Order of sequences 5' to 3': FRT; w[+] marker (opposite orientation to fru targeting sequence); RS(I-CreI); a mutated fragment of fru genomic sequence (targeting sequence); RS(I-CreI); FRT. The targeting sequence is about 7.5kb of fru intronic sequence starting 0.6kb 3' to the S-exon of fru and including the P2 promoter. An RS(I-SceI) site has been engineered into this sequence 4.1 Kb from its 5' end.