3-hydroxy-3-methylglutaryl-CoA synthase 2 (mitochondrial) is an enzyme in humans that is encoded by the HMGCS2 gene. The protein encoded by this gene belongs to the HMG-CoA synthase family. It is a mitochondrial enzyme that catalyzes the second and rate-limiting reaction of ketogenesis, a metabolic pathway that provides lipid-derived energy for various organs during times of carbohydrate deprivation, such as fasting, by addition of a third acetyl group to acetoacetyl-CoA, producing HMG-CoA. Mutations in this gene are associated with HMG-CoA synthase deficiency. Alternatively spliced transcript variants encoding different isoforms have been found for this gene.