An inverted loxP site was introduced upstream to exon 2 in a genomic framgment containing the Ext1 gene. The fragment was then subcloned into a vector upstream of the forward-facing loxP-flanked
neomycin resistance (Neo) cassette. The linearized targeti vector was electroporated into R1 ES cells. Cre-mediated recombination of this allele will result in inversion of exon2 of Ext1 and distruption of Ext1 function.