One loxP site was inserted 5" of exon 4. An FRT flanked neomycin selection cassette was inserted in intron 6. A second loxP site was also inserted in intron 6 downstream of the selection cassette and the selection cassette was subsequently removed from properly targeted mice by crossing to mice expressing flp recombinase leaving exons 4-6 flanked by loxP sites. Targeting was performed by inGenious Targeting Laboratory. (J:194700)