Subunit c is an intrinsic membrane component of ATP synthase, and in mammals it is encoded by two expressed nuclear genes, P1 and P2. Both genes encode the same mature c subunit, but the mitochondrial import pre-sequences in the precursors of subunit c are different. The DNA sequences of the human P1 and P2 genes are described. They occupy about 3.0 and 10.9 kb respectively of the human genome, and both genes are split into five exons. The human genome also contains about 14 related spliced pseudogenes, and the sequence of one such pseudogene related to P2 is described. Sequences flanking the 5' ends of the human P1 and P2 coding sequences each contain a CpG-rich island. Potential promoter elements (TATA and CCAAT boxes) are present in the 5' sequences of the P1 gene, but not that of P2, although there is no direct experimental evidence to show the involvement of these sequences in transcription of the genes.