A neomycin resistance cassette was inserted into exon 3, which encodes the activation site at the junction of the propeptide and mature enzyme. RT-PCR experiments demonstrated that exon 3 was absent from transcripts produced from this allele, but Western blot analysis revealed the presence of a truncated protein of a size predicted by this mutation. However, no mature processed protein was detected and there was no evidence of enzyme activity in homozygous mice.