Exon 9, encoding a proteolytic cleavage site, and flanking intronic sequence were replaced with a neomycin resistance gene cassette. A reduced level of mRNA was detectable by RT-PCR from this allele; sequence analysis revealed splicing from exon 8 to exon 10 without frameshift. However, Western blot analysis revealed that expression of the protein was abolished, and this allele is suggested to be a null.