1. California Institute for Quantitative Biosciences (QB3) and Department of Molecular and Cell Biology, University of California Berkeley, Berkeley, CA 94720, USA.
2. Howard Hughes Medical Institute, Chevy Chase, MD 20815, USA.
3. Department of Mechanical Engineering, University of California Berkeley, Berkeley, CA 94720, USA.
4. Department of Biological Sciences and Engineering, Life Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.