Engelke et al.8proposed this mechanism, citing evidence of raised CSF/plasma ratios of 3‐HIVA in an adult patient with MGA1 and progressive leukoencephalopathy, but also in other conditions affecting leucine degradation where neurological abnormalities occur (such as biotinidase deficiency).15 The gene discussed is BTD; the disease is hyperinsulinemic hypoglycemia, familial, 4.