1. University Departments of Medicine and Clinical Biochemistry (I.S.F., S.O.R.), Cambridge CB2 2QQ, United Kingdom
2. Department of Medicine, Singleton Hospital (M.K.J., M.E.), Swansea SA2 8QA, United Kingdom
3. Neurology (J.R.H.), Cambridge CB2 2QQ, United Kingdom
4. Medical Research Council Cognition and Brain Sciences Unit (J.R.H.), Addenbrooke’s Hospital, Cambridge CB2 2QQ, United Kingdom