Mitochondrial 3-hydroxy-3-methylglutaryl CoA synthase (mHS) is the first enzyme of ketogenesis, whereas the cytoplasmic HS isozyme (cHS) mediates an early step in cholesterol synthesis. We here report the sequence of human and mouse liver mHS cDNAs, the sequence of a HS-like cDNA from Caenorhabditis elegans, the structure of a partial human mHS genomic clone, and the mapping of the human mHS gene to chromosome 1p12-p13. The nucleotide sequence of the human mHS cDNA encodes a mature mHS peptide of 471 residues, with a mean amino acid identity of 66.5% with cHS from mammals and chicken. Comparative analysis of all known mHS and cHS protein and DNA sequences shows a high degree of conservation near the N-terminus that decreases progressively toward the C-terminus and suggests that the two isozymes arose from a common ancestor gene 400-900 million years ago. Comparison of the gene structure of mHS and cHS is also consistent with a recent duplication event. We hypothesize that the physiologic result of the HS gene duplication was the appearance of HS within the mitochondria around the time of emergence of early vertebrates, which linked preexisting pathways of beta oxidation and leucine catabolism and created the HMG CoA pathway of ketogenesis, thus providing a lipid-derived energy source for the vertebrate brain.