A targeting vector was used to introduce a loxP-flanked neomycin resistance cassette to intron 8, and an additional loxP site to intron 9 of the gene. The construct was electroporated into 129P2/OlaHsd-derived E14 embryonic stem (ES) cells. The neomycin cassette was excised with a C57BL/6 background EIIa-Cre, leaving loxP sites on either side of exon 9. (J:138683)