1. UCSF Diabetes Center, University of California, San Francisco, San Francisco, CA 94143–0669, USA.
2. Eli and Edythe Broad Center of Regeneration Medicine and Stem Cell Research, University of California, San Francisco, San Francisco, CA 94143–0669, USA.
3. Department of Cell and Tissue Biology, University of California, San Francisco, San Francisco, CA 94143–0669, USA.