Maple syrup urine disease (MSUD) (OMIM #24860), also known as leucinosis, is an inborn error of metabolism (IEM) caused by the deficiency of thiamine-dependent branched-chain α-ketoacid dehydrogenase (BCKD), composed of the subunits E1α, E1β, E2, and E3. The gene discussed is BCKDHB; the disease is maple syrup urine disease.