William L. Nyhan, MD, PhD, is Professor of Pediatrics and Founding Director of The William L. Nyhan Biochemical Genetics and Metabolomics Laboratory at the University of California, San Diego.Georg F. Hoffmann, MD, is Professor of Pediatrics and Chairman of the University Children’s Hospital and Head of the Center of Rare Diseases at the University Clinic Heidelberg in Heidelberg, Germany.Aida I. Al-Aqeel and Bruce A. Barshop have also provided contributions.