A neomycin expression cassette was inserted into exon 6, deleting 31 bp of sequence encoding the region of the active site and rendering the sequence out of frame after the insertion. Northern blot analysis on spleen RNA demonstrated an absence of the normal transcript in homozygous mice, and western blot analysis showed that the protein was not expressed in peritoneal macrophages of homozygous mice. This allele was generated in ES cells that lack protein expression of Casp11 (Casp129mt).