Homologous recombination inserted a floxed CMV-hygromycin-thymidine kinase expression cassette downstream of exon 5 and placed a loxP site in intron 3. Cre-mediated recombination in ES cells excised the expression cassette and left loxP sites flanking exons 4 and 5, which encode the localization motifs and the entire catalytic domain of the protein. (J:66733)