1. Department of Biomedical Engineering, University of Wisconsin—Madison, Madison, Wisconsin, USA
2. Department of Biochemistry, University of Wisconsin—Madison, Madison, Wisconsin, USA
3. Division of Biology and Biological Engineering, California Institute of Technology, Pasadena, California, USA
4. Howard Hughes Medical Institute, California Institute of Technology, Pasadena, California, USA
5. Department of Chemistry, University of Wisconsin—Madison, Madison, Wisconsin, USA