1. Division of Biology and Biological Engineering, California Institute of Technology, Pasadena, California 91106, United States
2. Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91106, United States
3. Howard Hughes Medical Institute, Janelia Research Campus, Ashburn, Virginia 20147, United States
4. Howard Hughes Medical Institute, University of California, San Diego, San Diego, California 92093, United States