Topoisomerase I unwinds the DNA during cell replication and is an enzyme that is found in Archaea, Bacteria, and Eukarya. Most every living thing has a homolog. To everyone's surprise, it is possible to create mutations that completely remove the activity of the enzyme, yet have cells capable of growth. These cells are sick, but they can grow. The latest research by Stockum et al. shows that these deletion mutants of topoisomerase I actually have compensating mutations, and in fact loss of topoisomerase I is lethal. This makes much more sense.