A neomycin resistance gene replaces 2.5 kb of intron 1 and all but the last 6 bp of exon 2. This allele deletes the sequences encoding the translation start site as well as the membrane-binding domain. The targeting event generated a truncated protein which localized to the cytoplasm and not present in microsomes. Cytochrome c reductase activity in liver microsomes isolated from adult heterozygous mice was 50% that found in wild-type mice.