1. Department of Biochemistry, Vanderbilt University School of Medicine, Nashville, TN 37232-0146, USA
2. Platform Technology and Science, GlaxoSmithKline, Medicines Research Centre, Gunnels Wood Road, Stevenage, Hertfordshire SG1 2NY, UK
3. York Structural Biology Laboratory, Department of Chemistry, University of York, York YO10 5DD, UK
4. Department of Medicine (Hematology/Oncology), Vanderbilt University School of Medicine, Nashville, TN 37232, USA
5. VA Tennessee Valley Healthcare System, Nashville, TN 37212, USA