The targeting construct introduced a neomycin resistance cassette (neor), flanked on each side by a loxP followed by a frt site, upstream of exon 2 and a single loxP site downstream of exon 2. The neor was deleted by FLP recombinase, leaving a single frt site in intron 1 and loxP sites flanking exon 2, which contains the translation initiation site.