LoxP sites were inserted around exon 4 with a Frt-flanked neomycin cassette included between the exon and the downstream loxP site. Chimeric mice were bred with Flpe transgenic mice to remove the neomycin cassette, leaving exon 4 floxed. Cre-mediated deletion of exon 4 will eliminate part of the catalytic domain of the protein product and will also result in a frame shift upon fusion of exon 3 to exon 5. (J:150709)