1. Department of Structural Biology, Stanford University School of Medicine, Stanford, CA 94305-5126, USA.
2. Department of Applied Physics, Stanford University, Stanford, CA 94305-4090, USA.
3. Schools of Biochemistry and Microbiology, University College Cork, Western Gateway Building, Western Road, Cork, Ireland.
4. Department of Human Genetics, University of Utah, Salt Lake City, UT 84112-5330, USA.