1. Clinical Neuroscience Branch, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland 20892; Children’s Hospital Division of Genetics, Harvard Medical School, Boston, Massachusetts 02115; The Diabetes and Nutrition Research Laboratory, St. Luke’s Hospital, Kansas City, Missouri 64111; UCL Medical School, Whittington Hospital, London NI9 3UA, Unit