Exon 28 was replaced with a modified exon 28 in which nucleotide substitution (GAT to TAA) resulted in the amino acid substitution of a stop codon for aspartic acid at the second processing site. This mutation prevents the expression of the C-terminus subunit. Western blot analysis confirmed the lack of expression of the C-terminal subunit and reduced expression of the N-terminus subunit. Cre-mediated recombination removed the neo cassette. (J:183048)