1 School of Physiology, Pharmacology and Neuroscience, University of Bristol, Biomedical Sciences Building, University Walk, Bristol BS8 1TD, UK. Electronic address: D.N.Sheppard@bristol.ac.uk.
2 Peter Gilgan Centre for Research and Learning, Programme of Molecular Structure and Function, Hospital for Sick Children, 686 Bay St., Rm. 209420 U-West, Toronto, ON M5G 0A4, Canada.
3 Department of Gastroenterology and Hepatology, Erasmus University Medical Center, 's-Gravendijkwal 230, 3015 CE Rotterdam, The Netherlands.