S205D (AGT->GAT) and K224A (AAA->GCA) point mutations were introduced to the catalytic subunit of the gene and an FRT-flanked neomycin selection cassette was inserted in intron 6. The neomycin selection cassette was excised through crosses with Tg(ACTFLPe)9205Dym deleter mice, leaving a single FRT site in intron 6/7.