The targeting vector was designed (by site-directed mutagenesis) to substitute a serine to histidine at amino acid position 270 and leucine to alanine at amino acid position 277 of exon 9. An FRT-flanked PGKneo cassette was inserted downstream of exon 9. Flp-mediated recombination removed the neo cassette and left a single FRT site downstream of exon 9.