Exon 1 was disrupted by the insertion of a neomycin selection cassette immediately following the start codon. Northern blot analysis showed an absence of transcript in homozygous mutant mice. The absence of protein produced from Ube2b was determined via Western blot analysis of testicular extracts. Further analysis showed that the level of protein produced from the homologous Ube2a locus was unaffected by the targeting event.