The attP site (5'-GTGCCCCAACTGGGGTAACCTttGAGTTCTCTCAGTTGGGGG-3') is one of the two target sites of the phiC31:int integrase. phiC31:int mediates recombination between two non-identical target sites, attP and attB, each of which contains a short integration core flanked by imperfect inverted repeats. Exchange between the attP/attB pair results in the formation of hybrid sites (attL and attR) that are no longer a substrate of the phiC31:int integrase, and thus the exchange reaction driven by phiC31:int is undirectional (reviewed in PMID:21195181, FBrf0231034). The recombination event catalyzed by phiC31:int results in genetic modification, the nature of which is influenced by the relative orientation (direct or inverted), and location of the two target sites. The types of possible modification include deletion of DNA, generation of chromosomal rearrangements, integration of DNA into the genome, and replacement of genetic material with that from a donor plasmid using recombination-mediated cassette exchange (RMCE) (reviewed in FBrf0231034). For integration of DNA into the genome, the 'docking site' in the genome typically contains the attP site and the donor plasmid containing the sequence to be integrated contains the attB site, since this directionality gives higher integration rates compared to the alternative combination where the attB site is integrated into the genome (PMID:11359900, PMID:12563279).