A topological filter in Hin-hix recombination. The ribbons represent double-stranded DNA. The Hin recombination complex consists of four Hin subunits bound in pairs to two hix sites, along with an enhancer sequence (green) bound by a host-encoded protein called Fis. Recombination cannot occur unless the entire complex comes together with the topology shown here. This particular alignment of hix sites occurs only if the sites are present in the same supercoiled DNA molecule and have the opposite orientation.