An FRT flanked selection cassette was inserted in intron 2 and the codon in exon 3 for Cys61 5'-TGT-3' was mutated to 5-GCT-3' encoding alanine via homologous recombination. Flp mediated recombination removed the selection cassette. The mutation in exon 3 disrupts the isopeptidase activity of the protein. However regulatory properties of the protein are intact. (J:217664)