1. Department of Chemical and Systems Biology, Stanford University School of Medicine, Stanford, CA 94305, USA.
2. Department of Biochemistry and Biophysics, University of California, San Francisco, San Francisco, CA 94158, USA.
3. Department of Cell and Developmental Biology, Medical University of Vienna, 1090 Vienna, Austria.
4. Department of Developmental Biology, Stanford University School of Medicine, Stanford, CA 94305, USA.
5. Institute for Stem Cell Biology and Regenerative Medicine, Stanford University School of Medicine, Stanford, CA 94305, USA.
6. Howard Hughes Medical Institute, Stanford University School of Medicine, Stanford, CA 94305, USA.