A targeting vector containing a FRT site flanked PGK-Neo selection cassette and a loxP site was utilized in the construction of this mutant. This selection cassette was inserted 227 bp upstream of exon 3 of the targeted gene, and another loxP site was inserted 225 bp downstream of exon 4. Flp-mediated recombination removed the neo cassette and left exons 3 and 4 floxed.