1. Division of Structural Biology, The Wellcome Trust Centre for Human Genetics, University of Oxford, Roosevelt Drive, Oxford, OX3 7BN, UK, Oxford Centre for Molecular Sciences, New Chemistry Building, University of Oxford, South Parks Road, Oxford, OX1 3QT, UK, GlaxoSmithKline Research and Development, Stevenage, Hertfordshire, U.K. and GlaxoSmithKline Research and Development, 5 Moore Drive, Research Triangle Park, North Carolina 27709