[1] Royal Belfast Hosp Sick Children, Dept Child Hlth, Belfast, Antrim, North Ireland
[2] Royal Belfast Hosp Sick Children, Dept Clin Biochem, Belfast, Antrim, North Ireland
[3] Royal Hosp Sick Children, Dept Child Hlth, Bristol BS2 8BJ, Avon, England
[4] Rijkshosp, Inst Biochem, Oslo, Norway
[5] Ullevaal Univ Hosp, Dept Paediat, Oslo, Norway
[6] Great Ormond St Hosp Sick Children, Metab Unit, London WC1N 3JH, England