A targeting vector was used to flank exon 8 of the endogenous gene with loxP sites. The vector also contained an FRT-flanked neomycin resistance cassette. Correctly targeted ES cells were injected into recipient blastocysts. The resulting chimeric animals were crossed to C57BL/6 mice. Heterozygous mice were crossed to the FlpER (129S4/SvJaeSor-Gt(ROSA)26Sortm1(FLP1)Dym) strain to remove the neo cassette, leaving a single FRT site upstream of exon 8 and the flanking loxP sites.