Maple syrup urine disease (MSUD, MIM #248600) is an autosomal recessive inherited metabolic disorder caused by the deficiency of branched-chain α-ketoacid dehydrogenase (BCKDH) complex, an enzyme responsible for the second step of branched-chain amino acids catabolism [1, 2]. The gene discussed is PPM1K; the disease is maple syrup urine disease.