The yeast mitochondrial outer membrane contains a major 70-kd protein which is coded by a nuclear gene. Two forms of this gene were isolated from a yeast genomic clone bank: the intact gene, and a truncated gene which had lost a large part of its 3' end during the cloning procedure. Upon transformation into yeast, both the intact and the truncated gene are expressed; the truncated gene generates a shortened protein missing 203 amino acids from the carboxy-terminus. This truncated polypeptide reacts with a monoclonal antibody against the authentic 70-kd protein and is transported to the mitochondrial outer membrane. By integrative transformation, we have constructed a yeast mutant which lacks the 70-kd protein and is unable to adapt to growth on a nonfermentable carbon source at 37 degrees C. This phenotypic lesion can be corrected by transforming the mutant with the intact, but not the truncated gene. The carboxy-terminal sequence of 203 amino acids is thus necessary for the function of the protein, but not for its targeting to the mitochondrion.