A neomycin resistance cassette replaced a genomic fragment containing exon 1, which encodes the translational initiation site and part of the GTP-binding domain. Northern blot and RT-PCR analysis demonstrated that no transcripts were detectable in total RNA derived from neutrophils or lymphocytes in homozygous mice. Western blot analysis confirmed an absence of protein produced from this allele in peritoneal exudate cells from homozygous mice.