Exon 2 was floxed by the following targeting strategy. A single loxP site was inserted into intron 1 at the AatII site and a loxP-flanked neomycin resistance cassette was inserted at ApaI site 3' of exon 2. The neomycin cassette was removed by transient expression of cre-recombinase, leaving exon 2 floxed.