1. Z. Galcheva-Gargova, I-H. Wu, T. Barrett, R. J. Davis, Program in Molecular Medicine, Department of Biochemistry and Molecular Biology, University of Massachusetts Medical School and Howard Hughes Medical Institute, Worcester, MA 01605, USA.
2. K. N. Konstantinov and F. G. Klier, Department of Molecular and Experimental Medicine, and Department of Cell Biology, Scripps Research Institute, La Jolla, CA 92037, USA.