Site-specific recombinases carry out a multitude of critical functions in nature ranging from gene rearrangement to genome segregation []. The Cre (Causes REcombination) recombinase protein is a member of the integrase family of site specific recombinases. It is a 38kD protein found in bacteriophage P1 that catalyzes the recombination between two identical double stranded sites of a particular sequence called loxP (Locus Of crossiver(X) in P1) sites []. This system has become one of the most valuable tools for the manipulation of DNA in vitro and in vivo today with many applications. The physiological role in bacteriphage P1, the binding of Cre recombinase to DNA, and a few examples of uses to manipulate DNA will be discussed. Here is a model of Cre Recombinase,