The PAI entry in FlyBase represents the serine recombinase encoded by Pseudomonas aeruginosa Int-2 (GB_protein:WP_023098492.1). PAI mediates recombination between two non-identical target sites, attP(PAI) and attB(PAI). Exchange between the attP(PAI)/attB(PAI) pair results in the formation of hybrid sites that are no longer a substrate of the PAI serine recombinase, thus the exchange reaction driven by PAI is unidirectional (FBrf0262209). The recombination event catalyzed by PAI 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 and integration of DNA into the genome.