A floxed neomycin resistance cassette was inserted upstream of exon 3. Exon 3 was replaced with one in which a nucleotide substitution(s) results(s) in the amino acid substitution of alanine for cysteine at position 103 (C103A). This mutation disrupts the deubiquitinating function of the OTU domain. Cre-mediated recombination removed the neomycin resistance cassette.