Exons 3 and 4 are deleted and are replaced by a PGK-hygromycin selectable marker. This prevents translation beyond residue 67 and deletes the entire catalytic domain (residues 271-484) including the catalytic cysteine at residue 301. Whole mount immunohistochemistry experiments confirmed that the encoded protein was undetectable in homozygous embryos. (J:75950)