A loxP site was inserted between exons 18 and 19 and a 1.8kb DNA fragment containing another loxP site as well as an frt flanked PGK-neomycin selection cassette was inserted between exons 20 and 21. Mice were crossed to Tg(EIF1A-flp)66Mim to remove the neomycin selection cassette. Exons 19 and 20 remain flanked by loxP sites.