1. Nuffield Department of Clinical Biochemistry and Imperial Cancer Research Fund Laboratories, Institute of Molecular Medicine, John Radcliffe Hospital, University of Oxford, Oxford OX3 9DS, UK.,
2. Laboratoire de Microbiologie et Génétique Moléculaire du Centre National de la Recherche Scientifique, 118 Route de Narbonne, 31062 Toulouse, France.