A loxP site was inserted upstream of exon 7 and a loxP-flanked neomycin selection cassette was inserted downstream of exon 8. Transient expression of cre recombinase removed the neomycin cassette, leaving exons 7 and 8 floxed. These exons encode in addition to the oligomerization domain, two highly conserved aspartic acid residues that are essential to the enzymatic activity of the gene product. Correct targeting was confirmed by genomic PCR.