Exon 1 of the Vim gene was disrupted by insertion of a LacZ neomycin resistance cassette. Northern analysis of organs from homozygous mutant animals did not detect Vim transcript. Western analysis, immunofluorescence microsopy, and immunohistochemical studies did not detect VIM protein in tissues of homozygous mutant mice. The endogenous Vim promoter controls expression of beta-galactosidase in the targeted allele.