Recently, enzymes of the valine degradation pathway have also been shown to cause LS [8, 9]; in particular, mutations in enoyl-CoA hydratase (ECHS1), a nuclear gene encoding a mitochondrial matrix enzyme catalyzing the second step of the ß-oxidation spiral of fatty acids, has been associated with LS in several patients [9–11]. The gene discussed is ECHS1; the disease is Leigh syndrome.