Topoisomerase II alpha is an essential nuclear enzyme involved in DNA replication and is a target for any anti-cancer drugs used for cancer therapy. Decreased expression of topoisomerase II alpha is a predominant  mechanis of resistance to several chemotherapeutic agents. A significant variation in the range of expression of this protein has been noted in many different tumours. Topoisomerase II alpha is predominant in proliferating cells and is modified in M phase by phosphorylation at specific sites. This appears to be critical for mitotic chromosome condensation and segregation. Topoisomerase II alpha may be a useful proliferation marker and may aid to identify a subgroup of neoplasms that respond to cytotoxic therapy with topoisomerase II inhibitors.