1. Boston Biomedical Research Institute, Watertown, MA 02472, USA.
2. Department of Medical Biochemistry and Microbiology, Uppsala University Biomedical Center, PO Box 582, S-75123, Uppsala, Sweden.
3. Department of Biomedicine, Division of Physiology, University of Bergen, Jonas Lies vei 91, 5009 Bergen, Norway.
4. Center for Stem Cell Biology, Vanderbilt University, Nashville, TN 37232,USA.