To create a knockout of both Cd80 and Cd86 in mice, an ES cell line in which the Cd80 gene was mutated was subsequently used in a second round of gene targeting. A hygromycin selection cassette replaced an exon encoding the IgV-like domain of the protein and flanking genomic sequences of the Cd86 gene. FACS analysis on splenocytes and LPS-stimulated B cells derived from homozygous mice confirmed that no functional protein was encoded by this allele.