The use of site-specific recombinases has revolutionized the genetic analysis of development and has made possible the precise engineering of genomes (1 ,2 ). InDrosophila, theFLP/FRTsystem, introduced by Golic and Lindquist (3 ), has been used (1) to generate genetic mosaics by mitotic recombination as well as by “flip-outs”(3 -5 ), and (2) to generate defined chromosomal rearrangements (6 ,7 ). In yeast and mammalian cells, site-specific recombination has also been used to mediate targeting of exogenous DNA to genomic docking sites (8 ). Although such targeted integration is by nature an inefficient process-as a result of the favoring of intramolecular over intermolecular recombination-this limitation has been overcome in these systems by the ability to introduce DNA into a large number of cells simultaneously and to select for rare integration events by chemical means. InDrosophila, such an approach is not currently available, although an approximation of targeted integration of exogenous DNA has been used, with varying efficiency, to mobilize FRT-flanked DNA already in the genome to a specificFRTtarget site elsewhere (9 ).
