Fig. 7. Site-specific integration in Drosophila. (A) C31 integrase-mediated transgenesis using single attP docking sites. Docking sites are transposons, such as P elements (Groth et al., 2004), piggyBac (Venken et al., 2006) or Mariner (Bischof et al., 2007), that contain a single attP recombination site and a marker 1, and that are integrated into the genome. A plasmid containing an insert, marker 2 and an attB recombination site, can then integrate into the docking site when C31 integrase is provided. Correct recombination events between attP and attB are identified using marker 2. They result in two hybrid sites, attL and attR, that are no longer a substrate for C31 integrase - the reaction is therefore irreversible. (B) Cre- and FLP-mediated RMCE. Docking site transposons (with 5' and 3' transposon termini), such as P (Oberstein et al., 2005) or piggyBac (Horn and Handler, 2005) elements, contain marker 1 flanked by heterotypic direct-oriented recombination sites (RS) `RS1' (loxP or FRT, gray) and `RS2' (such as lox2272 or F3, purple). The RMCE plasmid, containing marker 2 flanked by a similar configuration of heterotypic recombination sites, can integrate when Cre or FLP is provided. Correct recombination events are identified by the absence of marker 1 and presence of marker 2. Recombination can be partial (single integration events are not shown) and is reversible. (C) C31 integrase-mediated RMCE. A docking site P element transposon (5'P and 3'P element termini) (Bateman et al., 2006) contains a marker 1 flanked by inverted attP recombination sites. The RMCE plasmid, containing insert flanked by inverted attB recombination sites, can integrate when C31 integrase is provided. Correct recombination events, between both attP and attB sites, are identified through absence of marker 1 and result in hybrid sites, attL and attR, that are no longer substrates for C31 integrase. The integrated DNA can be in either orientation (arrows).