λInt is a tyrosine recombinase, and the mechanism of strand exchange follows the pathway described above for the Cre protein.Unlike Cre recombination, however, λ integration requires accessory proteins to help the required protein–DNA complex to assemble.These proteins control the reaction to ensure that DNA integration and DNA excision occur at theright time in the phage life cycle.We first consider the integration pathway and then look at how excision is triggered.