Three kinds of inborn error of branched-chain amino acids (valine, leucine isoleucine) are described with a focus on the findings in recent biochemistry and molecular genetics. Frequency of valinemia and leucinisoleucinemia is quite low but their significance is distinct because of our knowledge that at least two aminotransferases, one branched-chain keto acids dehydrogenase and two short-chain acyl-CoA dehydrogenases are present in human, is based on the observations of these limited cases. In MSUD, the sequential study of each subunit cDNA reveals a close relationship between the clinical phenotypes and the various sites of mutation.