A Fine Balance Intellectual and neurological disabilities can arise from diverse developmental aberrations. Novarino et al. (p. 394 , published online 6 September; see the Perspective by Beaudet ) have now determined the genetic basis for one such disorder for a small group of patients. Exome sequencing led to identification of mutations in a kinase BCKDK ( Branched Chain Ketoacid Dehydrogenase Kinase ) that regulates metabolism of branched-chain amino acids such as valine, leucine, and isoleucine. Mice with homozygous mutations in the BCKDK gene showed developmental and neurological abnormalities resembling those in certain mouse autism models. Analysis of transport mechanisms responsible for carrying amino acids across the blood-brain barrier revealed competition between the branched-chain amino acids and large neutral amino acids. Nutritional supplementation with extra branched-chain amino acids in the diet of mice carrying homozygous mutations in the BCKDK gene normalized their phenotype.