1. Nestlé Research Center, Nestec Ltd., Vers-chez-les-Blanc, 1000 Lausanne 26, Switzerland, Jack Birch Unit for Environmental Carcinogenesis, Department of Biology, University of York, Heslington, York Y01 5DD, U.K., York District Hospital, Wiggington Road, York Y03 7HE, U.K., and Molecular and Structural Biology Division and Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, California 94550