A neomycin cassette was inserted into exon 3 (codon 40 of the 439 codon reading frame) of the gene. This insertion truncated the peptide 13 amino acid residues downstream of the cleavage site for the N-terminal mitochondrial targeting sequence. Southern blots showed production of a truncated protein in mutant mice, and functional assasys showed lack of enzymatic activity of the truncated protein. (J:29269)