A very rare inherited metabolic disorder involving abnormal metabolism of branched chain amino acids (leucine, isoleucine and valine) and resulting in severe illness which generally leads to death if not treated. Even mild form can result in mental and physical retardation if untreated. Various types of maple syrup urine disease involve different genetic defects – type 1B specifically involves a defect in the E1-beta subunit gene. See also Maple syrup urine disease, type 1B