1. Department of Cellular and Molecular Pharmacology, Howard Hughes Medical Institute, and Center for Systems and Synthetic Biology, University of California, San Francisco, CA 94158, USA.
2. Department of Mechanical Engineering, Stanford University, Stanford, CA 94305, USA.
3. Present address: Eli and Edythe Broad CIRM Center for Regenerative Medicine and Stem Cell Research, University of Southern California, Los Angeles, CA 90033, USA.