1The Burnham Institute, Glycobiology Program, 10901 North Torrey Pines Road, La Jolla, CA 92037, USA, 1Department of Clinical Genetics, University Hospital Rigshospitalet, Copenhagen, Denmark, 2Center for Human Genetics, University of Leuven, Leuven, Belgium and 3Center for Metabolic Medicine, Great Ormond Street Hospital for Sick Children, Great Ormond Street, London WC1N 3JH, UK