A floxed neomycin resistance cassette was inserted upstream of a modified exon 4 in which nucleotide substitutions (CACAGGAGTCAGAAA to CACAGATCTCAAAGG) result in the amino acid substitution of arginine for lysine at position 253 (K253R). This mutation prevents ubiquitination. Cre-mediated recombination removed the neomycin resistance cassette. (J:201198)