P1cre corresponds to the recombinase encoded by the cre locus of the bacteriophage P1 genome. P1cre mediates recombination between a pair of specific target sites, with compatible target sites corresponding to the wild-type loxP site and some of its derivatives. The loxP target site is composed of two 13bp inverted repeats flanking an 8bp asymmetric spacer sequence (PMID:6954485). Recombination occurs between a pair of target sites oriented in the same direction; the 13bp repeats each act as binding sites for the recombinase, while the asymmetric 8bp spacer is the site of DNA strand exchange and determines the orientation of the target site (PMID:3856690). The recombination event catalyzed by P1cre results in genetic modification, the nature of which is influenced by the relative orientation (direct or inverted), location and composition of the two target sites. The types of possible modification include deletion of DNA and generation of chromosomal rearrangements (reviewed in FBrf0231034).