A frt-flanked neomycin cassette and an upstream loxP site was inserted upstream of exon 1. A second loxP site was inserted downstream of exon 2 at nucleotide position 1080. Founder mice were generated from homologously targeted clones and then crossed with Flp-deleter to remove the neomycin cassette and leave exons 1 and 2 floxed. Correct targeting was confirmed by Southern blotting.