Two cDNAs encoding different precursor proteins of the same mature proteolipid subunit of mitochondrial ATP synthase have been cloned from a bovine cDNA library. The hybridisation probe was a mixture of 17-mer oligonucleotides containing 256 discrete sequences. The coding sequences of the two cDNAs differ in 25 silent positions of codons and the 3' non-coding sequences are only weakly related. The precursor sequences, which direct the import of the proteolipid into the mitochondrion, are 61 and 68 amino acids long. They are related to each other in regions which probably are recognition signals for the processing protease. The corresponding genes are expressed differently in various tissues in a way that reflects their embryonic origin.