Exon 4, encoding the active site, was replaced with a loxP flanked hygromycin B resistance cassette. Tranfection of ES cells with a cre-expressing plasmid resulted in the excision of the hygro. Brain tissue from mutants was analyzed by RT-PCR to show the presence of the altered transcript. Sequence analysis revealed a frameshift and a premature stop codon after exon 3. (J:95522)