One loxAPA site was inserted in intron 5 240 bp upstream of exon 6 and an FRT flanked MC1-neomycin cassette followed by a second loxP site was placed 330 bp from exon 7. The neomycin selection cassette was excised by crossing to Tg(ACTFLPe)9205Dym mice. Exons 6 and 7, containing the essential zinc binding domain, were then removed by crossing to Tg(EIIa-cre)C5379Lmgd mice.