Insertion of a PGK-neomycin resistance cassette into an exon of the C3 gene deleted sequences that code for the C-terminal region of the beta chain and the N-terminal region of the alpha chain, including the site for processing the pro-C3 molecule. ELISA testing did not detect C3 protein in serum of homozygous mutant mice. A C3 hemolytic assay did not detect functional C3 activity. (J:30152)