Exon 3 of the endogenous allele was disrupted by the insertion of a neomycin selection cassette via homologous recombination. The deleted region encoded the G1 and G2 motifs of the GTPase domain. The insertion introduced a translational stop codon into the 5' region of exon 3. Protein was undetected by Western blot analysis of homozygous mutant embryos.