Exon 4 was floxed by introducing a loxP site upstream of the exon and inserting a loxP flanked PGK/TK neomycin cassette downstream of the exon. Chimeric mice were generated, crossed with a Cre-deleter strain, and offspring were selected that had the neomycin selection cassette removed but left exon 4 intact. Genotypes were confirmed by genomic PCR.