Order of sequences 5' to 3': FRT; w[+] marker (same orientation as fru targeting sequence); RS(I-CreI); a mutated fragment of fru genomic sequence (targetting sequence); RS(I-CreI); FRT. The targetting sequence consists of a mutated S-exon (lacking 261bp from the 3' end) flanked by 0.6kb of 3' intronic sequence (up to where the fru sequence of fruX-3'donor begins), and 7.1 kb of 5' intronic sequence into which an RS(I-SceI) site has been engineered 3.4 Kb 5' to the start of the S-exon.