1. Howard Hughes Medical Institute and Department of Human Genetics, University of Utah Medical Center, Salt Lake City, UT 84132.
2. Department of Biological Chemistry, California College of Medicine, Irvine, CA 92717.
3. St. Mary's Hospital Medical School, Department of Biochemistry, University of London, London W2 1PG, U.K.
4. Division of Gastroenterology, University of Utah Health Sciences Center, Salt Lake City, UT 84132.
5. St. Mark's Hospital, Salt Lake City, UT 84117.
6. Department of Biology, Utah State University, Logan, UT 84321.