Exons 5-7 encoding the Sec-insertion sequence element and the polyadenylation signal were flanked by loxP sites. A frt-flanked neomycin resistance cassette was placed immediately downstream of the 3' loxP site. Founder mice were crossed with FLPe deleter mice to remove the neomycin cassette leaving behind a single frt site. Correct targeting was confirmed by Southern blotting.