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