1. Department of Molecular Neuroscience, Institute of Neurology, Queen Square, London WC1N 3BG, U.K.
2. National Institute of Ageing, National Institutes of Health, 31 Center Drive, Bethesda, MD 20892, U.S.A.
3. Reta Lila Weston Institute of Neurological Sciences, Windeyer Building, University College London, 46 Cleveland Street, London W1T 4JF, U.K.