The targeting construct obtained from EUCOMM is composed of FRT-splice acceptor-IRES-lacZ-polyA-loxP-neomycin cassette-poylA-FRT-loxP inserted upstream of exon 4. A third loxP site is inserted downstream of exon 4. Flp-mediated recombination removed the FRT-flanked lacZ and neo cassette leaving exon 4 floxed. (J:260852)