1. Department of Biochemistry, Duke University School of Medicine, Durham, NC 27710, USA.
2. Departments of Biological Sciences, Chemistry, and Bioengineering, Lehigh University, Bethlehem, PA 18015, USA.
3. Genome Integrity and Structural Biology Laboratory, National Institute of Environmental Health Sciences, National Institutes of Health, Department of Health and Human Services, Research Triangle Park, NC 27709, USA.