A single missense mutation was found in exon 13 of the 24-exon gene locus (GenBank accession no. NM_01970). The mutation was an A-to-G transition at nucleotide 2041, predicting the amino acid substitution cysteine for tyrosine (Y496C). This mutation occurs on the C-terminal side of the protease domain. Allelic complementation assays suggest the activity of this hypomorph allele is less than half that of the wild-type allele.