Exon 7 was replaced with one in which nucleotide substitutions result in the amino acid susbtitutions of alanine for cysteine at positions 609 and 612 (C609A, C612A). This mutation disrupts the zinc finger 4-based E3 ligase and ubiquitin binding. Cre-mediated recombnation removed the floxed neomycin resistance cassette inserted downstream of the modified exon 7.