1. T. Leeuw, A. Fourest-Lieuvin, C. Wu, K. Clark, M. Whiteway, D. Y. Thomas, E. Leberer, Eukaryotic Genetics Group, Biotechnology Research Institute, National Research Council of Canada, 6100 Royalmount Avenue, Montreal, Quebec H4P 2R2, Canada.
2. J. Chenevert, Department of Biochemistry and Biophysics, University of California, San Francisco, CA 94143-0448, USA.