The sequences encoding the first 435 amino acids (including 16 out of 19 leucine repeats) corresponding to exon 1, intron 1, and part of exon 2 were replaced by a neomycin resistance cassette. Northern blot analysis showed an absence of gene transcripts in liver and kidneys of homozygous mutant animals. Plasma proteins from homozygous animals were unable to form cross-linking complexes which is consistent with deletion of the 16 leucine repeats.