxes only negative supercoils. DNA topoisomerase III (alpha) exists as a long and a short isoform, which are produced by alternative splicing. DNA topoisomerase III (alpha), which localizes to the nucleolus, is constitutively expressed and remains at high levels throughout the cell cycle in HL-60 cells. DNA topoisomerase III (beta) exists as three isoforms, namely (beta)1, (beta)2 and (beta)3, also produced by alternative splicing. DNA topoisomerase III (beta)1 is expressed in testes, heart and skeletal muscle, whereas (beta)2 is expressed in thymus, kidney and pancreas.