THE syndrome of biotin-responsive multiple carboxylase deficiency has two relatively distinct phenotypes: a neonatal form, with the onset of life-threatening lactic acidosis, seizures, and erythematous rash occurring in the first weeks of life,¹ and a juvenile form, with later onset of acute symptoms of ataxia, alopecia, and keratoconjunctivitis, in addition to seizures and lactic acidosis.^{2 , 3} Both forms are characterized by an elevation in metabolites proximal to the three biotin-dependent carboxylase reactions, and both show a dramatic response to the administration of pharmacologic doses of biotin. One patient with the neonatal form had deficient activity of biotin holocarboxylase synthetase,⁴ the enzyme . . .