The targeting vector used the gene from BAC RP24-167K18 and inserted the following elements around exon 2: a loxP site, an FRT site, a neomycin resistance gene cassette, an FRT site and a loxP site into intron 1, and a third loxP site into intron 2. The neo cassette was removed through subsequent flp-mediated recombination, and exon 2 was deleted through subsequent cre-mediated recombination.