A loxP site was inserted into the XbaI restriction site upstream of exon 1 and a frt-flanked neoymycin cassette with a 3' loxP site was inserted downstream of exon 1. Mice were crossed with transgenic Flipase to remove the neomycin cassette, leaving exon 1 floxed. Protein expression was unaffected as determined by immunoblot analysis of homozygote lymphocyte extracts. (J:147057)