One loxp site was inserted in intron 3 and an FRT flanked neomycin selection cassette, together with a second loxP site, was inserted in intron 5. The neo selection cassette was subsequently removed by expression of flp recombinase. Exons 4 and 5 containing the ATG start site are thus flanked by loxP sites. Cre recombinase expression then excised the two floxed exons. (J:211228)