Exon 4 was replaced with a modified exon 4 in which nucleotide substitutions (GAA to GGC) result in the amino acid substitution of alanine for phenylalanine at position 77 (F77A). This mutation abolishes DNA-dependent cooperative multimerization. Cre-mediated recombination removed the floxed neomycin resistance cassette inserted downstream of the modified exon. (J:209388)