1. Centre for Heavy Metals Research, School of Chemistry, The University of Sydney, New South Wales 2006, Australia, Laboratory of Cell Biology, National Cancer Institute, National Institutes of Health, 37 Convent Drive, Bethesda, Maryland 20893-4256, and Oxford Drug Resistance Group, Nuffield Department of Clinical Laboratory Sciences, John Radcliffe Hospital, University of Oxford, Oxford OX3 9DU, U.K.