Mammalian cells contain a DNA-recombining activity that can be dissociated from topoisomerase activity.

The role of the topoisomerase enzyme in DNA recombination was investigated by extracting chromosomal deoxyribonucleoproteins from a variety of cultured mammalian cells and assaying for the formation of recombinant DNA structures. Although each of the crude deoxyribonucleoprotein preparations contained topoisomerase activity, they did not all contain DNA-recombining activity. A distinct, perhaps novel, enzyme may therefore promote DNA recombination in these cell-free systems.