1. Molecular Oncology Program, Cross Cancer Institute, Edmonton, Alberta, Canada T6G 1Z2, Cancer Research Campaign Nitrosamine-Induced Cancer Research Group, Department of Biochemistry and Molecular Biology, University College London, Grower Street, London WC1E 6BT, England, Department of Biological Sciences, Molecular Section, University of Southern California, Los Angeles, California 90089-1340, and Istituto di Ricerche di Biologia Molecolare, P. Angeletti, Via Pontina km. 30.600, 00040-Pomezia (Roma), Italy