1. Howard Hughes Medical Institute, Worcester, MA 01605, USA.
2. Program in Molecular Medicine, University of Massachusetts Medical School, Worcester, MA 01605, USA.
3. Howard Hughes Medical Institute and Department of Immunobiology, Yale University School of Medicine, New Haven, CN 06520, USA.
4. Department of Medicine, Division of Endocrinology, Metabolism and Diabetes, University of Massachusetts Medical School, Worcester, MA 01605, USA.