Exon 4, intron 4, and part of exon 5 were replaced with a PGK-neo cassette via homologous recombination. Mutant gene transcripts were detected in homozygous mutant embryos by RNase protection assay and RT-PCR analysis. Sequencing of the RT-PCR product revealed the mutant transcript was spliced from exon 3 to exon 6 resulting in a frameshift and nonsense mutation in exon 7. Absence of protein product in homozygous mutants was confirmed by immunohistochemical analysis of E13.5 embryo sections and FAP-specific activity assays. (J:59998)