A targeting vector was designed to replace exons 10-13 with a loxP-flanked neo. The deleted exons encode 2 of 3 catalytic residues essential for serine protease activity. Quantitative RT-PCR failed to detect transcript of exons 11 and 13. Mutants expressed only a 20% reduction in transcripts spanning exons 3 and 4 or 5 and 6, however, mutants expressed 80% less transcript of the 3' UTR than wild-type mice. Sequencing revealed that mutant transcripts alternatively splice exon 9 to exon 14.