DNA topoisomerase IIα is a human enzyme encoded by the TOP2A gene. Topoisomerase IIα relives topological DNA stress during transcription, condenses chromosomes, and separates chromatids. It catalyzes the transient breaking and rejoining of two strands of duplex DNA which allows the strands to pass through one another. Two forms of this enzyme exist as likely products of a gene duplication event. The gene encoding this form, alpha, is localized to chromosome 17 and the beta gene is localized to chromosome 3. The gene encoding this enzyme functions as the target for several chemotherapy agents and a variety of mutations in this gene have been associated with the development of drug resistance.[citation needed] Reduced activity of this enzyme may also play a role in ataxia-telangiectasia. TOP2A has been shown to interact with SMURF2, HDAC1, CDC5L, Small ubiquitin-related modifier 1 and P53.