1. Department of Cell and Tissue Biology, University of California at San Francisco, San Francisco, CA 94143, USA.
2. Eli and Edythe Broad Center of Regeneration Medicine and Stem Cell Research, University of California at San Francisco, San Francisco, CA 94143, USA.
3. Department of Molecular Pathology, Graduate School of Medicine, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan.
4. Helen Diller Family Comprehensive Cancer Center, University of California at San Francisco, San Francisco, CA 94143, USA.
5. Department of Cellular and Molecular Pharmacology and Howard Hughes Medical Institute, University of California at San Francisco, San Francisco, CA 94143, USA.
6. Department of Anatomy, University of California at San Francisco, San Francisco, CA 94143, USA.