A loxP site was inserted upstream of exon 4 and a floxed neomycin cassette was placed downstream. After targeting and selection, transient expression of cre recombinase removed the neomycin cassette and left exon 4 floxed. Correct targeting was confirmed by Southern blotting. Expression of cre recombinase will remove exon 4, which encodes a zinc finger domain common to the three major protein isoforms of the gene locus.