A targeting vector is designed to contain (from 5' to 3') a frt site, an En2 splice acceptor, an internal ribosome entry site (IRES) fused to an EGFP sequence with a polyadenylation signal, and a floxed cassette containing a neo and a second frt site all upstream of exon 4. A third loxP was placed downstream of exon 6. Flp-mediated recombination removed the neo cassette and the 5' loxP site, leaving exons 4-6 floxed. (J:290857)