Exon 9 was replaced with a modified exon 9 in which nucleotide substitutions (AAGAAAAA>GCGGCCGC) result in the amino acid substitution of lysine with alanine at positions 316, 317, and 318. These substitutions disrupt nuclear localization signal 1 (NLS1). A loxP site flanked neomycin resistance gene cassette inserted downstream of the modified exon 9 was removed by Cre-mediated recombination. (J:175018)