Lox P (locus of X-over P1) is a site on the Bacteriophage P1 consisting of 34 bp which is recognised by the site specific Cre recombinase.

When cells that have loxP sites in their genome express Cre, a recombination event can occur between the loxP sites. The double stranded DNA is cut at both loxP sites by the Cre protein. The strands are then rejoined with DNA ligase in a quick and efficient process. The result of recombination depends on the orientation of the loxP sites. For two lox sites on the same chromosome arm, inverted loxP sites will cause an inversion of the intervening DNA, while a direct repeat of loxP sites will cause a deletion event. If loxP sites are on different chromosomes it is possible for translocation events to be catalysed by Cre induced recombination.