1. P. Ren, C.-S. Lim, R. Johnsen, P. S. Albert, D. L. Riddle, Molecular Biology Program and Division of Biological Sciences, 311 Tucker Hall, University of Missouri, Columbia, MO 65211, USA.
2. D. Pilgrim, Department of Biological Sciences, G-507 Biological Sciences Building, University of Alberta, Edmonton, Alberta, Canada T6G 2E9.