By homologous recombination in ES cells a single loxP site was inserted into intron 3 and FRT flanked PGK-neo cassette followed by loxP was inserted into intoron 4, resulting in Exon 4 flanked by loxP sites. Heterozygous Cyp4x1tm1Beld mice are crossed with cre deleter to remove Exon 4 and PGK-neo cassette, resulting in frame-shift mutaion. (J:254435)