We describe three patients, from two Spanish families, with 2-methyl-3-hydroxybutyryl-CoA dehydrogenase (MHBD) deficiency, a recently described X-linked neuro-degenerative inborn error of isoleucine metabolism. Two of them are males with severe lactic acidosis suggestive of a mitochondrial encephalopathy, and the third is a female who was less severely affected, suggesting skewed X-inac-tivation. Molecular studies revealed a new missensemutation, 740A→ G, in one family and a previously described mutation, 388C→ T, in the other, causing the amino acid substitutions N247S and R130C, respectively. Both male patients died, one of them despite treatment with an isoleucine-restricted diet, but the disease has remained stable in the female patient after 1 y of treatment.
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