Insertion of a neomycin resistance cassette into exon 13 disrupted the gene. RT-PCR studies of cultured tail cells using primers from exon 12 and exon 14 did not detect wild-type transcript in homozygous mutant mice. Enzyme activity assays of organs and cultured cells detected nearly no activity in homozygous mutant mice. Western blot analysis of several tissues using rabbit anti-human antibodies did not detect immunoprecipitated protein in homozygous mutant mice.