1 Division of Endocrinology and Diabetes (C.P.H., S.S., M.A.L.), The Children's Hospital of Philadelphia, and Department of Pediatrics (M.A.L.), Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19104; National Children's Research Centre (C.P.H.), Dublin 12, Ireland; and Department of Laboratory Medicine and Pathology (R.S.), The Mayo Clinic, Rochester, Minnesota 55905.