ACAT1 is a mitochondrially localized enzyme that catalyzes the reversible formation of acetoacetyl-CoA from two molecules of acetyl-CoA. Defects in this gene are associated with 3-ketothiolase deficiency, an inborn error of isoleucine catabolism characterized by urinary excretion of 2-methyl-3-hydroxybutyric acid, 2-methylacetoacetic acid, tiglylglycine, and butanone.The 427-amino acid precursor had a molecular mass of 45.2 kD. The sequence included a 33-residue leader peptide and a 394-amino acid subunit of the mature enzyme, which had a molecular mass of 41.4 kD. In all 4 cell lines, the T2 mRNA had the same 1.7-kb transcript as that of the control; however, content was reduced in 2 cell lines and normal in the other 2. Human T2 is a homotetramer of 41-kD subunits.