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. Cre-mediated recombination removed exons 2 and 3. (J:300510)