A targeting vector was designed to insert a loxP site upstream of exon 20 (alternative splice site 4) followed by a frt-flanked neomycin resistance (neo) cassette, and a second loxP site downstream of exon 20. The less common splice acceptor sequence of exon 20 (TGTTTTTTAAACTTTAAAG) was substituted with a much more common one (TGTTTTTTCGTCTTCCTAG) to make the exon always incorporated in the transcript.