A loxP site was introduce 5' of exon 1 and a loxP-flanked selection cassette was introduced to intron 3 via 129X1/SvJ-derived RW4 embryonic stem (ES) cells. ES cell clones containing the recombinant allele were expanded and transiently transfected with a plasmid encoding Cre recombinase under the control of the CMV promoter to remove exons 1-3 and the selection cassette.