1 Jeffrey A. Bluestone is Director, Hormone Research Institute, and holds the A. W. and Mary Margaret Clausen Distinguished Professorship in Metabolism and Endocrinology at the University of California, San Francisco, San Francisco, CA 94143, USA. E-mail: jeff.bluestone@ucsf.eduQizhi Tang is Associate Professor of Surgery and Director of the UCSF Transplantation Research Laboratory, University of California, San Francisco, San Francisco, CA 94143, USA.