To investigate substrate specificity of SCEH, activity measurements were performed with different substrates in cultured fibroblasts of the SCEH-deficient patients and fibroblasts of two patients with a deficiency of other mitochondrial enoyl-CoA hydratases: one patient with mitochondrial trifunctional protein (MTP) deficiency and one patient with 3-methylglutaconyl-CoA hydratase (MGH) deficiency. The gene discussed is AUH; the disease is hyperinsulinemic hypoglycemia, familial, 4.