Comparison of existing protein and cDNA data for human liver mitochondrial aldehyde dehydrogenase reveals deviations in two segments. They are shown to correspond exactly to localized frameshifts in the cDNA data, which are likely to have three reading errors. After correction of the cDNA frameshifts, a deduced amino acid sequence corresponds exactly to the data established at the protein level. In addition, extension of the shifted frame into the cDNA corresponding to the mitochondrial leader sequence allows reinterpretation of that sequence. The new leader sequence is consistent with characteristics of such segments of other mitochondrial protein pro-forms. Furthermore, the sequence displays a homology, when centered around the cleavage site, with the leader sequence of rat liver carbamyl phosphate synthetase I, suggesting a novel similarity between mitochondrial targeting sequences of two different enzymes.