Figure 4

Difference topology experiment. Mu represented by the cyan colored ball is shown bound to five DNA crossings. Cre is represented by the smaller pink ball. Before Cre recombination, the DNA is circular and unknotted. Cre recombination changes the DNA configuration outside of the Mu transpososome. Since four of the five crossings bound by Mu are trapped by Cre recombination, the DNA product configuration equals a four crossing link.