Exon 4 was replaced with a modified exon in which nucleotide substitutions result in the amino acid substitution of aspartic acid for serine at position 362 and 374 (S362D and S374D). Flp-mediated recombination removed the floxed neo cassette inserted downstream of the endogenous polyadenylation sequence and left an EGFP gene with a 5' loxP site intact. (J:175064)