A FRT site flanked targeting vector, with a loxP site at the downstream end and containing a PGK-neo selection cassette was utilized in the construction of this mutant. This selection cassette was inserted downstream of exon I (also called exon 2, when the first non-coding exon is included in the numbering scheme) of the targeted gene, and another loxP site was inserted upstream of exon III (also called exon 4, when the first non-coding exon is included in the numbering scheme). The PGK-neo selection cassette was removed by Flp-recombinase mediated excision. Exon II is flanked by loxP sites. A point mutation within the first loxP site was identified that can interfer with recombination. (J:147269, J:266012)