Topoisomerase-1 preferentially acts at nicks in DNA to cleave adjacent to the nick and then winds or unwinds the complex topologies associated with packed DNA. Here, the nick in the DNA serves as a marker for single strand breakage and subsequent unwinding. It is possible that this is not a highly conserved process. Topoisomerase may cause short deletions when it cleaves bonds, because both full-length DNA products and short deletion strands are seen as products of topoisomerase cleavage while inactive mutants only produced full-length DNA strands.