Exon 3 was replaced with neomycin/kanamycin cassette inserted by homologous recombination. The deleted region encoded residues 126 through 251 which make up a portion of the CoA and cobalamin binding domains. Transcript was undetected in homozygous mutant mice by RT-PCR analysis of hepatic RNA. [14C]propionate incorporation in primary fibroblast cultures from mutant mice revealed that enzyme activity was reduced to ~6% of wild-type levels.