A 0.5 kb genomic fragment encoding 62 amino acids including those that make up homology regions II and III was replaced with a neo cassette inserted by homologous recombination. Transcript was undetected by Northern blot analysis of homozygous mutant liver tissue. Activity assays indicated a near comlpete ablation of endogenous activity and showed that the activity of the microsomal isoform was unperturbed.