The Cre–lox structures reveal that recombination requires four subunits of Cre, with each molecule bound to one binding site on the substrate DNA molecules (Fig. 12-9).The conformation of the DNA is generally a square planar four-way junction (see discussion of Holliday junctions in Chapter 11) with each “arm” of this junction bound by one subunit of Cre.Although at first glance the structures appear to have fourfold symmetry, this is not really thecase.