1. Section of Islet Cell and Regenerative Biology, Joslin Diabetes Center, Department of Medicine, Brigham and Women’s Hospital, and Harvard Medical School, Boston, MA 02215, USA.
2. Department of Cell Biology, Program in Biological and Biomedical Sciences, Graduate School of Arts and Sciences, Harvard University, Cambridge, MA 02138, USA.
3. Present address: Discovery Research Division, Institute of Molecular and Cell Biology, Proteos, Singapore 138673, Singapore.
4. Present affiliation: School of Biological Sciences, Nanyang Technological University, Singapore 637551, Singapore.
5. Present affiliation: Department of Biochemistry, Yong Loo Lin School of Medicine, National University of Singapore, Singapore 117596, Singapore.