The gene for human cytochrome c oxidase subunit VIIa liver isoform (COX7AL) was isolated and its sequence determined and analyzed. The three introns of the gene are considerably larger than those of the heart isoform of subunit VIIa (COX7AH), but the position of the introns relative to the cDNA sequences is homologous between the two genes. Comparison with other isolated COX7AL genes suggests that the promoter region binding motifs for transcription factors have evolved along with the coding region. In fibroblasts cultured originally from a Leigh's disease patient, a shortened COX7AL cDNA was identified by RT-PCR, consisting of exon I joined to exon IV, omitting exons II and III. No mutation could be identified in COX7AL of the patient, suggesting that the shortened cDNA is due to an alteration of the genome during cell culture. A surprising transcription of COX7AH was observed in cultured fibroblasts, suggesting a potential utility of these cells for study of its gene expression.