A T to G nucleotide substitution was introduced into exon 6 at nucleotide 943 by homologous recombination. This point mutation causes the substitution of the active site serine 238 within the catalytic histidine-aspartate-serine triad with alanine (S238A) and renders it catalytically inactive. In addition, a loxP flanked neomycin cassette under the control of the 3-phosphoglycerate kinase promoter was inserted upstream of the substitution. The neomycin cassette was removed via Cre-mediated recombination by mating with Tg(EIIa-cre)C5379Lmgd transgenic mice. RT-PCR indicates that the introduction of the point mutation and the loxP site in intron 5 do not affect mRNA expression. Western blot analysis shows that protein is expressed at levels similar to wild-type and the presence of a faster migrating species in the kidney and lung.