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A floxed hygromycin cassette was inserted in intron 1 and a third loxP site was placed in intron 3. In vitro cre recombination removed the floxed hygromycin cassette from properly targeted ES cell clones. Exons 2 and 3 were then left flanked by loxP sites. Mice were crossed to a protamine deleter strain to confirm that the recombined allele had a null phenotype. (J:93140)