A neomycin resistance cassette replaced a genomic fragment containing exon 20, which encodes the first two disulfide loops. Northern blot analysis on RNA derived from salivary gland and kidney of homozygous mice demonstrated that negligible amounts of transcipt is produced from this allele. Western blot analysis on samples derived from salivary gland of homozygous mice confirmed that no detectable protein is expressed from this allele.