A PGK-neomycin resistance cassette replaced all of the Serpine1 coding sequence and part of the promoter region including the transcription initiation site. Northern blot analysis did not detect Serpine1 mRNA in endotoxin-treated homozygous mutant liver. Neither untreated nor endotoxin-treated homozygous mutant kidney produced detectable SERPINE1 immunoreactivity. A monoclonal antibody based immunofunctional assay did not detect protein levels in plasma of endotoxin-treated homozygous mutant mice.