During λ integration, recombination always occurs at exactly the same nucleotide sequencewithin two recombination sites, one on the phage DNA and the other on the bacterial DNA.Recombination sites carry two classes of sequence elements: sequences specifically bound by the recombinases and sequences where DNA cleavage and rejoining occur.Recombination sites are often quite short, 20 bp or so, although they may be much longer andcarry additional sequence motifs and protein-binding sites.