The targeting vector is designed to contain (from 5'; to 3') an 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 4. The frt-flanked neomycin cassette was removed in the germline via FLP-mediated recombination. (J:340358)